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Mate acquisition in the European blackbirdand its implications for sexual strategiesEmma Creighton aa Department of Biology, The Open University, Milton Keynes, UnitedKingdomVersion of record first published: 19 May 2010.

To cite this article: Emma Creighton (2001): Mate acquisition in the European blackbird and itsimplications for sexual strategies, Ethology Ecology & Evolution, 13:3, 247-260

To link to this article: http://dx.doi.org/10.1080/08927014.2001.9522774

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Mate acquisition in the European blackbird and its implications for sexual strategies

EMMA CREIGHTON

Department of Biology, The Open University, Milton Keynes, United Kingdom

Received 10 July 2000, accepted 17 May 2001

Female European blackbirds, Turdus merula have been shown to engage inmixed reproductive strategies and females do not seem to gain any material ben-efit from their extra-pair copulation behaviour. To test the hypothesis that suchbehaviour arises out of constraint on their choice of social mate, I investigate ifand how these females are constrained in their choice at mate acquisition. Irecorded patterns of territorial distribution and interactions between residentbirds both before pairing and during the breeding season. Patterns of observedbehaviour agreed with earlier descriptions of winter and breeding territorialityand provided clues to the underlying mechanism of mate acquisition. I concludethat the constraint on breeding success imposed by high nest predation pro-motes intrasexual competition for high quality nesting habitat between femalesprior to pair formation. Female distribution promotes intrasexual competitionbetween males for territorial area overlapping females or for habitat likely to beoccupied by females. Mate choice during pair formation is constrained by theoutcome of this intrasexual competition and, typically, overlapping territorialmales and females pair into social monogamy on a shared breeding territory.Although this ultimately leads to assortitive mating by competitive ability, lowquality females paired to low quality males may improve upon the quality oftheir genetic mate by engaging in extra-pair copulations. These data suggest thatthe process of mate acquisition in urban European blackbirds constrains thechoice of social mate and so generates sexual selection pressures to drive mixedreproductive strategies during the breeding season.

KEY WORDS: European blackbird, mate choice, territory, social constraint.

Introduction . . . . . . . . . . . . . . . . . 248Methods . . . . . . . . . . . . . . . . . . 249Results . . . . . . . . . . . . . . . . . . 251

Pairing dates . . . . . . . . . . . . . . . . 251Territorial distributions . . . . . . . . . . . . . 251Effects of vegetation structure . . . . . . . . . . . 252

Ethology Ecology & Evolution 13: 247-260, 2001

Correspondence: E. Creighton, Department of Psychology & Speech Pathology, ManchesterMetropolitan University, Elizabeth Gaskell Campus, Manchester, M13 0JA, UK; Tel. +44 161 2472558, Fax +44 161 247 6364 (E-mail: e.creighton@mmu.ac.uk).

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248 E. Creighton

Male-male territorial interactions . . . . . . . . . . 252Female-female territorial interactions . . . . . . . . . 255Territorial replacements and movements . . . . . . . . . 255

Discussion . . . . . . . . . . . . . . . . . 256Acknowledgements . . . . . . . . . . . . . . . 258References . . . . . . . . . . . . . . . . . 258

INTRODUCTION

The constrained female choice hypothesis predicts that females engaging inmixed reproductive strategies are making “the best of a bad job” (TRIVERS 1972,DAWKINS 1976) when ecological or social constraint has limited their choice ofsocial mate (MØLLER 1992, GOWATY 1996), and they may improve upon the geneticfitness of their off-spring by mating with superior males outside of their socialbond (MULDER et al. 1994, HASSELQUIST et al. 1996, KEMPENAERS et al. 1997, SHEL-DON et al. 1997). So, both the process of mate acquisition and the subsequentreproductive strategies adopted in the breeding season contribute to the sexualselection pressures acting upon individuals (MØLLER 1994a, 1998).

European blackbirds, Turdus merula are socially monogamous, althoughextra-pair courtship attempts by males are frequent (2.05 intrusions/hr during thefemales’ likely fertile periods) and may result in extrapair paternity [14% of chicks(2/14) in 29% of broods (2/7)] when females occasionally accept the courtshipadvances of intruding males (12% of intrusions) (CREIGHTON 2000a). There is littleevidence that females gain material benefits from their extra-pair copulation (EPC)strategies as blackbirds do not engage in courtship feeding and females rarely ven-ture from their own territories to benefit from the resources offered on the territo-ries of EPC males (DAVIES 1985, GRAY 1997a). Nor are they likely to be strengthen-ing extra-pair bonds as an investment for future mating (COWELL & ORING 1989,WAGNER 1991a, SHELDON 1994, GRAY 1997b) as divorce and re-mating in blackbirdsis influenced by breeding site quality rather than mate quality (DESROCHERS &MAGRATH 1993, 1996). Females may engage in EPCs to insure against low spermcounts in their mate (WAGNER 1991b, WETTON & PARKIN 1991, SHELDON 1994, GRAY

1997b) as between 8-10% of blackbird eggs fail without any apparent embryo devel-opment (MAGRATH 1989, CREIGHTON 2000a), but see BIRKHEAD et al. (1995).

In the light of this lack of evidence for material benefits to account for femaleblackbird EPC behaviour, the females may be engaging in mixed reproductivestrategies to improve upon the quality of their genetic mate when they have beenconstrained in their choice of social mate (MULDER et al. 1994, HASSELQUIST et al.1996, KEMPENAERS et al. 1997, SHELDON et al. 1997). In this paper I explore themate acquisition process in the Oxford Botanic Garden population of Europeanblackbirds to determine if females are constrained in their choice of social mate,the nature of these constraints, and the implications they may have for sexualselection and subsequent sexual strategies in this species.

Behaviour associated with pair formation in blackbirds has been described inthe literature by several authors, but little evidence for the process of mate acquisi-tion has been discussed. Descriptions of excited gatherings of displaying and/orfighting (usually male) blackbirds in late winter and early spring have been report-ed (MORLEY 1937, LACK 1941, LACK & LIGHT 1941), and have been suggested toserve a function in mate acquisition (MORLEY 1937). More detailed work (JACKSON

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249Mate acquisition in blackbirds

1954; SNOW 1956, 1958; LACK 1966; EDWARDS 1983) however, suggested that matedpairs develop from “apparent pairs” of resident males and females whose winterterritories overlap (SNOW 1956, 1958). Apparent pairs changed throughout the win-ter as females moved territories or the mosaic of male territories shifted due to theinsertion of a new male territory, though previously mated birds tended to staytogether on the territories they defended during the previous breeding season.However, beyond the observation that a territory was a prerequisite for breeding inboth sexes and an implicit assumption that females joined males on their territo-ries (SNOW 1956, 1958), the actual process of mate acquisition has not beenaddressed.

Studies of mate acquisition in other territorial species suggest a number of dif-ferent patterns are found. Where pairs form from mate choice during communaldisplays over winter, then already mated pairs may settle together (e.g. WILLIAMS

1983; SORENSON 1992, 1994). However, in migratory species males typically arrive onthe breeding site before the females, with the highest quality males arriving first toclaim the best quality habitat (MØLLER 1994a, 1994b; KOKKO 1999) and the femalesarriving a few days later (presumably also competitively) to choose between them(competitive mate choice, ALTMAN et al. 1977, HALLIDAY 1983). Though whetherfemale choose on the basis of characteristics of the male (e.g. CATCHPOLE 1980, LIF-JELD & SLAGSVOLD 1988, HASSELQUIST et al. 1996, MOUNTJOY & LEMON 1996), or onthe basis of his territory (e.g. GOTTLANDER 1987, RADESATER et al. 1987) is difficult todetermine, due to the inevitable correlation between the qualities of males and theresources they defend (DAVIES 1978, SEARCY 1982). In more resident species mateacquisition has been found to arise as a consequence of over-lapping territorialmales and females forming social bonds at the start of the breeding season (DAVIES

1992). Here females are proposed to settle in competition with each other for suit-able breeding habitat and independently of the territorial patterns of males (inde-pendent female settlement). Males are proposed to impose themselves on the femaledistribution and compete to monopolise female territories.

In this study I observed territorial distribution and territorial defense behav-iour in both sexes of urban European blackbirds both before and after pair forma-tion and I compare them with earlier description of pairing behaviour in this popu-lation. From this I deduced patterns of territorial settlement and pair formation,and the nature of the ecological and social constraints shaping them. I concludewith a discussion of how these constraints may shape the sexual selection pressuresand subsequent sexual strategies of the birds during the breeding season.

METHODS

Behavioural observations

I studied the territorial behaviour of a colour ringed population of European blackbirdsin Oxford Botanic Garden, UK (51°44’N, 1°16’W) from January to June 1993 as part of awider study of the birds’ reproductive behaviour.

From January to mid-March I made daily systematic searches of the study site, usuallyin the morning when territorial activity is at its highest (EDWARDS 1983). The search routeensured all areas were equally covered, taking into account the topographical features andvegetation structure of the garden. I recorded on a map of the site the identity and first

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250 E. Creighton

observed position of each territorial bird, identified by their dominant behaviour over non-territory holders and intruders, including assertive body postures, beak wipes, flight displays,boundary displays with territorial neighbours, chasing intruders, and fights (SNOW 1958). Irecorded subsequent positions and the sex and identity of opponents if the bird moved morethan 5 m or engaged in territorial interactions with conspecifics. Disputes at territorialboundaries were recorded as three positions corresponding to the extremities and mid-pointof a boundary display, or the area over which a fight took place.

During the breeding season I recorded the positions of territorial birds during 20 minfocal watches of breeding pairs and during infrequent systematic searches of the garden asdescribed above.

The positional data were transcribed into six figure grid references for analysis usingthe radio tracking data analysis programme Wildtrack (TODD 1993).

Removal experiment

In January and February I trapped known territorial but unpaired individuals andremoved them from their territory for 3 days, a period within which blackbirds react to nat-ural territorial vacancies (SNOW 1958, pers. obs). The removed birds were held in a well-venti-lated and undisturbed shed near the study site, in cages measuring 100 × 50 × 50 cm, and fedad libitum with soft bill bird food, fresh fruit and water.

I collated territorial patterns for the 3 days before, during and after the removal toassess the effect of removal on the territorial distribution of the birds that remained. For thisassessment, any bird seen exhibiting overtly dominant behaviour in the experimental area fora minimum of half a day in each of the 3 day stages of a trial was regarded as territorial forthat stage.

Statistical methods

The positional data I collected for each bird were essentially independent fixes, weight-ed by a factor of three towards territorial boundary disputes (three positions correspondingto the extremities and mid-point of a boundary display, or the area over which a fight tookplace). The temporal separation between initial fixes of the birds’ positions — a minimum of2 hr up to a maximum of several days — ensured their independence (SWAIHART & SLADE

1985a). The inclusion of fixes after a move greater than 5 m or on engaging in a territorialinteraction added a source of temporal autocorrelation to the data set (SWAIHART & SLADE

1985b), but this has been demonstrated to have no effect on the nonparametric minimumconvex polygon technique that I used here (ANDERSEN & RONGSTAD 1989).

I used minimum convex polygons as a simple but effective method of determining terri-tory outlines from positional data that included indications of boundaries (KENWARD 1987). Asa non-parametric technique it is unaffected by mild autocorrelation in the data (see above)and is robust when the number of data points are low (HARRIS et al. 1990). Unless otherwisestated, I included only boundary outlines with sufficient points to provide a stable estimate ofterritory area, indicated by the plot of number of fixes against enclosed area reaching a stableplateau (KENWARD 1987). For direct comparisons of overlapping territory areas, I used equalnumbers of data points for each territory to avoid any possible confounding effects due tocorrelations between the number of data points and territory area.

The different methods of data collection I used before and after pairing preventeddirect comparisons of rates of behaviour and I have restricted the analyses to frequencies ofindividuals observed expressing the behaviour during specified periods of the study. Theseanalyses included all territorial birds in the study site, including those whose territories werenot included in the territorial analysis due to restricted observational access (e.g. territoriesextending onto private land at the edges of the study site).

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251Mate acquisition in blackbirds

Vegetation cover

In February I plotted the vegetation structure of the garden onto a 1 m2 grid map ofthe study site by indicating the areas of lawns and flowerbeds, paths and buildings, andshrubbery and climbing plants. Although a great deal of information was lost by plottingthree dimensional structures onto a two dimensional map, the frequency of grid squarestaken by the different vegetation structures provided an index of the habitat type of the occu-pied and unoccupied areas.

RESULTS

Pairing dates

The process of pair formation in blackbirds takes place over several days(SNOW 1958). Prior to pair formation, the two sexes generally ignore each other,though males will aggressively chase females when the two meet. During pair for-mation male behaviour towards females switches from aggression to courtship, towhich pairing females become increasingly receptive and copulations may occur.Once paired, the birds associate together on their territory with the female domi-nant over the male (SNOW 1956, 1958; LACK 1966; CREIGHTON 2000a).

Six of the eight breeding pairs formed in the last week in February, one in thefirst week in March and one at the end of the first week in April. Accordingly, I col-lated data over the 6 weeks from the 10th January to the end of February as pre-pairing data and over 6 weeks from the 7th April until the 24th May for compari-son as breeding data. Prior to pairing, all interactions I observed between overlap-ping birds involved the male directing aggression towards the female (6/10 residentmales) whilst after pair formation all instances of aggression I observed betweenpaired birds was directed at the male by the female (5/10 breeding females).

Territorial distributions

Prior to pair formation both sexes defended exclusive winter territories withintheir sex with overlap between the territories of males and females (Fig. 1). All ofthe female territories were overlapped by males but not all of the male territoriesoverlapped by females. After correction to an equal number of data points for eachpair-wise comparison there was no difference in the areas of overlapping male andfemale territories and the majority of their statistical centres lay within 10 m ofeach other (median = 4.8 m, range = 3.2-19.3 m) (Table 1).

Like the winter territories, breeding territories were exclusive within each sexand the territories of overlapping males and females were congruent (Fig. 2). Allfemales occupied a breeding territory with a male, but one male remainedunpaired throughout the breeding season. After correction to the same number ofdata points for each pair-wise comparison, overlapping males and females occupiedsimilar areas and all of their statistical territory centres lay within 10 m of eachother (median = 6.8 m, range = 3.2-10.1 m) (Table 1).

There was no difference in the before and after pairing distances between thecentres of overlapping male and female territories (Table 1).

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252 E. Creighton

Effect of vegetation structure

Female winter territories were not evenly distributed over the study site (Fig.1). The occupied areas contained a higher proportion of lawns and flowerbeds, over3 times the proportion of shrubbery and climbers, but a lower proportion of build-ings and pathways than found in the unoccupied areas (Chi Squared test of the fre-quency of 1 m2 grid squares for each vegetation type and each area: χ2 = 53.9, df =2, P < 0.001). Male winter territories, however, occupied all parts of the study siteincluding the areas not favoured by females (Fig. 1).

Male-male territorial interactions

On winter territories I observed territorial males ousting intruders of bothsexes, but during the breeding season I observed them oust mainly other males(Table 2). Territory boundary disputes were observed only with other males both

Fig. 1. — Overlap of male (shaded) and female (clear) territories prior to pair formation. Territoriesare shown for the last resident bird in the area, fine shading and outlines indicate insufficient fixesto give a good estimate of territory area or territories with boundaries outside the study site: (1)floater male failed to force a new territory between existing male territories; (2) male remainedunpaired, but gained ground to the south when the neigbouring male damaged his foot; (3) maleabandoned this territory at pair formation to join an unpaired female outside the study site; (4)male moved north when the above male abandoned his territory, but then returned to this areawhen an unpaired female settled.

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253Mate acquisition in blackbirds

Table 1.

Territorial areas of overlapping males and females after correction to equal numbers of data points for each pair-wise comparison, and the distances between their statistical centres.

Before pairing Breeding season

Area (m2)1Centres No. Area (m2)2

Centre No.Males Females dist. (m)3 fixes Males Females dist. (m)3 fixes

2756 2418 3.2 65 4099 4079 9.0 851577 1597 3.2 64 1910 1782 3.2 462336 1759 4.5 42 2181 2144 6.4 551185 884 5.1 30 2998 193 3.6 311889 2458 14.4 46 1533 4495 6.8 441331 604 19.3 29 4168 3912 10.1 46

5437 3671 9.8 111

1 Difference in areas of overlapping pre-pairing territories, Wilcoxon signed ranks test, Z6 = 5, P =0.313. 2 Difference in areas of overlapping breeding territories, Wilcoxon signed ranks test, Z7 = 7, P= 0.296. 3 Difference in the distances between centres of overlapping territories before and afterpairing, Mann-Whitney U test, U6,7 = 40, P = 0.418.

Fig. 2. — Overlap of male (shaded) and female (clear) territories during the breeding season. Fineshading and outlines indicate territories with boundaries outside the study site. The hatched territo-ry is that of the unpaired male who persistently intruded into the neighbouring territory to thesouth where the resident male had damaged his foot.

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254 E. Creighton

before and after pairing, though during the breeding season males were sometimesaccompanied by their mate when engaging in boundary disputes with neighbouringpairs (Table 3).

This boundary display analysis assumed that each combination of neighboursgenerated statistically independent data, even though individuals were includedmore than once when interacting with different neighbours. However, conservativeanalyses using each individual only once show similar results to the full analysisand indicate that the data were not biased by repeated measures [G-test withWilliams correction (SOKAL & ROHLF 1995); before pairing, G2 = 10.17, P < 0.01;after pairing, G2 = 5.484, P > 0.05].

Table 2.

Territorial birds seen ousting intruding birds only of the same sex, only of the opposite sex and ofboth sexes, before pairing and during the breeding season. Data are the frequency of birds observed

ousting intruders (with percentage of all territorial birds). Statistics are G-tests, df = 2.

Sex of ousted individuals

Only same sex Only opposite sex Both sexes N G P

Before pairingMales 5 (42%) 1 (8%) 3 (25%) 12 2.7 > 0.200Females 3 (38%) 0 0 8 5.4 < 0.050

Breeding seasonMales 6 (60%) 0 2 (20%) 10 7.7 < 0.050Females 4 (44%) 0 0 9 7.6 < 0.050

Table 3.

The observed occurrence of boundary displays between neighbouring birds before and after pair formation (with percentage of all possible interactions). Statistics are Chi Squared tests, df = 2.

Observed territory boundary interactions between neighbouring birds

Male-male Female-female Male-female χ2 P

Before pairing1

Observed interactions 14 (67%) 0 0 25.4 < 0.001All possible interactions 21 11 26

Breeding seasonObserved interactions 11 (58%) 8 (62%) 10 (30%)2 3.1 > 0.100All possible interactions 19 13 33

1 Overlapping males and females were not observed to co-operate in joint boundary displays. 2 Allmale-female interactions took place during interactions between pairs.

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255Mate acquisition in blackbirds

Female-female territorial interactions

On both winter and breeding territories I observed resident females oust onlyfemale intruders (Table 2). I never observed females on winter territories engage inboundary displays with their neighbours (Table 3). However, during the breedingseason females were just as likely to be observed in boundary displays with neigh-bouring birds as their mates, though displays with neighbouring males onlyoccurred when the female cooperated with her mate in a joint defense of their ter-ritory boundary (Table 3).

Territorial replacements and movements

Due to the difficulty in trapping previously handled birds I caught only fourringed territorial individuals for experimental removal prior to pair formation. Inthree trials involving the removal of a female, the overlapping pattern of male terri-tories did not change and the experimentally removed females were replaced byfloaters. On release, two of the three experimental females regained their territory,actively displacing the replacement female who rejoined the floater population.

Two cases of natural replacement of females were observed. Although noactive competition was observed between the new and old residents, there was noevidence of either territory being abandoned by the original owners prior to thearrival of the replacement. Both displaced owners were later seen in the floaterpopulation.

In the single male-removal trial the vacancy was not filled, nor did the over-lapping female move and I did not observe her in any other areas of the gardenduring the male’s removal. This was in contrast to observations of attempted natur-al replacements in males, where floater males were frequently observed to chal-lenge the territorial boundaries of established males. One male succeeded indefending a small territory between two established territories over 3 weeks in lateJanuary and early February (Fig. 1), but despite almost continuous challenging ofhis neighbours boundaries, he did not succeed in forcing a new territory betweenthem.

I observed further natural territorial movements during pair formation. Sixbreeding pairs formed between six of the resident winter males and overlappingfemales. A seventh formed between a resident male and a new female who replacedthe winter resident female. The eighth was formed late in the season when a femalefrom the floating population tried and failed to establish herself on the southernedge of the study territories, before moving to the area that had been abandoned byone of the unpaired males five weeks earlier. Immediately on her arrival the nearbyunmated male moved his territory to overlie hers and the two paired (Fig. 1).

Two of the remaining winter male residents abandoned their territories. Onedisappeared from the study area and the other moved out of the study site to joinan unmated female across the river (Fig. 1). One of the abandoned territories wasannexed by a neighbouring male, and the other remained vacant until the arrival ofthe eighth female (described above). The final male remained unpaired on his win-ter territory throughout the breeding season, but persistently intruded into a neigh-bouring territory who’s resident male had a damaged foot. The damaged foot pre-vented this male from defending his full winter territory into the breeding seasonand he lost ground to the neighbouring unpaired male. The resident female contin-

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ued to roam over the whole of the territory she had defended during the winter,coming to overlap both males (Fig. 2). However, she built all her nests in a densethorny tree in the damaged foot mate’s territory and only he was seen to provisionthe nestlings from the clutch that hatched.

DISCUSSION

Prior to pair formation both males and female blackbirds defended exclusivewithin-sex late winter territories and, as SNOW (1956, 1958) and EDWARDS (1983)reported in earlier descriptions of pair formation in this population, the majority ofbreeding pairs were formed between over-lapping territory holders. There was noevidence of already mated pairs settling on a territory together, refuting suggestionsmade in the early literature that blackbirds pair during late-winter gatherings ofdisplaying and fighting birds (MORLEY 1937, LACK 1941, LACK & LIGHT 1941).

As reported by SNOW (1956, 1958) males defended their late winter territoriesagainst intruders of both sexes, including directing aggression at the overlappingfemale, and they engaged in often protracted boundary disputes with their maleneighbours. This behaviour suggests that males were in competition with eachother for a territorial area, as illustrated by accounts of males attempting to insertnew territories between those of established males by challenging and forcing backthe territorial boundaries (SNOW 1956, 1958; EDWARDS 1983).

In contrast to males, and again in agreement with SNOW (1956, 1958), femaleswere not observed to engage in boundary disputes on their late winter territoriesbut did aggressively chase away any female intruders. Combined with the observa-tions of experimentally removed females regaining their territories from replace-ments and of natural territorial replacements (SNOW 1956, 1958), this behavioursuggests that females were in competition with each other for resources, but thatthese resources did not depend upon territorial area per se.

Female late winter territories were confined to areas of the garden with high-er proportions of shrubbery and climbers. A review of the breeding ecology ofEuropean blackbirds has shown the central importance to territory dispersion andreproductive success of dense nesting cover and alternative sites for repeat nestingattempts after predation (CREIGHTON 2000a). Females defending winter territories inareas of dense vegetation appear to have been defending future nesting resources.

This was unlikely to be true for males since nesting cover was clumped anddid not therefore depend upon territory area. Whilst foraging resources may havebeen related to territory area, the birds collected much of the food for nestlingsand fledglings from nearby playing fields (SNOW 1956, 1958; EDWARDS 1983, pers.obs) and food availability on the territory has been found to have little effect uponbreeding success (EDWARDS 1983).

It is not immediately obvious whether the congruency of over-lapping maleand female late winter territories was due to females competing to settle within anarea defended by a single male (competitive mate choice; ALTMAN et al. 1977, HALLI-DAY 1983) as earlier work assumes (SNOW 1956, 1958), or whether males were com-peting to defend an area occupied by a settled female (independent female settle-ment; DAVIES 1992). However, two observations of territorial movements by malesto join already settled females during pair formation suggest independent femalesettlement (Fig. 1). Similarly the independence of female territories is suggested by

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257Mate acquisition in blackbirds

the observation that the female paired to the male with the damaged foot contin-ued to use the full extent of her territory even when her mate was pushed into asmaller area by the neighbouring unpaired male (Fig. 2). SNOW (1958) adds furthersupport with one report of an unmated male moving to pair with an already settledfemale and two cases of apparent polygyny where two exclusive female territoriescame to lie within an area defended by a single male in a pattern reminiscent ofpolygyny arising out of independent female settlement in dunnocks, Prunella modu-laris (DAVIES 1992).

In either case, mate acquisition in urban European blackbirds arises from theoutcome of competitive settlement of females on late winter territories offeringhigh quality nesting habitat and the competitive settlement of males on late winterterritories either overlapping female territories or in areas where females might set-tle. Breeding pairs form between overlapping territorial males and females betweenlate January and early March. As a result there is little scope for mate choice byeither sex due to the constraints imposed by competition from members of thesame sex and the sexual selection pressures operating during mate acquisition willbe heavily biased towards intrasexual competition.

The largely resident nature of the birds provides ample opportunity for com-petition within each sex and the highest quality individuals would be expected togain ownership of the highest quality territories (DAVIES 1978, SEARCY 1982). Thereis some evidence to support this hypothesis from changes of territorial ownershipin this and other urban populations of blackbirds (EDWARDS 1983; DESROCHERS &MAGRATH 1993, 1996; CREIGHTON 2000a) suggesting a tendency towards assortitivemating by competitive ability. However, lower quality females paired to lower quali-ty males may be able to improve upon the quality of their genetic mate by engagingin extra-pair copulations with higher quality males during the breeding season(MØLLER 1992, GOWATY 1996).

This is supported to some extent here by the observed changes in patterns ofterritorial interactions before and after pair formation. As SNOW (1956, 1958) alsofound, males continued to aggressively defend their territory boundaries againsttheir male neighbours, but they ceased to direct aggression at female intruders dur-ing the breeding season and I have shown elsewhere that males will engage incourtship or attempted copulation with extra-pair females whenever the opportuni-ty arises (CREIGHTON 2000a).

Females continued to aggressively oust only female territorial intruders, butin contrast to their pre-pairing behaviour, began to engage in territorial boundarydisputes with their female neighbours during the breeding season (SNOW 1956,1958). This suggests a shift in emphasis to defense of territorial area, though asdiscussed above this is unlikely to reflect a defense of material breeding resources.In other species female defense of territory area has been interpreted as a strategyto prevent polygyny or mate switching and the accompanying loss of paternal careand so female reproductive success, by preventing other females from formingsocial bonds with the resident male (ACRES 1989, DUNN & HANNON 1991, DAVIES

1992, SLAGSVOLD & LIFJELD 1994). I have shown elsewhere (CREIGHTON 2000b) thatin contrast to males (CREIGHTON 2001) females do not actively guard their mateagainst taking part in EPCs, but they may be using defense of territorial area dur-ing the breeding season to prevent polygyny and limit the reproductive strategies oftheir mates to social monogamy and extra-pair copulations (CREIGHTON 2000a).

In conclusion, both sexes are constrained in their choice of social mate byintrasexual competition for late winter territories leading to the formation of social

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pairs between overlapping territorial males and females. However, females mayimprove upon the quality of their genetic mate by engaging in extra-pair copula-tions with males of their choice after pair formation (CREIGHTON 2000a). Males mayimprove upon their reproductive success by seeking EPCs with neighbouringfemales (CREIGHTON 2000a), but female territorial defense during the breeding sea-son appears to limit their opportunity for polygyny. These data suggest that theprocess of mate acquisition in urban European blackbirds constrains the choice ofsocial mate and so generates sexual selection pressures to drive mixed reproductivestrategies during the breeding season, thus offering support for the social con-straint theory of sperm competition (MØLLER 1992, GOWATY 1996).

ACKNOWLEDGEMENTS

This study was supported by SERC as part of a Ph.D. studentship at the Open Universi-ty, Milton Keynes, and was conducted in Oxford Botanic Garden by kind permission of theTrustees. I thank my supervisors Tim Halliday and Ben Hatchwell for their support, andPatricia Gowaty and Tore Slagsvold for invaluable comments on the manuscript.

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