RESEARCH PAPER

Are the Costs of Site Unfamiliarity Compensated With Vigilance?A Field Test in Eurasian SiskinsJordi Pascual, Juan Carlos Senar & Jordi Dom�enech

Evolutionary and Behavioural Ecology Research Unit (CSIC), Natural History Museum of Barcelona, Barcelona, Spain

Correspondence

Jordi Pascual, Evolutionary and Behavioural

Ecology Research Unit (CSIC), Natural History

Museum of Barcelona, Pg. Picasso s/n, 08003

Barcelona, Spain.

E-mail: jpascualsala@gmail.com

Received: January 2, 2014

Initial acceptance: February 12, 2014

Final acceptance: March 23, 2014

(M. Herberstein)

doi: 10.1111/eth.12243

Keywords: residence, dispersal, vigilance

behaviour, site unfamiliarity, dominance,

social foraging

Abstract

Determination of fitness differentials between individuals adopting differ-

ent migratory and dispersal strategies is basic to understand the evolution

of migration. In the Eurasian siskin Carduelis spinus, both resident and

transient birds forage within the same wintering area, providing the rare

opportunity to compare their foraging behaviour in the same area and

habitat. The aim of this study was to test the predictions associated to the

different hypothesized costs of transience by studying the vigilance and

foraging behaviour of wild wintering siskins foraging at three bird tables

with different predation risk and interference competition levels. Tran-

sient siskins showed longer scan durations than residents, either because

of site unfamiliarity or subordination (i.e. prior-occupancy effect). How-

ever, residents and transients did not differ in aggression rates, contrary to

the dear-enemy effect. Transient siskins did not show a higher allocation

of time to vigilance, contrary to the hypothesis of compensation vigilance

to reduce predation risk by dispersing animals. Moreover, transients

increased pecking rate with increasing predation risk, showed lower scan

rates, longer foraging bouts and, in males, presented marginally higher

proportions far from cover. Altogether these results strongly support the

hypothesis that transients incur a predation cost due to a less efficient vigi-

lance and foraging system.

Introduction

Migration and dispersal are widespread behaviours in

many animal taxa (Clobert et al. 2001; Milner-Gul-

land et al. 2011). A great amount of individual vari-

ability in migratory strategies coexists in many species

and is maintained because of the changing ecological

conditions and food availability over time and space

(Johnson & Gaines 1990). Wintering survival of resi-

dent and migratory individuals is highly affected by

weather conditions, food availability and also by their

own experience, so that it might show great variations

between years and between cohorts (Sanz-Aguilar

et al. 2012).

The study of the costs and benefits of migration

and dispersal is an important topic in zoology which

has received much attention (Johnson & Gaines

1990; B�elichon et al. 1996; Coulton et al. 2011). The

main reported costs of migration are related to access

to food and predation risk (Alerstam 2011). Winter

food availability at wintering grounds and stop-over

locations cannot be predicted from food availability

at the breeding area, and therefore, migration entails

some level of uncertainty in the access to food.

Additionally, even when food is abundant, it might

be defended by resident holders, which are domi-

nant over transients (see Matthysen1993 for a

review) owing to the prior-occupancy effect (Sandell

& Smith 1991). Moreover, unfamiliarity with flock-

mates may involve a cost for transients in terms of

higher aggression rates while feeding between

strangers than between familiar individuals (Chaves-

Campos et al. 2009). Finally, unfamiliarity with local

food might also involve a cost to transients. These

individuals might be slower in both searching for

and handling food, thereby reducing the efficiency

of their routine vigilance while foraging (Baker et al.

2011).

Ethology 120 (2014) 1–13 © 2014 Blackwell Verlag GmbH 1

Ethology

ethologyinternational journal of behavioural biology

Dispersing animals have also been shown to suffer

higher predation rates because of their higher move-

ment rate and their site unfamiliarity (Metzgar 1967;

Yoder et al. 2004). The costs of transience related to

site unfamiliarity have been attributed to increased

time to reach a refuge (Clarke et al. 1993; Hoogland

et al. 2006), to worse knowledge of predators (Frair

et al. 2007; Robinson & Merrill 2013), to higher use

of risky habitats (Koivunen et al. 1998) or to a combi-

nation of all (Yoder et al. 2004). Lind & Cresswell

(2006) suggested that migratory birds could compen-

sate their higher risk of predation by joining larger

flocks, changing patches and/or allocating more time

to antipredatory behaviour.

One central problem in testing the costs and bene-

fits of migration is that we cannot compare the behav-

iour of migratory and non-migratory individuals as

they are found in different locations. Wintering Eur-

asian siskins (Carduelis spinus) provide us with the rare

opportunity for such a test. Siskins have a social sys-

tem in which there are both resident and transient

individuals within the same wintering area (Senar

et al. 1992). Residents are present in a given locality

for extended periods, making only short-range move-

ments (usually <3 km). In contrast, the majority of

birds (77%) are transients, staying for short periods

and being very mobile (making movements of

10–40 km in a single day; Senar et al. 1992). There-

fore, in this species, we have a subpopulation of

dispersing individuals feeding together with a subpop-

ulation of resident individuals within the same area

and even the same habitat.

Residence and transience in siskins are probably

evolutionary stable strategies involving both costs and

benefits (B�elichon et al. 1996; Kokko 2011). Resident

siskins are familiar with local predators, food and resi-

dent flock mates at wintering grounds, and they are

dominant over transients (Senar et al. 1990b). Siskins

are gregarious birds which form small groups with

tight social bonds. The individuals of resident groups

know each other, and they are expected to show tol-

erance with familiar birds (Senar et al. 1990a). On the

other hand, transient groups do not know each other,

and therefore, they should be more prone to fight to

access food.

Vigilance and foraging are the main activities of

birds during the non-breeding season. Hence, the

study of these behaviours is highly relevant to deter-

mine the possible evolutionary costs of transience.

Lind (2004) in a theoretical study pointed out to the

key importance of vigilance to predators, more than

body mass regulation, on survival of migratory birds,

and stressed the need of studies that focused on

vigilance during fuelling. In a recent paper, resident

elks were shown to be more efficient than migrants in

adjusting their vigilance levels to spatial variation in

wolf predation risk, and they were found to be also

better at mitigating the foraging costs of vigilance by

synchronizing vigilance with chewing (Robinson &

Merrill 2013). Feeding and vigilance behaviours have

been shown to be dependent on dominance rank, so

that subordinates are forced by dominants to feed in

riskier patches (Koivula et al. 1994) and also to be

more vigilant in order to keep dominants, in addition

to predators, under surveillance (Waite 1987).

The aim of this paper is to determine the possible

costs of transience in wintering siskins by comparing

the vigilance and foraging behaviour of resident and

transient birds while feeding together and to deter-

mine whether transients compensate for their site

unfamiliarity with increased vigilance. We designed

an experiment with three feeders differing in preda-

tion risk and interference competition (Pascual &

Senar 2013), and we compared for both subpopula-

tions the vigilance variables, pecking rate, foraging

bout length, departure reasons, percentage of time

spent in aggressions and proportion of birds at each

feeder. According to the site unfamiliarity effect, we

should expect transient siskins to have a greater use

of the high predation risk feeder (Koivunen et al.

1998), to have longer foraging bouts and less distur-

bance-related departures (especially at this feeder)

and to show longer scan durations (and hence more

vigilance non-compatible with feeding; Robinson &

Merrill 2013) due to their ignorance of the direction

of a possible attack (Desportes et al. 1991). According

to the prior-occupancy effect (Sandell & Smith 1991),

we should expect transient siskins to be displaced to

the high predation risk feeder by dominant residents

(Koivula et al. 1994), to display longer scan durations

to keep dominant competitors under surveillance

(Knight & Knight 1986; Pascual & Senar 2013) and to

show lower pecking rates, shorter foraging bouts and

more aggression-related departures, especially at the

high competition feeder. According to the unfamiliar-

ity with local food effect (Baker et al. 2011), we

should also expect transients to have longer scan

durations because of an increase in handling times,

and according to the unfamiliarity with flockmates

effect (Chaves-Campos et al. 2009), we should expect

transients to have higher aggression rates. Finally,

according to the compensation hypothesis, we should

expect transient siskins to allocate more time to vigi-

lance to avoid the predation costs of dispersal (Lind

2004; Lind & Cresswell 2006), to reduce interscan

durations (i.e. to reduce the detection time to a

Ethology 120 (2014) 1–13 © 2014 Blackwell Verlag GmbH2

Vigilance, Foraging and Residence J. Pascual, J. C. Senar & J. Dom�enech

predator attack; Hart & Lendrem 1984; Whittingham

et al. 2004) and/or to have shorter foraging bouts and

more disturbance-related departures (especially at the

high predation risk feeder).

Methods

Model Species and Study Site

Our model species was the Eurasian siskin (C. spinus),

a socially foraging finch (Senar et al. 1992). This spe-

cies was selected because at the study area wintering

siskin populations are formed by resident and tran-

sient birds (Senar et al. 1992). The study was carried

out in a permanent ringing station at Sarri�a

(41°24021″N, 2°06046″E), in the suburbs of Barcelona

city (Catalonia, NE Spain), in an area surrounded by

orchards, small pine woods (Pinus halepensis) and gar-

dens. Siskins were captured from 19 October 1996 to

22 March. 1997, coinciding with their wintering per-

manence at the area. Birds were captured under the

special authorization for scientific capture 206/97

from the Sudirecci�o General de Conservaci�o de la Na-

tura from the Departament d’Agricultura, Ramaderia i

Pesca of the Generalitat de Catalunya. After capture,

siskins were ringed and measured by expert bird ring-

ers (JCS and JD) under the authorization of the Orni-

thological Catalan Institute, and after that they were

immediately released back to the field.

The risk of predation at the area was real and high

because we witnessed six attacks of a male sparrow-

hawk (Accipiter nisus) to the birds foraging at the

experimental feeders and we found the remains of

over 20 predated siskins in a pine wood at <20 m from

them (Pascual et al. 2014).

Bird Ringing and Videotaping

We trapped siskins weekly throughout the wintering

season using Yunik platform traps, mist and clap nets

(Senar 1988), and we marked them with numbered

aluminium rings. Birds were removed immediately

after capture at traps and clap nets, and mist nets were

revised every half an hour. Siskins recaptured more

than 15 d after their first capture (i.e. considered as

‘residents’; Senar et al. 1992) were additionally given

unique colour ring combinations, allowing long dis-

tance identification. Because of the high trapping

effort, we categorized birds as ‘transients’ if they were

not marked with colour rings. The 104 residents were

captured (�x � SE) 4.79 � 0.26 times, whereas the

479 transients were captured 1.29 � 0.03 times

(Mann–Whitney U = 1722, p < 0.0001).

We placed three feeders in one single site at a height

of 1 m from the ground. They had a border of 1.5 cm

and were filled with turnip (Brassica rapa) seeds to

0.5 cm every day, so we can assume the same density

of food for all. Birds foraging at the feeders were vid-

eotaped from a hide with a S-VHS-C movie camera

Panasonic NV-S7E equipped with digital zoom 916.

In total, 133 flocks were recorded at feeders on 28 dif-

ferent days from 10 January to 11 March 1997,

between 11.00 AM and 17.00 PM. During the weeks

of video recordings, the mean temperature was mild

(�x � SE; data from the Fabra Observatory, located at

1.7 km from the ringing station): 9.9 � 0.4°C in Jan.,

11.3 � 0.3°C in Feb. and 14.1 � 0.6°C in Mar. The

minimum temperature for this period was 4.4°C, andthe maximum temperature was 21.6°C. Most days

were sunny, and no video recordings were made on

the few rainy days.

Experimental Design

As we wanted to discriminate between the effects of

competition and predation risk on the vigilance

behaviour of resident and transient siskins, we

designed an experiment with three bird tables differ-

ing in their feeding surface (i.e. expected interference

competition; Elgar 1987) and distance from protective

cover (i.e. expected predation risk; Lima 1987). Two

feeders differing in surface were placed below an

almond tree, at 1.6 m from a dense Rhamnus alaternus

bush 2.2 m tall covering an area of 10 m2 and at 4 m

from a pine wood edge. The inner large feeder was

0.75 9 0.5 m, while the inner small feeder was

0.08 9 1 m. The third feeder was placed at 4.7 m

from the tree and the bush and at 8.5 m from the pine

wood edge and was the same size and shape as the

inner small feeder. The feeders were aligned NE, and

apart from the pine wood to the NE, there was an

almond tree orchard to the SE at 7 m from all the

feeders, a line of R. alaternus bushes at more than 5 m

from the outer small feeder to the SW and an area of

scrubs at 7 m from the feeders to the NW. Elsewhere

(Pascual & Senar 2013) we studied the foraging bout

lengths, departure reasons, aggression rates and num-

ber and density of birds at the three feeders and we

found: (1) that the foraging bout lengths were higher

at the inner large than at the inner small and outer

small feeders; (2) that the aggression rates were

higher at the inner small than at the outer small fee-

der and higher at the outer small than at the inner

large feeder (where almost no aggressive behaviours

were observed); (3) that at the inner large feeder the

most frequent departures were individual based (i.e.

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J. Pascual, J. C. Senar & J. Dom�enech Vigilance, Foraging and Residence

no apparent reason for leaving the feeder), at the

inner small feeder were aggressions received by the

focal birds and at the outer small feeder were distur-

bances (i.e. sudden departures of most or all of the

feeding birds); (4) that the number of birds foraging

on the inner large feeder (�x = 14.6 birds, SE = 1.29)

was higher than the mean number of birds feeding on

the inner small feeder (�x = 9.10 birds, SE = 0.38) and

the outer small feeder (�x = 7.87 birds, SE = 0.44),

while the mean density was lower on the inner large

feeder (�x = 38.84 birds/m2, SE = 3.43) than on the

inner small (�x = 113.75 birds/m2, SE = 4.81) and

outer small (�x = 98.33 birds/m2, SE = 5.45) feeders;

and (5) that the inner large and inner small feeders

had many birds around, perched on the almond tree,

at <1.2 m, so that the effective group size for them

was big and very similar, while the outer small feeder

did not have birds around it, so that its effective group

size was much lower. According to these results, we

concluded that the inner large feeder was a low pre-

dation risk and low competition feeder; the inner

small feeder was a low predation risk and high compe-

tition feeder, and the outer small feeder was a high

predation risk and intermediate competition feeder.

For more details, see Pascual & Senar (2013).

While recording at the feeders, the low predation

and low competition feeder was emptied of food to

force the birds to feed either at the low predation and

high competition or at the high predation and inter-

mediate competition feeders. Conversely, the low pre-

dation and high competition and the high predation

and intermediate competition feeders were emptied of

food when recording at the low predation and low

competition feeder from the same hide. We filmed

only half the length of the feeders when videotaping

the low predation and high competition and the high

predation and intermediate competition feeders, as

they were too long to be recorded in the same image.

Hence, to prevent any biases and increase the number

of individuals recorded, we shifted the video camera

every 2 min from one half of the feeder to the other.

Data Obtained From Tapes

When analysing the video recordings, we counted, for

each foraging group at each feeder, the sex and resi-

dence status of all the individuals for which we had

identified these traits in the field. For the low preda-

tion and high competition and the high predation and

intermediate competition feeders, we could obtain the

residence status of a big proportion of the birds forag-

ing at any time, as there were few birds and their posi-

tion in the feeder allowed us to easily check whether

they were colour ringer or not. However, at the low

predation and low competition feeder, there were

many birds, for most of them it was impossible to see

whether they were ringed or not, and during the

recordings, we were mainly concerned in finding col-

our ringed individuals, so we could not estimate the

percentage of residents for this feeder. For the com-

parison of the percentage of residents between feed-

ers, in males, we obtained 147 individuals at the low

predation and high competition feeder and 96 indi-

viduals at the high predation and intermediate com-

petition feeder, and in females, we obtained 119

individuals at the low predation and high competition

feeder and 125 individuals at the high predation and

intermediate competition feeder.

As we wanted to investigate the effect of residence

on vigilance behaviour, we looked for pairs of resident

and transient siskins foraging simultaneously in order

to avoid the confusion effects of flock size and other

time-related variables (Elgar 1989). We identified 62

pairs of residents and transients in the tapes (28 pairs

at the low predation and low competition feeder, 20

pairs at the low predation and high competition fee-

der and 14 pairs at the high predation and intermedi-

ate competition feeder). These pairs of birds were

selected so that they were foraging without fighting

or moving around for at least 30 s of recordings, the

time we analysed for the estimation of the vigilance

variables, and therefore, this subsample was called

‘undisturbed’. We did this selection because the vigi-

lance variables to be comparable between birds need

to be calculated when they only peck and scan. We

selected roughly the same number of pairs of males

and females at each feeder: 15 pairs of males and 13

pairs of females at the low predation and low competi-

tion feeder, 8 pairs of males and 12 pairs of females at

the low predation and high competition feeder and 6

pairs of males and 8 pairs of females at the high preda-

tion and intermediate competition feeder. As resident

birds were individually marked, we could check that

no resident bird was present in more than two pairs at

the same feeder. Transient siskins were not marked

with colour rings. However, the probability that we

repeated one transient in different pairs was very low

given their short stay in the area (compared with the

28 d of recordings throughout 2 mo) and the fact that

they comprised approximately three quarters of the

population (Senar et al. 1992), which by trapping

operations was shown to be of hundreds of birds (J.

Pascual, J. C. Senar & J. Dom�enech, pers. obs.). We

analysed the behaviour of a bird of a dyad using the

frame by frame function of the video (25 frames/s).

We then rewound the tape to the start of the focal pair

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Vigilance, Foraging and Residence J. Pascual, J. C. Senar & J. Dom�enech

and observed the other member of the dyad. This

allowed the same observer to follow both birds. We

recorded the percentage of time spent scanning (i.e.

with the tip of the beak raised to eye level or higher;

Lendrem 1983), the mean interscan duration (in sec-

onds), the mean scan duration (in seconds) and the

scan rate (as number of scans per second). We also

recorded the pecking rate (as an estimation of food

intake rate) of the birds during this period of time.

The selection of birds with at least 30 s of simulta-

neous feeding almost without fighting and moving

around could suppose the selection of relatively

peaceful birds or periods of time, and this could affect

the comparison between resident and transient

siskins. Therefore, we selected another subsample of

birds, called ‘random’, with no restriction of time on

feeder, aggression rate or movements. For each fee-

der, we registered all the different resident and tran-

sient birds that we found on the recordings and we

randomly selected one period of time for each. Then,

we paired each selected resident with the selected

transient that was foraging closer in time with it

(always within the same day and almost always,

except 11 pairs at the low predation and low competi-

tion feeder, within the same foraging group). We

found 27 pairs of males and 29 pairs of females at the

low predation and low competition feeder, 17 pairs of

males and 25 pairs of females at the low predation

and high competition feeder and 10 pairs of males and

14 pairs of females at the high predation and interme-

diate competition feeder. For them we analysed, for

the entire time spent on the feeder (or the total time

the bird was visible on the recordings), the percentage

of time spent in aggressions (considering aggressions

the agonistic interactions between individuals where

the focal bird either attacked or received the attack of

a flockmate) and the total time spent on the feeder (in

seconds). For one bird at the low predation and low

competition feeder, 11 birds at the low predation and

high competition feeder and 9 birds at the high preda-

tion and intermediate competition feeder, the times

on feeder were incomplete. The reason for this at the

low predation and high competition and at the high

predation and intermediate competition feeders was

that we shifted the video camera from one side of the

feeder to the other (see above) before the birds

departed. The single case at the low predation and

low competition feeder was due to the video tape fin-

ishing during the recording of this bird.

Additionally, we recorded the presumed reasons for

the departure of 221 siskins from the random subsam-

ple (low predation and low competition feeder: 55

residents and 53 transients; low predation and high

competition feeder: 35 residents and 36 transients;

and high predation and intermediate competition fee-

der: 22 residents and 20 transients), which were clas-

sified as follows: ‘aggression’ if the bird departed upon

being attacked, ‘disturbance’ if the focal bird and

other flock members departed suddenly and quickly

(usually after an alarm call emitted by a conspecific or

heterospecific bird) or ‘individual based’ if they were

not forced either by aggressions or by sudden distur-

bance departures.

Data Analysis and Transformation

We compared the percentages of residents between

feeders, both for all the sexes and for each one inde-

pendently, with the Pearson’s chi-square test of the

software Statistica 8.0 (StatSoft, Inc.). The same test

was used to compare the departure reasons between

residents and transients at all the feeders and at each

one independently.

For the comparison of times on feeder of residents

and transients, as choosing only birds with complete

times on feeder (i.e. recorded from arrival to depar-

ture) would have been biasing the data towards birds

with short values for this variable, we computed the

survival scores (with the survival analysis of the pro-

gram STATISTICA 8.0 (StatSoft Inc.)) for time on fee-

der of all the resident and transient focal siskins

together, treating incomplete times on feeder as cen-

sored data. As these scores did not fit the normal dis-

tribution of frequencies and they could not be

normalized using logarithmic or other related trans-

formations, we ranked them so that we could apply

parametric statistics (Conover 1981). Then, we com-

pared the ranked survival scores of time on feeder for

resident and transient siskins with a repeated mea-

sures ANOVA. To test whether there was an interac-

tion feeder 9 residence for this variable, as we could

not use a ranked variable (Blair et al. 1987), we

applied a one-way ANOVA with the difference in

scores of time on feeder between residents and tran-

sients as the dependent variable and feeder as the cat-

egorical factor. Then, we tested whether there was a

linear relationship between the difference in time on

feeder of residents and transients and the difference in

group size between them (because for many pairs

they did not feed simultaneously). Moreover, we

tested the effect of group size on time on feeder sepa-

rately for the low predation and low competition fee-

der and for the low predation and high competition

and the high predation and intermediate competition

feeders, as the number of birds highly differed

between the large and the small feeders, and we

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J. Pascual, J. C. Senar & J. Dom�enech Vigilance, Foraging and Residence

wanted to assess the effect of group size and not the

effect of feeder. For the low predation and low compe-

tition feeder, we computed the Spearman rank order

correlation between group size and the scores of time

on feeder (because on this feeder the scores did not fit

the normal distribution of frequencies). For the low

predation and high competition and the high preda-

tion and intermediate competition feeders, we applied

an ANCOVA with the scores of time on feeder as the

dependent variable, feeder as the categorical factor

and group size as the covariate.

We compared the percentage of time spent in

aggressions between residents and transients for the

three feeders together and for each one independently

(only the low predation and high competition and the

high predation and intermediate competition feeders)

by applying the Wilcoxon matched-pairs test, as the

distribution of these variables was highly skewed to

zero or to low values near zero (especially at the low

predation and low competition feeder, where only

15% of birds were involved in agonistic interactions,

compared with the 87% at the low predation and

high competition feeder and the 79% at the high pre-

dation and intermediate competition feeder). We also

computed an ANCOVA with percentage of time spent

in aggressions as dependent variable, residence as cat-

egorical factor and group size as covariable for only

the low predation and high competition and the high

predation and intermediate competition feeders. To

normalize the distribution of frequencies for percent-

age of time spent in aggressions and to avoid the effect

of the few zeros, we used the reciprocal of one plus

the percentage of time spent in aggressions.

Interscan durations and pecking rates did not fit the

assumption of normality in the distribution of fre-

quencies, and we applied the following transforma-

tions to normalize their distributions: minus inverse

of mean interscan durations and logarithm of pecking

rates. As we wanted to compare the values of the vigi-

lance and feeding variables between the two individu-

als of each pair at each feeder, we computed repeated

measures ANOVAs for each variable (i.e. percentage

of time spent scanning, logarithm of pecking rate,

mean scan duration and minus inverse of mean inter-

scan duration). Moreover, we analysed the effect of

group size on the differential values of transients and

residents for the vigilance variables, and we did it sep-

arately for the low predation and low competition fee-

der and the low predation and high competition and

the high predation and intermediate competition

feeders, as they highly differed in group size. For the

low predation and low competition feeder, we com-

puted a regression analysis with the four variables of

differential vigilance as dependents and the group size

as the categorical factor. For the low predation and

high competition and the high predation and interme-

diate competition feeders, we computed an ANCOVA

with the four variables of differential vigilance as

dependents, feeder as categorical factor and group size

as covariate. We applied the same regression analysis

and ANCOVA to analyse the effect of group size on

the differential pecking rate of transient and resident

siskins at the low predation and low competition fee-

der and at the low predation and high competition

and the high predation and intermediate competition

feeders, respectively.

To determine the possible effect of husking time on

vigilance behaviour of resident and transient siskins,

we studied the frequency distribution of mean scan

durations for both subpopulations. We estimated the

skewness � SE and the kurtosis � SE of the two

curves with the Descriptive Statistics function of the

program STATISTICA 8.0 (StatSoft Inc.), and we

tested the fit of the two curves to the normal, gamma,

log-normal, exponential and chi-square distributions

with the distribution fitting function of the same pro-

gram.

Results

Comparison of Resident and Transient Frequencies at

the Feeders

The percentage of resident siskins did not differ

between the low predation and high competition

feeder (22%) and the high predation and intermedi-

ate competition feeder (17%) when considering

both sexes together (Pearson v21 = 1.62, p = 0.20,

n = 487). However, when considering the sexes sep-

arately, we found almost significant differences

between feeders in males (Pearson v21 = 2.94,

p = 0.09, n = 243) and no differences in females

(Pearson v21 = 0.01, p = 0.72, n = 244). In males,

the percentage of residents was marginally higher

near than far from cover (low predation and high

competition feeder: 21%; high predation and inter-

mediate competition feeder: 12%), as expected both

by the site familiarity and the prior-occupancy

effects.

Comparison of Aggression Rates

Contrary to the prediction of the unfamiliarity with

flockmates effect, resident and transient siskins did

not differ in percentage of time spent in aggressions

either at all the feeders (Wilcoxon matched-pairs test:

Ethology 120 (2014) 1–13 © 2014 Blackwell Verlag GmbH6

Vigilance, Foraging and Residence J. Pascual, J. C. Senar & J. Dom�enech

T = 1300, Z = 0.84, p = 0.40, n = 122) or at the low

predation and high competition feeder (Wilcoxon

matched-pairs test: T = 388, Z = 0.03, p = 0.98,

n = 42) and high predation and intermediate compe-

tition feeder (Wilcoxon matched-pairs test: T = 73,

Z = 1.48, p = 0.14, n = 24). Aggression rates incre-

ased with group size at the low predation and high

competition and at the high predation and intermedi-

ate competition feeders (F1,128 = 18.39, p < 0.001),

but this increase did not differ between residents

and transients (interaction group size 9 residence:

F1,128 = 0.20, p = 0.65).

Comparison of Departure Reasons

Contrary to the site unfamiliarity, prior-occupancy

and compensation hypotheses, resident and transient

siskins did not differ in departure reasons when con-

sidering all the feeders (Pearson v22 = 0.14, p = 0.93)

and when considering the low predation and low

competition and the high predation and intermediate

competition feeders alone (Table 1). However, at the

low predation and high competition feeder, we found

marginally significant differences between groups in

departure reasons. Residents had higher percentages

of disturbance-related departures, while transients

had higher percentages of individual-based depar-

tures, giving some support to the site unfamiliarity

effect. However, and contrary to the prior-occupancy

effect, both groups had similar and high percentages

of aggression-related departures at this feeder

(Table 1).

Comparison of Times on Feeder and Pecking Rate

In accordance with the site unfamiliarity effect, and

contrary to the compensation hypothesis and the

prior-occupancy effect, transient siskins had longer

foraging bouts than residents, and this difference did

not change between feeders (Table 2, Fig. 1a). There

was a negative correlation between the differential

times on feeder of the transient and resident of a

pair and the differential group size experienced by

them (r = �0.20, F1,120 = 5.09, p = 0.026), so that

the member of the pair that foraged with more birds

had a shorter foraging bout length. At the low pre-

dation and high competition and at the high preda-

tion and intermediate competition feeders, the times

on feeder decreased with group size (F1,128 = 25.68,

p < 0.001), and this decrease was similar at both

feeders (interaction feeder 9 group size: F1,128 =0.80, p = 0.37). Conversely, at the low predation

and low competition feeder, we did not find a corre-

lation between group size and time on feeder

(Spearman r = 0.009, p = 0.92). Contrary to the

prior-occupancy effect, pecking rates did not differ

by residence when considering all the feeders

(Table 2, Fig. 1b). However, the difference between

groups was marginally affected by the feeder, so

that transients increased pecking rate when increas-

ing predation risk (i.e. high predation and interme-

diate competition feeder versus low predation and

low competition feeder) while residents did the

opposite (Fig. 1b). At the low predation and low

competition feeder, we found a negative correlation

between the differential pecking rate of transients

and residents and the group size (r = �0.49,

F1,26 = 8.22, p = 0.008; Fig. 2) so that residents

increased the pecking rate with the group size while

the transients did not, as expected by the prior-

occupancy effect. However, the differential pecking

rate and group size were not correlated at the low

predation and high competition and at the high pre-

dation and intermediate competition feeders (group

size: F1,30 = 0.55, p = 0.46; interaction feeder 9

group size: F1,30 = 0.08, p = 0.54).

Table 1: Observed frequencies (and row percentages) of the different reasons for departure of birds foraging at the three feeders

Individual based (%) Disturbances (%) Aggressions received (%) n row v22 p

Low predation and low competition

Res 42 (76) 12 (22) 1 (2) 55 1.00 0.61

Tran 36 (68) 16 (30) 1 (2) 53

Low predation and high competition

Res 3 (9) 8 (23) 24 (69) 35 5.26 0.07

Tran 9 (25) 3 (8) 24 (67) 36

High predation and intermediate competition

Res 5 (23) 11 (50) 6 (27) 22 0.29 0.86

Tran 4 (20) 9 (45) 7 (35) 20

p Values from Pearson’s chi-square tests with 2 degrees of freedom. Significant differences (p < 0.05) are marked with an asterisk, and tendencies

(p < 0.10) are shown in italics.

Ethology 120 (2014) 1–13 © 2014 Blackwell Verlag GmbH 7

J. Pascual, J. C. Senar & J. Dom�enech Vigilance, Foraging and Residence

Comparison of Vigilance Variables

Contrary to the compensation hypothesis, percentage

of time spent scanning did not differ between resi-

dents and transients at none of the feeders (Table 2,

Fig. 3a). However, and in accordance with the prior-

occupancy and the unfamiliarity with local food

effects, transients showed longer scan durations than

residents (Table 2, Fig. 3c), and contrary to the com-

pensation hypothesis, they also showed lower scan

rates (Table 2, Fig. 3b) and longer interscan durations

(Table 2, Fig. 3d). We did not find a significant

interaction between feeder and residence for any of

the vigilance variables, but there was a marginally

significant interaction for interscan durations, so that

the differences between residents and transients were

higher at the low predation and high competition fee-

der (Table 2, Fig. 3d). The differences between tran-

sients and residents for the four vigilance variables

were not affected by group size when considered

together either at the low predation and low competi-

tion feeder (r = �0.13, F1,26 = 0.45, p = 0.51) or at

the low predation and high competition and at the

high predation and intermediate competition feeders

(group size: F4,27 = 0.19, p = 0.94; interaction feeder

9 group size: F4,27 = 0.67, p = 0.62), and they neither

were affected by group size when considered alone

(p > 0.20 for all the comparisons).

Frequency Distribution of Mean Scan Durations

The frequency distribution of residents and transients

clearly differed. Residents presented a platykurtic dis-

tribution (kurtosis = �0.90 � 0.60) with almost no

skew (skewness = �0.02 � 0.30), while transients

presented a leptokurtic distribution (kurtosis = 0.85

� 0.60) with a positive skew (skewness = 0.91 �0.30). The frequency distribution of residents fitted

the normal (v25 = 7.25, p = 0.20) and gamma

(v25 = 6.83, p = 0.23) distributions and did not fit the

log-normal distribution (v25 = 12.78, p = 0.03),

whereas the frequency distribution of transients fitted

the normal (v25 = 4.37, p = 0.36), gamma (v25 = 0.50,

p = 0.92) and log-normal (v25 = 1.11, p = 0.78) distri-

butions. None of them fitted the exponential and chi-

square distributions (p < 0.0001 for all the tests). The

number of mean scans durations increased sharply

from 0.1 to 0.5 s in residents, and from 0.2 to 0.7 s for

transients (Fig. 4). From this point on, the frequencies

stabilized to 0.9 s in residents and to 1.1 s in tran-

sients and then dropped sharply to 1.2 s in residents

and smoothly to 2.3 s in transients. The delay in 0.2 s

from the frequencies of transients as compared to resi-

dents probably reflects the different husking speed of

both subpopulations, supporting the unfamiliarity

with local food effect. However, the much longer right

tail of the distribution of transients is probably related

to non-compatible vigilance, supporting the site unfa-

miliarity and prior-occupancy effects (i.e. longer vigi-

lances either to scan a broader area and/or to keep

competitors under surveillance).

Discussion

In this study, we found that the percentage of resident

siskins was marginally higher at the low than at the

high predation risk feeder in males. We could expect

Table 2: Results of repeated measures ANOVAs with vigilance and for-

aging variables as dependent variables, feeder as categorical predictor

and residence (paired values of residents and transients) as within-

effects

df (df Error) F p

Ranked survival scores time on feeder

Feeder 2 (119) 17.56 <0.0001*

Residence 1 (119) 4.38 0.0385*

Feeder 9 Residence 1 (119) 0.15 0.8644a

Logarithm pecking rate

Feeder 2 (59) 9.43 0.0003*

Residence 1 (59) 0.05 0.8301

Feeder 9 Residence 2 (59) 2.83 0.0670

% of time scanning

Feeder 2 (59) 12.57 <0.0001*

Residence 1 (59) 0.47 0.4935

Feeder 9 Residence 2 (59) 1.29 0.2827

Scan rate

Feeder 2 (59) 6.63 0.0025*

Residence 1 (59) 19.84 <0.0001*

Feeder 9 Residence 2 (59) 0.52 0.5992

Mean scan duration

Feeder 2 (59) 9.55 0.0003*

Residence 1 (59) 14.85 0.0003*

Feeder 9 Residence 2 (59) 0.36 0.6994

Minus inverse mean interscan duration

Feeder 2 (59) 6.06 0.004*

Residence 1 (59) 5.56 0.0218*

Feeder 9 Residence 2 (59) 1.62 0.0858

Time on feeder: n = 56 pairs for the low predation and low competition

feeder, n = 42 for the low predation and high competition feeder and

n = 24 for the high predation and intermediate competition feeder. All

other dependent variables: n = 28 for the low predation and low com-

petition feeder, n = 20 for the low predation and high competition fee-

der and n = 14 for the high predation and intermediate competition

feeder. Significant differences (p < 0.05) are marked with an asterisk

and tendencies (p < 0.10) are shown in italics.aThe interaction for time on feeder was tested with a one-way ANOVA

with the scores of transients minus the scores of residents as depen-

dent variable and feeder as categorical predictor.

Ethology 120 (2014) 1–13 © 2014 Blackwell Verlag GmbH8

Vigilance, Foraging and Residence J. Pascual, J. C. Senar & J. Dom�enech

this result according to the prior-occupancy effect

(Sandell & Smith 1991), as resident siskins are domi-

nant over transients (Senar et al. 1990b) and in many

species dominants displace subordinates to feed to

food patches more exposed to predation (Koivula

et al. 1994; Carrascal & Alonso 2006). However, in

our study, male resident and transient siskins suffered

similar aggression rates and had a similar proportion

of aggression-driven departures at the high competi-

tion feeder. Therefore, we cannot say whether tran-

sients were displaced to feed to the high predation risk

feeder or whether they did not avoid it as much as

residents did because of their unfamiliarity with the

area (Koivunen et al. 1998). Interestingly, in females,

the resident birds had similar percentages at both

feeders, suggesting that the dominance effect may be

important in explaining the differences found in

males.

We found that aggression rates did not differ

between resident and transient siskins, contrary to

the hypothesis of a cost to transience related to the

unfamiliarity with flock mates (i.e. the dear-enemy

effect; Chaves-Campos et al. 2009). However, this

lack of difference between subpopulations could be

related to the residents having high aggression rates

due to their agonistic interactions with unfamiliar

transients, which were three quarters of the foraging

birds (Senar et al. 1992).

We found that resident siskins had shorter foraging

bout lengths than transients. At the small feeders (low

predation and high competition and high predation

and intermediate competition feeders), with high

aggression rates (especially at low predation and high

competition feeder; Pascual & Senar 2013), time on

feeder decreased with group size because of the

increase in aggression rates. However, the reduction

in time on feeder was the same for resident and tran-

sient siskins, as both groups had similar aggression

rates. Nonetheless, at the low predation risk and high

competition feeder, we found that residents had

higher frequencies of departure related to distur-

bances than transients, which presented higher fre-

quencies of individual-based departures. We suggest

that this difference could reflect a differential percep-

tion of predation risk between residents and transients

(a) (b)

Fig. 1: Mean (�x) � SE of (a) time on feeder (in s) and (b) pecking rate (number of pecks per s) for resident and transient siskins foraging at three differ-

ent feeders. Time on feeder: n = 56 pairs for the low predation and low competition feeder, n = 42 pairs for the low predation and high competition

feeder and n = 24 pairs for the high predation and intermediate competition feeder; pecking rate: n = 28 pairs for the low predation and low compe-

tition feeder, n = 20 pairs for the low predation and high competition feeder and n = 14 pairs for the high predation and intermediate competition

feeder.

Fig. 2: Linear regression between the differential pecking rate of tran-

sient and resident siskins and group size. Pecking rate transients minus

residents = �0.03 – 0.49 * group size. n = 28 pairs, p = 0.008.

Ethology 120 (2014) 1–13 © 2014 Blackwell Verlag GmbH 9

J. Pascual, J. C. Senar & J. Dom�enech Vigilance, Foraging and Residence

at the study area, in line with the hypothesis of site

unfamiliarity (Robinson & Merrill 2013).

Resident and transient siskins did not differ in peck-

ing rates when considering all the data, but residents

had higher pecking rates at the low predation risk and

low competition feeder, probably explaining why

transients had higher foraging bout lengths at that

feeder. Moreover, there the residents but not the

transients increased pecking rate with group size. This

finding could reflect the differential effect of competi-

tion over the dominant residents and the subordinate

transients, supporting the prior-occupancy effect

(Sandell & Smith 1991). Pecking rate was not corre-

lated with group size at the long, small area feeders,

probably because there an increase in group size was

associated to an increase in aggression rates (Beau-

champ 1998). Interestingly, while transient siskins

increased pecking rate when increasing predation risk

(i.e. from the low predation and low competition to

the high predation and intermediate competition

feeders), residents did the opposite. As at the high pre-

dation and intermediate competition feeder aggres-

sion rates were higher than at the low predation and

low competition feeder (Pascual & Senar 2013), this

result could not be explained by the effect of domi-

nance relationships. Conversely, we suggest that it

could reflect a differential perception of predation risk

between residents and transients, or alternatively it

could result from a different foraging strategy by both

subpopulations.

As predicted by different hypotheses, transient

siskins showed longer scan durations than residents.

In our study, we found a 0.2-s delay in scan duration

between subpopulations, supporting a longer han-

dling time of transients due to unfamiliarity with local

food (Baker et al. 2011). However, the differences

(a)

(c)

(b)

(d)

Fig. 3: Mean (�x) � SE of (a) percentage of time spent scanning, (b) scan rate (number of scans per s), (c) mean scan duration (in s) and (d) mean inter-

scan duration (in s) for resident and transient siskins foraging at three different feeders. Low predation and low competition feeder: n = 28 pairs; low

predation and high competition feeder: n = 20 pairs; high predation and intermediate competition feeder: n = 14 pairs.

Ethology 120 (2014) 1–13 © 2014 Blackwell Verlag GmbH10

Vigilance, Foraging and Residence J. Pascual, J. C. Senar & J. Dom�enech

between residents and transients in mean scan dura-

tion were much more related to the much longer

scans of several transients, which were associated to

non-compatible vigilance. A higher proportion of

non-compatible vigilance was also found by Robinson

& Merrill (2013) in migrant than in resident elks (Cer-

vus canadensis) foraging in a sympatric range. Tran-

sient siskins could show longer non-compatible scans

either to scan a broader area because of their igno-

rance about the direction of a possible attack (Despor-

tes et al. 1991), to keep dominant residents under

surveillance (Knight & Knight 1986; Knight & Skagen

1988) or because they had a lower predator detection

ability (Sirot & Pays 2011).

Resident and transient siskins did not differ in the

percentage of time allocated to vigilance at none of

the feeders, which probably explains why they did

not differ in pecking rate. This finding is opposite to

the prediction of a higher allocation of time to vigi-

lance by transient siskins to compensate for their

unfamiliarity with the area (Lind 2004; Lind & Cres-

swell 2006). We could hypothesize that, contrary to

the models of Lind (2004), siskins might rely more on

body mass reduction than on vigilance to reduce pre-

dation risk. However, elsewhere we found that resi-

dents and transients did not differ in body mass

during the months of the study, and that residents

better adjusted this variable to the presence/absence

of a sparrowhawk (A. nisus) hunting at the area

(J. Pascual and J. C. Senar, own data). Additionally,

transient siskins showed a vigilance system with

lower scan rates and longer interscan durations,

which should increase their reaction time to a preda-

tor attack (Hart & Lendrem 1984; Whittingham et al.

2004), and as we saw above, they also showed mar-

ginally higher proportions far from cover (in males)

and longer foraging bouts, which should increase

their probability of encounter with predators. To sum

up, transient siskins should suffer a higher predation

risk than residents according to their vigilance and

foraging system.

Why did not transients increase their vigilance to

compensate for the high predation risk associated to

dispersal? Maybe they prioritized the accumulation of

fat reserves over the reduction of predation risk, as

ruddy turnstones (Arenaria interpres) do prior to

migration (Metcalfe & Furness 1984). It is also possi-

ble that they relied on the protection offered by the

high number of birds foraging together in the study

area (i.e. the many eyes, confusion and dilution

effects; Miller 1922; Pulliam 1973; Bertram 1978). In

fact, Lind & Cresswell (2006) suggested that joining a

larger flock could be a possible compensatory behav-

iour to reduce an increased predation risk in migra-

tory birds, in addition to changing paths and

allocating more time to antipredation behaviours. In

siskins, it could explain the high tendency of tran-

sients to join resident flocks after hearing their contact

calls (Senar & Metcalfe 1988). Hence, our study sup-

ports joining groups, and not increasing vigilance, as a

way to reduce predation risk at stopover locations by

dispersing birds.

Residency and transience are alternative evolution-

ary stable strategies (Maynard Smith 1984; Kokko

2011) related to migratory behaviour. Therefore, both

strategies must involve costs as well as benefits (B�eli-

chon et al. 1996). In this study, we found a predation

cost to dispersal, which must be compensated with

some advantages over philopatry, such as the avoid-

ance of foraging areas declining in quality or over-

crowded (B�elichon et al. 1996), the acquisition of

better territories (Coulton et al. 2011) or a reduced

vulnerability to temporal variation in food availability

(Johnson & Gaines 1990). Future investigations will

be needed to determine the possible benefits of tran-

sience to wintering siskins.

Acknowledgements

We are most grateful to David Bon�e, Anna Serra and

Esther Vilamaj�o for field assistance and to Nuria Mal-

l�en and Mª Luisa Arroyo for laboratory assistance. This

work was supported by the Spanish Research Council,

Ministerio de Ciencia e Innovaci�on (BOS 2000-0141

and CGL-2012-38262).

Fig. 4: Frequency distribution of mean scan durations for resident and

transient siskins foraging on artificial feeders. Bars around the values

represent the frequencies of residents (left) and transients (right) that

have a mean scan duration comprised between this value and the next

value (no siskin is comprised between 0 and 0.1 s).

Ethology 120 (2014) 1–13 © 2014 Blackwell Verlag GmbH 11

J. Pascual, J. C. Senar & J. Dom�enech Vigilance, Foraging and Residence

Literature Cited

Alerstam, T. 2011: Optimal bird migration revisited.

J. Ornithol. 152, 5—23.

Baker, D. J., Stillman, R. A., Smart, S. L., Bullock, J. M. &

Norris, K. J. 2011: Are the costs of routine vigilance

avoided by granivorous foragers? Funct. Ecol. 25, 617—

627.

Beauchamp, G. 1998: The effect of group size on mean

food intake rate in birds. Biol. Rev. 73, 449—472.

B�elichon, S., Clobert, J. & Massot, M. 1996: Are there dif-

ferences in fitness components between philopatric and

dispersing individuals? Acta Oecol. 17, 503—517.

Bertram, B. C. R. 1978: Living in groups: predators and

prey. In: Behavioural Ecology: An Evolutionary

Approach (Krebs, J. R., Davies, N. B., eds). Blackwell

Scientific Publications, Oxford, pp. 64—96.

Blair, R. C., Sawilowsky, S. S. & Higgins, J. J. 1987: Limita-

tions of the rank transform statistic in tests for interac-

tions. Commun. Stat. - Simul. C. 16, 1133—1145.

Carrascal, L. M. & Alonso, C. L. 2006: Habitat use under

latent predation risk. A case study with wintering forest

birds. Oikos 112, 51—62.

Chaves-Campos, J., Araya-Ajoy, Y., Lizana-Moreno, C. A.

& Rabenold, K. N. 2009: The effect of local dominance

and reciprocal tolerance on feeding aggregations of ocel-

lated antbirds. Proc. R. Soc. Lond. B 276, 3995—4001.

Clarke, M. F., Dasilva, K. B., Lair, H., Pocklington, R.,

Kramer, D. L. & Mclaughlin, R. L. 1993: Site familiarity

affects escape behaviour of the eastern chipmunk,

Tamias striatus. Oikos 66, 533—537.

Clobert, J., Danchin, E., Dhondt, A. A. & Nichols, J. D.

2001: Dispersal. OUP, Oxford.

Conover, W. J. 1981: Rank transformations as a bridge

between parametric and nonparametric statistics. Am.

Stat. 35, 124—129.

Coulton, D. W., Clark, R. G., Howerter, D. W., Anderson,

M. G. & Wassenaar, L. I. 2011: Costs and benefits of

natal dispersal in yearling mallards Anas platyrhynchos.

J. Avian Biol. 42, 123—133.

Desportes, J. P., Gallo, A. & C�ezilly, F. 1991: Effet de la

familiarisation avec l’environnement sur le comporte-

ment de vigilance de la tourterelle rieuse Streptopelia ris-

oria. Behav. Process. 24, 177—183.

Elgar, M. A. 1987: Food intake rate and resource

availability: flocking decisions in house sparrows. Anim.

Behav. 35, 1168—1176.

Elgar, M. A. 1989: Predator vigilance and group size in

mammals and birds: a critical review of the empirical

evidence. Biol. Rev. 64, 13—33.

Frair, J. L., Merrill, E. H., Allen, J. R. & Boyce, M. S. 2007:

Know thy enemy: experience affects elk translocation

success in risky landscapes. J. Wildlife Manage. 71, 541

—554.

Hart, A. & Lendrem, D. W. 1984: Vigilance and scanning

patterns in birds. Anim. Behav. 32, 1216—1224.

Hoogland, J. L., Cannon, K. E., DeBarbieri, L. M. &

Manno, T. G. 2006: Selective predation on Utah prairie

dogs. Am. Nat. 168, 546—552.

Johnson, M. L. & Gaines, M. S. 1990: Evolution of

dispersal: theoretical models and empirical tests

using birds and mammals. Annu. Rev. Ecol. Syst. 21,

449—480.

Knight, S. K. & Knight, R. L. 1986: Vigilance patterns of

bald eagles feeding in groups. Auk 103, 263—272.

Knight, R. L. & Skagen, S. K. 1988: Agonistic asymmetries

and the foraging ecology of bald eagles. Ecology 69,

1188—1194.

Koivula, K., Lahti, K., Rytk€onen, S. & Orell, M. 1994: Do

subordinates expose themselves to predation? Field

experiments on feeding site selection by willow tits.

J. Avian Biol. 25, 178—183.

Koivunen, V., Korpimaki, E. & Hakkarainen, H. 1998: Ref-

uge sites of voles under owl predation risk: priority of

dominant individuals? Behav. Ecol. 9, 261—266.

Kokko, H. 2011: Directions in modelling partial migration:

how adaptation can cause a population decline and why

the rules of territory acquisition matter. Oikos 120,

1826—1837.

Lendrem, D. W. 1983: Predation risk and vigilance in the

blue tit (Parus caeruleus). Behav. Ecol. Sociobiol. 14, 9—

13.

Lima, S. L. 1987: Distance to cover, visual obstructions,

and vigilance in house sparrows. Behaviour 102, 231—

238.

Lind, J. 2004: What determines probability of surviving

predator attacks in bird migration?: the relative

importance of vigilance and fuel load. J. Theor. Biol.

231, 223—227.

Lind, J. & Cresswell, W. 2006: Anti-predation

behaviour during bird migration; the benefit of

studying multiple behavioural dimensions. J. Ornithol.

147, 310—316.

Matthysen, E. 1993: Nonbreeding social organization in

migratory and resident birds. Curr. Ornithol. 11, 93—

141.

Maynard Smith, J. 1984: Game theory and the evolution

of behaviour. Behav. Brain Sci. 7, 95—101.

Metcalfe, N. B. & Furness, R. W. 1984: Changing priorities:

the effect of pre-migratory fattening on the trade-off

between foraging and vigilance. Behav. Ecol. Sociobiol.

15, 203—206.

Metzgar, L. H. 1967: An experimental comparison of

screech owl predation on resident and transient white-

footed mice (Peromyscus leucopus). J. Mammal. 48, 387—

391.

Miller, R. C. 1922: The significance of the gregarious habit.

Ecology 3, 122—126.

Ethology 120 (2014) 1–13 © 2014 Blackwell Verlag GmbH12

Vigilance, Foraging and Residence J. Pascual, J. C. Senar & J. Dom�enech

Milner-Gulland, E. J., Fryxell, J. M. & Sinclair, A. R. E.

2011: Animal Migration: A Synthesis. OUP, Oxford.

Pascual, J. & Senar, J. C. 2013: Differential effects of preda-

tion risk and competition over vigilance variables and

feeding success in Eurasian siskins (Carduelis spinus).

Behaviour 150, 1665—1687.

Pascual, J., Senar, J. C. & Dom�enech, J. 2014: Plumage

brightness, vigilance, escape potential, and predation

risk in male and female Eurasian siskins (Spinus spinus).

Auk 131, 61—72.

Pulliam, H. R. 1973: On the advantages of flocking.

J. Theor. Biol. 38, 419—422.

Robinson, B. G. & Merrill, E. H. 2013: Foraging-vigilance

trade-offs in a partially migratory population: compar-

ing migrants and residents on a sympatric range. Anim.

Behav. 85, 849—856.

Sandell, M. & Smith, H. 1991: Dominance, prior occu-

pancy, and winter residency in the great tit, Parus major.

Behav. Ecol. Sociobiol. 29, 147—152.

Sanz-Aguilar, A., Bechet, A., Germain, C., Johnson, A.

R. & Pradel, R. 2012: To leave or not to leave:

survival trade-offs between different migratory

strategies in the greater flamingo. J. Anim. Ecol. 81,

1171—1182.

Senar, J. C. 1988: Trapping finches with the Yunick

platform trap: the residency bias. J. Field Ornithol. 59,

381—384.

Senar, J. C. & Metcalfe, N. B. 1988: Differential use of local

enhancement for finding food by resident and transient

siskins. Anim. Behav. 36, 1549—1550.

Senar, J. C., Camerino, M. & Metcalfe, N. B. 1990a:

Familiarity breeds tolerance: the development of social

stability in flocking Siskins (Carduelis spinus). Ethology

85, 13—24.

Senar, J. C., Copete, J. L. & Metcalfe, N. B. 1990b: Domi-

nance relationships between resident and transient win-

tering siskins. Ornis Scand. 21, 129—132.

Senar, J. C., Burton, P. J. K. & Metcalfe, N. B. 1992: Varia-

tion in the nomadic tendency of a wintering finch

Carduelis spinus and its relationship with body condition.

Ornis Scand. 23, 63—72.

Sirot, E. & Pays, O. 2011: On the dynamics of predation

risk perception for a vigilant forager. J. Theor. Biol.

276, 1—6.

Waite, T. A. 1987: Vigilance in the white-breasted nut-

hatch: effects of dominance and sociality. Auk 104, 429

—434.

Whittingham, M. J., Butler, S. J., Quinn, J. L. & Cresswell,

W. 2004: The effect of limited visibility on vigilance

behaviour and speed of predator detection: implications

for the conservation of granivorous passerines. Oikos

106, 377—385.

Yoder, J. M., Marschall, E. A. & Swanson, D. A. 2004: The

cost of dispersal: predation as a function of movement

and site familiarity in ruffed grouse. Behav. Ecol. 15,

469—476.

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J. Pascual, J. C. Senar & J. Dom�enech Vigilance, Foraging and Residence