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Journal of Ornithology ISSN 2193-7192Volume 154Number 1 J Ornithol (2013) 154:73-82DOI 10.1007/s10336-012-0872-9

The relationship between brood sizeand prey selection in a Peregrine Falconpopulation located in a strategic region onthe Western European Flyway

Iñigo Zuberogoitia, José EnriqueMartínez, José Antonio González-Oreja,José Francisco Calvo & Jabi Zabala

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ORIGINAL ARTICLE

The relationship between brood size and prey selectionin a Peregrine Falcon population located in a strategicregion on the Western European Flyway

Inigo Zuberogoitia • Jose Enrique Martınez •

Jose Antonio Gonzalez-Oreja • Jose Francisco Calvo •

Jabi Zabala

Received: 7 December 2011 / Revised: 6 June 2012 / Accepted: 21 June 2012 / Published online: 11 July 2012

� Dt. Ornithologen-Gesellschaft e.V. 2012

Abstract In raptors, brood size seems to be closely

related to the size of prey brought to the nest, the delivery

rate and the degree of parental effort. In the case of Pere-

grine Falcons (Falco peregrinus), any increase in the size

of prey is considered to be linked to the increased role of

the female in hunting. We investigated the possible effects

of differences between sexes in prey composition on the

brood size of a Peregrine Falcon population in northern

Spain during 1998–2010. The study area was located on the

Gulf of Biscay, in the middle of the Western European

Flyway, hence a wide range of prey species were available

during the breeding season. We monitored a total of 320

Peregrine nests, which produced 603 fledglings (average

brood size = 2.67) and identified 2,832 prey, from 128

different bird species. Our results indicate that brood size

was negatively related to bad weather (e.g. rainfall in

April), but not with the body mass of the prey species

delivered to the nest. There were no significant differences

in body mass between attacked versus captured prey, nor

was mass affected by the sex of the attacking Peregrine,

and gender had no significant effect on the probability of a

successful capture. Therefore, males and females hunted

prey species of similar body size. Our data suggest that

prey size is not related to the number of fledglings,

although this may play an important role; Peregrines can

compensate by hunting for more or larger prey.

Keywords Body mass � Brood size � Falco peregrinus �Peregrine Falcon � Prey selection � Parental effort

Zusammenfassung

Die Beziehung zwischen Brutgroße und Beutewahl in

einer Wanderfalkenpopulation in einer strategischen

Region im westeuropaischen Zugweg

Bei Greifvogeln steht die Brutgroße anscheinend in enger

Beziehung zur Große der ins Nest eingetragenen Beute, der

Futterrate und dem Ausmaß der elterlichen Brutfursorge.

Im Fall von Wanderfalken (Falco peregrinus) wird jegli-

che Zunahme der Beutegroße mit der verstarkten Rolle des

Weibchens bei der Jagd in Verbindung gebracht. Wir ha-

ben die moglichen Effekte von Geschlechtsunterschieden

in der Zusammensetzung der Beute auf die Brutgroße in

einer Wanderfalkenpopulation in Nordspanien von 1998

bis 2010 untersucht. Das Untersuchungsgebiet lag am Golf

von Biskaya, mitten im westeuropaischen Zugweg,

weshalb wahrend der Brutsaison eine große Auswahl von

Beutearten verfugbar war. Wir haben insgesamt 320

Wanderfalkennester uberwacht, aus denen 603 Jungvogel

Communicated by P. H. Becker.

I. Zuberogoitia (&)

Estudios Medioambientales Icarus, Pintor Sorolla,

6, 1�C, 26007 Logrono, Spain

e-mail: zuberogoitia@icarus.es

J. E. Martınez

Bonelli0s Eagle Study and Conservation Group, Apdo. 4009,

30080 Murcia, Spain

e-mail: jemartinez64@gmail.com

J. E. Martınez � J. F. Calvo

Departamento de Ecologıa e Hidrologıa, Universidad de Murcia,

Campus de Espinardo, 30100 Murcia, Spain

J. A. Gonzalez-Oreja

Seccion de Vertebrados, Sociedad de Ciencias Naturales de

Sestao, PO Box 41, 48910 Sestao, Spain

e-mail: jgonzorj@hotmail.com

J. Zabala

Sebero Otxoa 45, 5. B, 48480 Arrigorriaga, Bizkaia, Spain

123

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DOI 10.1007/s10336-012-0872-9

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ausflogen (durchschnittliche Brutgroße = 2,67), und 2832

Beutetiere identifiziert, die 128 verschiedenen Vogelarten

angehorten. Unsere Ergebnisse deuten darauf hin, dass die

Brutgroße in negativer Beziehung zu schlechtem Wetter

(z.B. Regenfall im April) stand, jedoch in keiner Beziehung

zur Korpermasse der ins Nest eingetragenen Beutearten.

Die Korpermasse attackierter und gefangener Beute

unterschied sich nicht signifikant und hing auch nicht mit

dem Geschlecht des angreifenden Falken zusammen, und

das Geschlecht eines Falken hatte keinen signifikanten

Einfluss auf die Wahrscheinlichkeit eines erfolgreichen

Fangs. Daher jagten Mannchen und Weibchen Beutearten

ahnlicher Korpergroße. Unsere Daten deuten darauf hin,

dass die Beutegroße nicht mit der Anzahl der Flugglinge

zusammenhangt, obwohl dies eine wichtige Rolle spielen

konnte; Wanderfalken konnen kompensieren, indem sie

mehr oder großere Beute jagen.

Introduction

Resource availability is an important limiting factor for many

raptor populations (Newton 1979). In general terms, preda-

tors making optimal foraging decisions are expected to rec-

ognize conditions of high prey availability and adjust their

hunting choices accordingly (Daan 1982). However, during

the breeding season, brood size should influence parental

effort, provisioning rate and hunting behaviour (Simmons

2002; Olsen and Tucker 2003), since parents are limited in

their ability to provide food for nestlings by environmental

conditions and physiological constraints (Newton 1986).

In the case of Peregrine Falcons (Falco peregrinus),

prey delivery rate has been related to nestling age and

brood size (Olsen et al. 1998; Jenkins 2000a). In general,

the rate of delivery increases gradually until the nestlings

are about 3 weeks old and then steadily declines near

fledging. As long as brood size increases, parental effort

also tends to increase, whereas parental investment usually

decreases. Moreover, the number of deliveries per nestling

tends to decrease in larger broods (Olsen et al. 1998).

Peregrines can prey on species weighing from\10 g up

to as much as 1,200–1,500 g, including birds which are

larger than themselves. The optimal prey mass is around

100–150 g (Ratcliffe 1993). Redpath and Thirgood (1997)

estimated ca. 188 g of prey mass per day as being neces-

sary for an adult and calculated a prey delivery rate of

222 g per nestling per day. Hence, we would expect a

breeding Peregrine to increase its hunting efficiency by

hunting the most energetically cost-effective prey species,

especially when brood size is large. However, Peregrines

may suffer injury, particularly when capturing large, dan-

gerous prey species, which could imply that adults reduce

their energetic outputs and mortality risk by hunting as

little as possible (Zuberogoitia et al. 2002a; Olsen and

Tucker 2003).

The general tendency of female Peregrines to hunt larger

birds than males is consistent with theories on the selective

advantage of reversed sexual dimorphism (Jenkins 2000b).

While results suggest that the ability of individual females

to capture large prey can have dramatic effects on the

amount of biomass brought to nests, the fact that a broad

range of different-sized prey can be hunted by the species

suggests that Peregrine Falcons may behave quite flexibly

in meeting the nutritional requirements of nestlings

(Dawson et al. 2011). It has been widely suggested that in

species such as the Peregrine, size differences between the

sexes must have some consequences on feeding behaviour

(Ratcliffe 1993). However, several authors have reported

on the capacity of males for hunting large prey species and

some authors have reported no difference between the

sexes with regards to the size of the hunted prey (see

Treleaven 1977; Ratcliffe 1993; Zuberogoitia et al. 2002a;

Dekker and Taylor 2005).

Most papers addressing reproductive performance in

Peregrine Falcons are short-term studies and only a few

encompass long-term periods (Verdejo and Lopez-Lopez

2008), although it seems that what is interpreted as being

long-term research varies for different users of the term (Rull

and Vegas-Vilarrubia 2011). Nonetheless, extended, long-

term approaches are needed to test hypotheses on the factors

controlling breeding success and other fundamental ques-

tions in biology and ecology (Collins 2001). In this paper,

considering a scenario of high abundance of optimally-sized,

medium-weight birds as well as small passerines, provided by

the stream of passage migrants using the Western European

Flyway, we explore the relationship between brood size and

differential prey consumption in a Peregrine Falcon popula-

tion in northern Spain during the period 1998–2010. Taking

into account the above theories, we tested whether parents

with large broods deliver larger items than parents with

smaller broods, and vice versa (after Palmer et al. 2004), and

also whether, independently of the season, females catch

larger prey than males, in which case larger prey items found

in the nest during the breeding season can be deemed as

evidence of a greater hunting effort by the female.

Methods

Study area

The study area (2,384 km2) was the entire province of

Biscay (northern Spain), close to the Cantabrian Sea. Man-

made forests consisting of pine and eucalyptus plantations,

pastures, small villages and densely populated cities make

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up the bulk of the landscape. The terrain is rugged, and

elevation ranges from sea level to ca. 1,500 m in the

Basque Mountains. The weather is temperate, with an

annual rainfall of between 1,000 and 2,000 mm and mean

annual temperatures of 11–12 �C (Loidi 1987).

The study area is located on the Gulf of Biscay, in the

middle of the Western European Flyway. Pre-breeding

migration occurs differentially between species in our

study area. The first migrating species are Northern Lap-

wings (Vanellus vanellus), Golden Plovers (Pluvialis atri-

capillia), Wood Pigeons (Columba palumbus), Skylarks

(Alauda arvensis), Meadow Pipits (Anthus pratensis), Tree

Pipits (Anthus trivialis), wagtails (Motacilla spp.), thrushes

(Turdus spp.), Common Starlings (Sturnus vulgaris) and

fringilids, which start to migrate at the end of February and

continue during March and the first days of April (Galarza

1997; Zuberogoitia and Torres 1998, 2002). These species

are the main prey of the Peregrine during the pre-hatching

and hatching periods (Zuberogoitia et al. 2002a). From

then on, in April and May, the bulk of migrating birds are

Grey Plovers (Pluvialis squatarola), godwits (Limossa

spp.), Whimbrels (Numenius phaeopus), redshanks (Tringa

spp.), terns (Sterna spp.), Turtle Doves (Streptopelia tur-

tur), Common Swifts (Apus apus), Barn Swallows (Hir-

undo rustica), House Martins (Delichon urbica), Northern

Wheatears (Oenanthe oenanthe), Red-backed Shrikes

(Lanius collurio) and fringilids (Galarza 1997; Zubero-

goitia and Torres 1998, 2002). The occurrence of migration

depends on weather conditions. In fact, as the European

anticyclone is well established by April, migrants follow-

ing a trans-continental route in spring tend to be more

regular and to proceed at a faster rate than those using the

western seaboard, since the latter are more affected by

frontal systems approaching from the Atlantic (Elkins

2004).

Territories and breeding success

The Peregrine Falcon population in Biscay was systemat-

ically surveyed each year from 1998 to 2010. During this

period, the population fluctuated between 42 and 52

breeding territories, although we focused our monitoring

sample on 37 of these (see Zuberogoitia et al. 2009). Every

year, we started searching for falcons 30 days before the

earliest local laying date recorded for the population (20

February; Zuberogoitia et al. 2002a). At this time of year,

breeding pairs can be readily located because they fre-

quently engage in courtship displays. Nests can also be

found by observing displaying individuals near the crags

and sea cliffs where they will eventually breed.

To minimise the risk of nest desertion, the number of

eggs laid was not documented. In order to gather data on

productivity and ring the nestlings, each nest was visited at

least twice per breeding season before the young fledged.

We first climbed to the nest when the nestlings were

20–30 days old. This was from 20 April onwards

depending on the hatching time (nestlings\15 days old are

rarely ringed because the tarsus is not wide enough,

whereas those aged 30 days plus are capable of jumping

out of the nest). Each territory was visited again later in

order to monitor fledging success.

Following the terminology proposed by Steenhof

(1987), a breeding pair was one that laid eggs, and a suc-

cessful pair was one that fledged at least one young. Pro-

ductivity was expressed as both breeding success (i.e., the

percentage of the nests which fledged at least one chick)

and brood size (i.e., the mean number of young fledged per

breeding pair and per successful pair).

Prey data collection

On a yearly basis, during the visits to each nest (from 20

April to 30 May), all the pellets or prey remains were

collected and labelled. We paid special attention to col-

lecting small feathers in order to avoid biases against small

passerines. As a norm, the prey remains found in the nests

belong to the previous 7–10 days, depending on the age of

the nestlings. Typically, nests with nestlings younger than

15 days old lack prey remains and pellets because the

adults provide plucked birds (Zuberogoitia et al. 2002a).

From then on, adults provide semi-plucked birds and the

young eat flesh with feathers, although adults do pluck the

prey whilst feeding the offspring. We assumed that the prey

remains found in nests had been fed to young because the

adults feed away from the nests (Zuberogoitia et al. 2002a).

Prey remains were identified by comparing feathers,

bills, skulls, legs and other anatomical features with spec-

imens from a private reference collection (Aranzadi Sci-

ence Society). All remains were identified to species level,

except for some Calidris, Limosa and Anthus specimens

which were identified to genus level. We identified and

counted each prey item based on the most commonly found

bone, body part or feather representing one individual, in

order to give the minimum number of individuals (MNI)

present in the sample (see Martınez and Zuberogoitia

2001). Although some authors have expressed concern

about the usefulness of using prey remains or pellets to

estimate diet in raptors, there is often high correlation

between the direct observation of prey species hunted and

those identified from prey remains (Dawson et al. 2011).

Using data from the Aranzadi Science Society ringing

scheme database (287,698 ringed birds from 295 species;

Crespo and Iraeta 2012), we estimated the average body

mass of prey at species level. For those bird species that

showed sexual size dimorphism (for instance, the Yellow-

legged Gull, Larus michahellis), we calculated the average

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body mass of the two sexes. One basic method of esti-

mating raptor prey size is to use the arithmetic mean prey

weight. However, this ignores the fact that for most raptors

the frequencies of prey weights do not follow normal dis-

tributions and are usually heavily skewed. This method is

therefore of limited value since skewing is not accounted

for. A more accurate result is obtained by using the Geo-

metric Mean Prey Weight (GMPW), proposed by Jaksic

and Braker (1983). This is obtained by summing the

products of the number of prey items in each category

multiplied by the loge-transformed weight and dividing the

value obtained by the total number of prey.

Hunting data

Throughout the whole study period, mainly during the

breeding seasons, we observed 267 complete attacks by

Peregrines in which their gender could be determined. Of

these attacks, we only considered the 205 events where the

target bird species could be identified with certainty.

Statistical analyses

We applied a generalized linear mixed model (GLMM; Gail

et al. 2007; Zuur et al. 2007) to study the effect of both

biological and environmental factors influencing variations

in Geometric Mean Prey Weight (GMPW), the response

variable, which followed a normal distribution. To avoid

pseudoreplication and the non-independence of data, we

considered repeated measures of GMPW obtained during the

whole study period from the same nest (i.e., breeding terri-

tory) as a random effects factor. In addition, we considered

brood size (i.e., number of chicks/successful nest) as a fixed

effects factor, and the year of study (i.e., 1998–2010) as a

covariable. In addition, we used the above-mentioned field

and hunting data and applied a permutational analysis of

variance (Anderson 2001) for differences in the body weight

of prey species between attacked versus hunted prey. We

also included the sex of the hunting Peregrine (males vs.

females) and the interaction of both factors in the analysis. In

this way, a crossed design, 2-way permutational ANOVA

with interaction was used. As a resemblance measure we

used the Euclidean distance, so that corresponding sums of

squares were similar to traditional (i.e., non-permutational)

sums of squares (Anderson et al. 2008). Statistical analyses

were performed by running SPSS v.18 (SPSS, Chicago, IL,

USA) or PAST v.2.10 (two-way NP-MANOVA; Hammer

et al. 2001).

Results

Over the course of the study period, we monitored 320

Peregrine nests which produced 603 fledglings (Fig. 1). We

collected the prey remains from 185 of these nests and

identified a total of 2,832 birds belonging to 128 different

species (see ‘‘Appendix’’).

Does the number of nestlings condition prey size?

The average brood size of the Peregrines studied was 2.67

(SD = 0.87, n = 164 successful nests, Table 1). There was

no statistically significant relationship between prey body

mass (GMPW) and brood size (GLMM: F3,153 = 1.850,

P = 0.140; Fig. 2). A high variability in the body mass was

detected in every brood size category (from 1 to 4 nestlings),

which determined the low coefficient of determination

associated with the linear regression (R2 = 1.25 %; Fig. 2).

However, the effect of ‘‘year’’ on GMPW was statistically

significant (GLMM; F1,159 = 10.386, P = 0.02).

0

5

10

15

20

25

30

1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010

Nu

mb

er o

f m

on

ito

red

pai

rs0

0.5

1

1.5

2

2.5

3

3.5

Ave

rag

e n

um

ber

of

fled

glin

gs

successful pairs unsuccessful pairs non-breeding pairs

productivity brood size

Fig. 1 Temporal changes

(bars) in the number of

territories of Peregrine Falcon

monitored in Bizkaia, North of

Spain, during the whole study

period (1998–2010), including

successful, unsuccessful and

non-breeding pairs, and (lines)

average productivity (large

young/active nest) and mean

brood size (large young/

successful nest)

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Summarizing, parents with large broods do not deliver larger

items than parents with smaller broods, although we did find

some small variations between different years.

Do females hunt larger prey than males?

During the study period, we observed a total of 102 attacks

by female Peregrines (76 of them on identified prey spe-

cies) and 140 attacks by males (129 on known species).

The overall success rate was 20.6 % for females and

31.4 % for males, though conversely, when only attacks on

identified species were considered, the hunting success

rates were 28.4 and 25 %, respectively.

Females attacked identified birds with an arithmetic

average weight of 209.4 g (SD = 449.6), from which they

caught 21 prey whose mean weight was 99.1 g (SD = 107.7).

Males, on the other hand, attacked birds of known species

weighing an average of 121.0 g (SD = 306.1), from which

they trapped 44 prey with a mean weight of 138.8 g (SD =

44.0). Our field and hunting data reveal no statistically sig-

nificant differences in prey body mass between attacked

versus captured prey (two-way NP-MANOVA: P = 0.583;

Fig. 3), or between the two sexes (P = 0.181; Fig. 3), and

there was no statistically significant interaction between the

two factors (P = 0.499).

Finally, differences between the body mass of captured

prey species and that of prey found in the nests were statis-

tically significant (Mann–Whitney test, P \ 0.001). In gen-

eral, Peregrines brought the largest prey species to the nests.

Discussion

Peregrines in our study area demonstrated generalist hunting

behaviour during the breeding season, including as many as

129 bird species in their diet. It has been suggested that

Peregrine Falcons in temperate environments are usually

specialised feeders though as conditions become more

extreme they become more generalised (Jenkins and Avery

1999). This theory was not borne out in our study, since the

Peregrines exploited not only local resources but also migrant

bird species, e.g. waders (29 species, 11.5 % of the total birds

preyed upon) during the pre-nuptial migration. This broad

spectrum of prey species is conditioned by the stream of

passage migrants using the Western European Flyway during

the Peregrine breeding season, which favours the high pro-

ductivity of the studied population.

Lopez-Lopez et al. (2009) found that one group of

species, pigeons, accounted for 43.78 % of the prey and up

to 68.70 % of the total biomass of the Peregrine’s diet. A

similar preference for columbiforms has been recorded in

many other areas of the Peregrine falcon’s distribution

where pigeons and doves are available (Ratcliffe 1993;

Jenkins and Avery 1999), whereas in other areas, waders

become the most common group preyed upon (Palmer

et al. 2004; Castellanos et al. 2006). In our case study,

pigeons and doves only accounted for 14.76 % of the prey

and 32.17 % of the total biomass. We did, however, detect

a preference in the case of certain individuals for hunting

particular species or groups of species. For example, we

observed one male which was highly specialised in hunting

Yellow-legged Gulls, one female which kleptoparasitised

Carrion Crows (Corvus corone) and Black Kites (Milvus

migrans), and one pair that specialized in Blackbirds

(Turdus merula) (Zuberogoitia et al. 2002a, b; Zuberogoitia

2005).

The local weather, in our case the rainfall in April, was

the main environmental factor affecting brood size at

fledging (see Zuberogoitia et al. 2002a). According to

Bionda and Brambilla (2012), the effect of rainfall might

be most important during chick-rearing, whereas before-

rearing rainfall usually has no detectable effects on

Table 1 (Above) Number of nests of Peregrine Falcon (Falco pere-grinus) monitored in Bizkaia, northern Spain, during the whole study

period (1998–2010), and number of prey items collected in them, per

brood size class (1–4); (below) mean body weight (in grams) of the

prey species found in the nests (arithmetic mean with standard

deviation in parentheses), and range (min–max), in the same brood

size classes

1 2 3 4

Brood size

Number of nests 15 48 72 29

Number of prey

items

215 635 1,241 583

Body weight of the prey species

Average

(g) (SD)

114.7

(224.6)

107.7

(129.3)

134.9

(153.2)

129.2

(144.7)

Range (g) 6–2,800 7–816 5–1,000 5–816

y = 0.0618x + 4.0651R

2 = 0.0125

3

3.5

4

4.5

5

5.5

6

4321

Brood size

GM

PW

Fig. 2 Relationship between Geometric Mean Prey Weight (GMPW)

and Peregrine Falcon (Falco peregrinus) brood size (1–4) found in

Bizkaia, northern Spain, in the 164 nests studied during the whole

period (1998–2010)

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reproduction. Prolonged heavy rain is thought to prevent

Peregrine Falcons from hunting (Ratcliffe 1993), although

some raptor species, including Peregrines, may be able to

hunt even under harsh weather conditions (Sergio 2003).

Nonetheless, as a general rule, the availability of some prey

species would be considerably reduced during bad weather

conditions (Olsen and Olsen 1992). In our study area, spells

of adverse conditions can be recurrent and may last longer

than a week. In bad weather, the passage of waders stops or

is delayed, and this provokes Peregrines to search for other

prey species. These tend to be mostly local birds which

normally become active quickly after rain (see Olsen and

Olsen 1992), thus providing Peregrines with the possibility

of hunting them. This may explain, in part, the variability

detected between different years in the prey species

recovered.

Parental effort and provisioning behaviour must be

adjusted to accommodate brood size, rather than vice versa.

Once the brood size is established, adjustments in the

provisioning rate and prey size will be made by the adult

birds in response to the solicited requirements of the brood.

Some authors have found that the average mass of prey

items increases with brood size (Palmer et al. 2004;

Dawson et al. 2011). In a brood manipulation study with

Peregrine Falcons, Olsen and Tucker (2003) showed that,

in reduced broods, the male continued to bring small prey

to the nest whereas the female ceased hunting, whilst

parents of larger broods increased their capture of large

prey, compared to the pre-manipulation phase or controls.

The main differences between these and our results may

be due to (1) a super-abundance of optimally-sized,

medium-weight birds, provided by the stream of passage

migrants using the Western European Flyway, and (2) the

above-mentioned individual specialization, which might

appear counterintuitive. For example, we observed some

pairs of Peregrines raising four fledglings which specialised

in capturing small birds (passerines). These are obviously

less profitable in an energetic sense, unless in high supply.

At the other end of the scale, we had a pair with only one

fledgling which was fed with large gulls and even one

Northern Gannet (Morus bassana), the heaviest of the prey

species recorded in our study. In this case, we do not know

whether the Peregrines actually killed the Gannet or found

the carcass and scavenged it, although Zuberogoitia et al.

(2002a) described the unsuccessful attack of two Pereg-

rines on a juvenile Gannet. Whatever the case may be, the

fact that a large array of prey species, with very diverse

size and biomass, were actually captured by the Peregrine

Falcons in the study area can be interpreted as a result of

the behavioural and ecological flexibility of this raptor

species in meeting their nutritional requirements (Dawson

et al. 2011). No doubt, this adaptability can help Peregrine

Falcons to survive under contrasting environmental

conditions.

When hunting medium-sized and small birds, the overall

success rate of male Peregrines is higher than that of

females, presumably due to the male’s smaller size and

greater aerial manoeuvrability (Dawson et al. 2011).

However, Dekker and Taylor (2005) observed males

hunting up to 22.9 % of a total of 62 attacked gulls, even

when their success rate in relation to 32 small passerines

was very high (84.4 %). In our study, considering the

0

20

40

60

80

100

120

140

0 100 200 300 400 500

Frequency

Males0

20

40

60

80

100

120

140

0 100 200 300 400 500

AttackedCaptured

Females

Prey weight [g]

Fig. 3 Bootstrapped frequency

distributions of the mean weight

(in grams) of attacked (red) and

captured (blue) prey of known

species, by both male (left) and

female (right) Peregrine

Falcons. Real observed averages

are also indicated (arrows). For

males, 130 real values were

repeatedly re-sampled for

attacked prey and 44 for

captured prey. For females, 76

real data values were considered

for attacked species, and 21 for

captured species. All bootstrap

analyses were run 1,000 times

with replacement. Although

females tended to attack larger

prey than those successfully

captured and males effectively

captured slightly larger prey

than those attacked, all

differences were statistically

non-significant (P [ 0.05)

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overall picture, we found that both sexes captured simi-

larly-sized prey. Females attacked more large species but

males tended to be more efficient than females when

attacking medium-sized prey. Additionally, males attacked

more small passerines, which were abundant and which

allowed them to increase their attack rate.

We also found that prey items in the nests (i.e. from prey

species captured during the breeding season) were larger

than those hunted by the Peregrines when considering the

year as a whole (i.e, including data from all seasons). This

could be related to the increased effort made to provide

energetically profitable prey for the offspring. We observed

that most of the prey brought to the nest during the

breeding season was captured by male Peregrines, which

agrees with observations by Olsen et al. (1998). As a

general rule, during the first half of the nestling phase,

when the male hunts for both the female and the brood (i.e.

females do not hunt), large prey are found in the territory.

However, later on, in the second half of the nestling phase,

when young are able to thermoregulate (and females can

leave the nest), large prey may be caught by both sexes

(Olsen 1992). On the other hand, Rosenfield et al. (1995)

observed that larger prey, such as waterfowl, generally do

not appear in the diet of nestlings until the female begins to

forage, after the young have reached thermoregulatory

independence. Nevertheless, we observed that females

rarely took an active part in hunting whilst they were

looking after the brood, even when the broods consisted of

four fledglings. Instead, the females waited for their part-

ners and then flew out of the eyrie to collect the prey

provided. Sometimes, this exchange occurred a long way

from the nest, which could lead to misinterpretation.

According to Olsen et al. (1998), male Peregrine Falcons

contribute about 80 % of all prey captured during the

courtship, incubation, nestling and fledgling periods.

Therefore, we assume that most of the prey items found in

the nests, even the largest, were caught by males.

Acknowledgments We thank L. Astorkia, Inaki Castillo, A. Iraeta,

F. Ruiz Moneo, J. Zuberogoitia, S. Hidalgo, C. Gonzalez de Buitrago,

J. Elorriaga, J. Isasi, J. Fernandez, I. Palacios, I. Gomez and

J. Iturralde for their field assistance. Imanol Gomez helped us in

identifying the prey items. The Aranzadi Ringing Scheme let us

review the body mass of all the birds ringed in the 60 years’ history of

scientific ringing in the Basque Country. This research was partially

funded by the Agriculture Department of the Diputacion Foral de

Bizkaia, which also issued the licences to work with this species.

Thanks to Philip Thomas and specially Alexandra Farrell for the

linguistic revision. Pascual Lopez Lopez and two anonymous referees

made useful comments on earlier drafts of this manuscript.

Appendix

See Table 2.

Table 2 Complete list of the bird taxa identified (mainly to species

level) from 2,832 prey remains collected from Peregrine Falcon nest

sites in Bizkaia, northern Spain (1998–2010)

Prey type Averagemass (g)

Numberof items

% numberof prey

% ofpreymass

Leach’s Storm Petrel,Oceanodromaleucorhoa

45 1 0.04 0.01

European StormPetrel, Hydrobatespelagicus

27 2 0.07 0.02

Northern Gannet,Morus bassanus

2,800 1 0.04 0.78

Little Egret, Egrettagarzetta

450 2 0.07 0.25

Cattle Egret, Bubulcusibis

350 1 0.04 0.10

Eurasian Wigeon,Anas penelope

700 1 0.04 0.19

Gadwall, Anasstrepera

760 1 0.04 0.21

Mallard, Anasplatyrhynchos

965 2 0.07 0.54

Common Teal, Anascrecca

325 2 0.07 0.18

Northern Shoveler,Anas clypeata

630 1 0.04 0.18

Hen Harrier, Circuscyaneus

425 4 0.14 0.47

Montagu’s Harrier,Circus pygargus

325 10 0.35 0.90

EurasianSparrowhawk,Accipiter nisus

200 6 0.21 0.33

Common Kestrel,Falco tinnunculus

200 6 0.21 0.33

Merlin, Falcocolumbarius

200 2 0.07 0.11

Eurasian Hobby,Falco subbuteo

200 19 0.67 1.06

Red-legged Partridge,Alectoris rufa

500 1 0.04 0.14

Common Quail,Coturnix coturnix

100 10 0.35 0.28

Common Pheasant,Phasianus colchicus

1,000 1 0.04 0.28

Common Moorhen,Gallinula chloropus

300 2 0.07 0.17

EurasianOystercatcher,Haematopusostralegus

540 1 0.04 0.15

Black-winged Stilt,Himantopushimantopus

180 5 0.18 0.25

Pied Avocet,Recurvirostraavosetta

275 1 0.04 0.08

Eurasian Stone-curlew, Burhinusoedicnemus

422 2 0.07 0.23

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Table 2 continued

Prey type Averagemass (g)

Numberof items

% numberof prey

% ofpreymass

Common RingedPlover, Charadriushiaticula

36 7 0.25 0.07

European GoldenPlover, Pluvialisapricaria

220 1 0.04 0.06

Grey Plover, Pluvialissquatarola

250 22 0.78 1.53

Northern Lapwing,Vanellus vanellus

200 2 0.07 0.11

Red Knot, Calidriscanutus

165 2 0.07 0.09

Sanderling, Calidrisalba

60 1 0.04 0.02

Little Stint, Calidrisminuta

30 3 0.11 0.03

Curlew Sandpiper,Calidris ferruginea

80 3 0.11 0.07

Dunlin, Calidrisalpina

39 33 1.17 0.36

Calidris spp. 60 7 0.25 0.12

Ruff, Philomachuspugnax

150 1 0.04 0.04

Common Snipe,Gallinago gallinago

104 1 0.04 0.03

Eurasian Woodcock,Scolopax rusticola

280 5 0.18 0.39

Black-tailed Godwit,Limosa limosa

350 16 0.56 1.56

Bar-tailed Godwit,Limosa lapponica

350 30 1.06 2.92

Limosa spp. 350 17 0.60 1.66

Whimbrel, Numeniusphaeopus

425 77 2.72 9.11

Eurasian Curlew,Numenius arquata

850 1 0.04 0.24

Spotted Redshank,Tringa erythropus

170 7 0.25 0.33

Common Redshank,Tringa totanus

115 30 1.06 0.96

Common Greenshank,Tringa nebularia

200 4 0.14 0.22

Green Sandpiper,Tringa ochropus

73 1 0.04 0.02

Common Sandpiper,Actitis hypoleucos

48 17 0.60 0.23

Ruddy Turnstone,Arenaria interpres

120 2 0.07 0.07

Black-headed Gull,Larus ridibundus

300 3 0.11 0.25

Lesser Black-backedGull, Larus fuscus

750 1 0.04 0.21

Yellow-legged Gull,Larus michahellis

816 32 1.13 7.27

Sandwich Tern, Sternasandvicensis

250 17 0.60 1.18

Table 2 continued

Prey type Averagemass (g)

Numberof items

% numberof prey

% ofpreymass

Common Tern, Sternahirundo

130 6 0.21 0.22

Little Tern, Sternaalbifrons

55 2 0.07 0.03

Black Tern,Chlidonias niger

75 1 0.04 0.02

Common Guillemot,Uria aalge

850 1 0.04 0.24

Rock Dove, Columbalivia

300 299 10.56 24.97

Common WoodPigeon, Columbapalumbus

450 25 0.88 3.13

Eurasian CollaredDove, Streptopeliadecaocto

200 24 0.85 1.34

European Turtle Dove,Streptopelia turtur

140 70 2.47 2.73

Amazona spp. 700 1 0.04 0.19

Budgerigar,Melopsittacusundulatus

30 14 0.49 0.12

Common Cuckoo,Cuculus canorus

115 10 0.35 0.32

Barn Owl, Tyto alba 250 4 0.14 0.28

Tawny Owl, Strixaluco

400 1 0.04 0.11

European Nightjar,Caprimulguseuropaeus

75 2 0.07 0.04

Common Swift, Apusapus

40 30 1.06 0.33

Common Kingfisher,Alcedo atthis

40 1 0.04 0.01

Eurasian Hoopoe,Upupa epops

67 2 0.07 0.04

Wryneck, Jynxtorquilla

37 1 0.04 0.01

Green Woodpecker,Picus viridis

180 15 0.53 0.75

Great SpottedWoodpecker,Dendrocopos major

85 11 0.39 0.26

Woodlark, Lullulaarborea

30 1 0.04 0.01

Eurasian Skylark,Alauda arvensis

38 32 1.13 0.34

Crag Martin,Ptyonoprognerupestris

23 6 0.21 0.04

Barn Swallow,Hirundo rustica

19 37 1.31 0.20

Common HouseMartin, Delichonurbicum

19 17 0.60 0.09

Tree Pipit, Anthustrivialis

24 3 0.11 0.02

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Table 2 continued

Prey type Averagemass (g)

Numberof items

% numberof prey

% ofpreymass

Meadow Pipit, Anthuspratensis

19 11 0.39 0.06

Rock Pipit, Anthuspetrosus

24 1 0.04 0.01

Water Pipit, Anthusspinoletta

24 4 0.14 0.03

Anthus spp. 20 62 2.19 0.35

Yellow Wagtail,Motacilla flava

19 2 0.07 0.01

Grey Wagtail,Motacilla cinerea

19 52 1.84 0.28

White Wagtail,Motacilla alba

19 21 0.74 0.11

Eurasian Wren,Troglodytestroglodytes

9 1 0.04 0.00

Dunnock, Prunellamodularis

20 5 0.18 0.03

European Robin,Erithacus rubecula

18 19 0.67 0.10

Common Nightingale,Lusciniamegarhynchos

20 6 0.21 0.03

Black Redstart,Phoenicurusochruros

16 18 0.64 0.08

Common Stonechat,Saxicola torquatus

15 18 0.64 0.08

Northern Wheatear,Oenanthe oenanthe

24 10 0.35 0.07

Common Blackbird,Turdus merula

100 394 13.91 10.97

Fieldfare, Turduspilaris

100 1 0.04 0.03

Song Thrush, Turdusphilomelos

70 227 8.02 4.42

Redwing, Turdusiliacus

60 5 0.18 0.08

Mistle Thrush, Turdusviscivorus

120 54 1.91 1.80

Common GrasshopperWarbler, Locustellanaevia

13 1 0.04 0.00

Melodious Warbler,Hippolais polyglotta

11 3 0.11 0.01

Dartford Warbler,Sylvia undata

10 1 0.04 0.00

Common Whitethroat,Sylvia communis

16 1 0.04 0.00

Blackcap, Sylviaatricapilla

20 64 2.26 0.36

Common Chiffchaff,Phylloscopuscollybita

7 14 0.49 0.03

Firecrest, Regulusignicapilla

6 3 0.11 0.01

Spotted Flycatcher,Muscicapa striata

15 2 0.07 0.01

Table 2 continued

Prey type Averagemass (g)

Numberof items

% numberof prey

% ofpreymass

Pied Flycatcher,Ficedula hypoleuca

15 2 0.07 0.01

Long-tailed Tit,Aegithalos caudatus

8 1 0.04 0.00

Blue Tit, Paruscaeruleus

11 4 0.14 0.01

Great Tit, Parus major 18 5 0.18 0.03

European Nuthatch,Sitta europaea

22 1 0.04 0.01

Golden Oriole,Oriolus oriolus

70 4 0.14 0.08

Red-backed Shrike,Lanius collurio

29 3 0.11 0.02

Eurasian Jay, Garrulusglandarius

170 12 0.42 0.57

Common Magpie,Pica pica

210 37 1.31 2.16

Carrion Crow, Corvuscorone

500 10 0.35 1.39

Common Starling,Sturnus vulgaris

80 16 0.56 0.36

Spotless Starling,Sturnus unicolor

85 16 0.56 0.38

House Sparrow,Passer domesticus

28 44 1.55 0.34

Tree Sparrow, Passermontanus

21 1 0.04 0.01

Common Chaffinch,Fringilla coelebs

22 120 4.24 0.74

Brambling, Fringillamontifringilla

25 3 0.11 0.02

European Serin,Serinus serinus

12 14 0.49 0.05

European Greenfinch,Carduelis chloris

26 132 4.66 0.96

European Goldfinch,Carduelis carduelis

15 239 8.44 1.00

Eurasian Siskin,Carduelis spinus

14 50 1.77 0.19

Common Linnet,Carduelis cannabina

20 43 1.52 0.24

Eurasian Bullfinch,Pyrrhula pyrrhula

22 19 0.67 0.12

Hawfinch,Coccothraustescoccothraustes

55 9 0.32 0.14

Cirl Bunting,Emberiza cirlus

25 1 0.04 0.01

Rock Bunting,Emberiza cia

25 1 0.04 0.01

Reed Bunting,Emberizaschoeniclus

21 2 0.07 0.01

Total 25,575 2,832 100 100

Data provided are the average mass of each taxon and the totalnumber of items identified

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