Mixed Reproductive Strategy Research Articles

Extra-pair matings, context-dependence and offspring quality: a brood manipulation experiment in pied flycatchers

Extra-pair paternity (EPP) in socially monogamous species has been associated with the genetic benefits incurred by females through extra-pair mate choice. There is conflicting evidence in the literature concerning the importance of such benefits possibly due to their context-dependence. To ascertain if there are such context-dependent genetic benefits, we conducted a brood manipulation experiment in pied flycatchers (Ficedula hypoleuca) breeding in central Spain. We found that extra-pair mates (EPM) are frequently not nearest neighbours. Cuckolded males and EPM did not differ from other males in any measured trait, and were not different either when cuckolds and cuckolders were compared within broods. Within-pair offspring (WPO) and extra-pair offspring (EPO) did not differ with respect to heterozygosity and no association of heterozygosity of either offspring or males and females with EPP could be detected. Moreover, the competitive context in the nest induced by brood manipulations affected the growth of nestlings and showed an interaction with paternity differences in nestling mass but in the opposite direction to the prediction of context-dependence, i.e., EPO were lighter than WPO in enlarged broods but not in control broods. Furthermore, EPO had shorter tarsi than WPO in all treatments. Mortality in the nest showed an association with brood manipulation treatment, but not with paternity or its interaction with treatment. Thus, we have not found any evidence of genetic benefits, context-dependent or otherwise, of EPP in our study population. The evidence of poor quality EPO does not support a mixed reproductive strategy of females in our population.

Dust record over Central Europe and the Holocene history of Sahara

Male fulmars, Fulmarus glacialis, employ a mixed reproductive strategy, copulating frequently with their mates while also taking opportunities to inseminate other females by extra-pair copulation. This study combines a complete record of copulation behaviour with paternity determined by DNA finger-printing in order to establish whether extra-pair copulations result in extra-pair paternity and to investigate how the fertilizing male achieves paternity. Despite the regular occurrence of extra-pair copulation (2·4% of all copulations involved an extra-pair male), no extra-pair paternity was detected in the study population. The upper 95% confidence limit for the population as a whole was 4·6% extra-pair paternity. The fertilizing male in all cases achieved both the greatest number of copulations relative to any other male and the last copulation before egg laying. The possible mechanism of paternity attainment in this species is discussed.

Mixed reproductive strategies of the Common moorhen on a microscale as revealed by genetic data

External factors shaping reproductive strategies within a population are still poorly understood. How individuals use space and where they decide to build a nest may influence reproductive strategies, as individuals that are close in space may more frequently interact socially. Here, we investigated a population (n=58 from 15 nests) of the Common moorhen in the Loir river (Western France) using microsatellite data (54 alleles). We found a surprisingly low level of genetic monogamy, a low relatedness among offspring in some nests and a low relatedness between the social parents and the offspring. Nest heterozygosity was highest close to the geographic center of the population. The mating strategies of the Common moorhen were highly variable, despite the social monogamy of the species, and were, to some extent, influenced by the microspatial structure. We discuss how our results contribute to the understanding of parent–offspring and offspring–offspring relationships.

Low Levels of Extra-Pair Paternity in a Neotropical Duetting Songbird, the Rufous-and-white Wren (Thryothorus rufalbus)

Abstract. Molecular analyses have confirmed that the majority of socially monogamous birds follow a genetically promiscuous reproductive strategy. Most such studies, however, have addressed species of the North Temperate Zone; tropical species are grossly underrepresented, although most of Earth's biodiversity is concentrated in the tropics. In this study, we describe the genetic mating strategy of the Rufous-and-white Wren (Thryothorus rufalbus), a socially monogamous duetting neotropical songbird. Over 8 years of studying a marked population in Costa Rica, we recorded genetic data from 51 broods. Microsatellite analysis of four variable loci revealed that the Rufous-and-white Wren follows a mixed reproductive strategy, where extra-pair young are found in a small minority of broods. Three of 158 nestlings (2%) were the result of extra-pair fertilizations; three of the 51 broods (6%) contained a nestling with alleles mismatched to its social father. We assigned paternity of two of the extra-pair nestlings...

DIRECT BENEFITS OF GENETIC MOSAICISM AND INTRAORGANISMAL SELECTION: MODELING COEVOLUTION BETWEEN A LONG-LIVED TREE AND A SHORT-LIVED HERBIVORE

We model direct fitness benefits of genetic mosaicism for a long-lived tree in coevolution with a short-lived herbivore to test four hypotheses: that mosaicism reduces selection on the herbivore for resistance to plant defenses; that module-level selection allows the individual tree to adapt to its herbivore; and that this benefits the tree population, increasing average tree fitness and reducing local adaptation of the herbivore. We show that: mosaicism does not sufficiently reduce selection for resistance in the herbivore to benefit the tree; that individual trees do benefit from module-level selection when somatic mutation introduces new defenses; and that mosaicism does reduce local adaptation in the herbivore, which increases average tree fitness. These results are robust to varying genetic assumptions of dominance and the somatic mutation rate, but only hold for sufficiently long-lived trees with relatively strong selection. We also show that a mixed reproductive strategy of primarily asexual reproduction interspersed with occasional sexual reproduction is effective in coevolving with the herbivore, as it maintains beneficial allele combinations. Finally, we argue that intraorganismal genetic heterogeneity need not threaten the integrity of the individual and may be adaptive when selection acts concordantly between levels.

Environmental control of reproductive phenology and the effect of pollen supplementation on resource allocation in the cleistogamous weed, Ruellia nudiflora (Acanthaceae)

Mixed reproductive strategies may have evolved as a response of plants to cope with environmental variation. One example of a mixed reproductive strategy is dimorphic cleistogamy, where a single plant produces closed, obligately self-pollinated (CL) flowers and open, potentially outcrossed (CH) flowers. Frequently, optimal environmental conditions favour production of more costly CH structures whilst economical and reliable CL structures are produced under less favourable conditions. In this study we explore (1) the effect of light and water on the reproductive phenology and (2) the effect of pollen supplementation on resource allocation to seeds in the cleistogamous weed Ruellia nudiflora. Split-plot field experiments were carried out to assess the effect of shade (two levels: ambient light vs. a reduction of 50 %) and watering (two levels: non-watered vs. watered) on the onset, end and duration of the production of three reproductive structures: CH flowers, CH fruit and CL fruit. We also looked at the effect of these environmental factors on biomass allocation to seeds (seed weight) from obligately self-pollinated flowers (CL), open-pollinated CH flowers and pollen-supplemented CH flowers. CH structures were produced for a briefer period and ended earlier under shaded conditions. These conditions also resulted in an earlier production of CL fruit. Shaded conditions also produced greater biomass allocation to CH seeds receiving extra pollen. Sub-optimal (shaded) conditions resulted in a briefer production period of CH structures whilst these same conditions resulted in an earlier production of CL structures. However, under sub-optimal conditions, plants also allocated more resources to seeds sired from CH flowers receiving large pollen loads. Earlier production of reproductive structures and relatively larger seed might improve subsequent success of CL and pollen-supplemented CH seeds, respectively.

Morphometric techniques allow environmental reconstructions from low-diversity continental ostracode assemblages

Quantification of intra-specific morphological variability of aquatic biota along environmental gradients can produce biological proxies that can be applied to paleoenvironmental reconstructions. This morphology-derived proxy information can be especially valuable when dealing with low-diversity fossil assemblages, i.e. in situations when paleoenvironmental inference based on species composition of the assemblage is less effective. We analyzed valve size and outline shape of the widespread and highly environmentally tolerant ostracode species Limnocythere inopinata collected in 15 lakes and ponds of Western Mongolia. We quantified shape variability among and within these living populations in relation to water chemistry and physical habitat variables. Our results indicate that: (1) a population’s mean valve outline is related to habitat type, (2) surface water temperature, the alkalinity to sulphate ratio, specific conductance and total phosphorus together explain a high portion of the variance in mean valve outline between populations, and (3) a quantitative model inferring the alkalinity to sulphate ratio from mean valve outline has an R² of 0.88 and RMSEP of 0.17. These results corroborate the hypothesis that high morphological variability in this ostracode species is due to both ecophenotypic variance and high clonal diversity associated with a mixed reproductive strategy (a combination of sexual and parthenogenetically reproducing lineages), and underline the value of morphometric techniques in paleoecology.

Direct Benefits and Genetic Costs of Extrapair Paternity for Female American Crows (Corvus brachyrhynchos)

The idea that extrapair paternity (EPP) in birds is part of a mixed reproductive strategy driven primarily by females is controversial. In cooperatively breeding American crows, we compared predictions of four female benefits hypotheses-the genetic diversity, good genes, genetic compatibility, and direct benefits hypotheses-to our predictions if EPP was primarily male driven. We found that genetically diverse broods were not more successful, extrapair young were not in better condition and did not have a higher survival probability, and, contrary to prediction, offspring sired by within-group extrapair males were more inbred than within-pair offspring. There was evidence of direct benefits, however: provisioning rate and number of surviving offspring were higher in groups containing within-group extrapair sires. Females therefore derived no apparent benefits from extragroup extrapair males but both direct benefits and genetic costs from within-group extrapair males. We suggest that males and females both influence the distribution of EPP in this system.

Female Mate Preferences Among Pan troglodytes schweinfurthii of Kanyawara, Kibale National Park, Uganda

Studies of reproduction among chimpanzees traditionally have focused on the mating strategies of males. However, less is known about the mating strategies of female chimpanzees and whether they demonstrate mate choice. I investigated sexual behavior and female mate preference in the chimpanzees of the Kanyawara community. To estimate mate preferences, I analyzed female proceptivity and resistance rates of 6 estrous females toward a total of 13 males as well as male solicitation and aggression rates toward females. Males solicited some females more often than others for mating and preferred them throughout estrus, not only during the periovulatory period (POP), when conception was most likely. In contrast, though females had strong mate preferences in both non-POP and POP, their mate preferences were not consistent between the 2 phases. The shift in mate preferences is evidence of a promiscuous yet tactical mating strategy to confuse paternity. Further, females were more proceptive and generally less resistant toward eschewed males in non-POP and more proceptive and less resistant toward preferred males in POP. Hence, the results indicate that females attempted to mate selectively during the fertile phase. Kanyawara female chimpanzees appear to change their mating strategies and selectivity during estrus and thus may pursue a mixed reproductive strategy. The tactic may allow females to deceive males, indicating that promiscuity among chimpanzee females may be more strategic than previously thought.

Older male reed buntings are more successful at gaining extrapair fertilizations

The importance of extrapair paternity (EPP) as an aspect of mixed reproductive strategies is currently the focus of many studies. Since females have at least some control over the occurrence of inseminations , they are expected to engage in extrapair copulations only if they benefit, for instance through gaining high-quality or compatible genes for their offspring, or insurance against infertility. To distinguish between these benefits, we studied characteristics related to intermale variation in fertilization success as well as differences in fitness between half-siblings in the reed bunting, Emberiza schoeniclus , a socially monogamous passerine with high levels of EPP (50% of young). We found that older males were more successful at siring offspring in other broods and a nonsignificant tendency for them to be less cuckolded in their own broods. The success of older males does not support the fertility insurance hypothesis: a male's sperm storage capacity did not predict his fertilization success, the occurrence of infertile eggs was unrelated to the occurrence of EPP in a nest and older males had more infertile eggs in nests with no EPP. Extrapair young had longer tarsi at fledging than their maternal half-siblings, but we found no other differences between maternal or paternal half-siblings in several presumed quality traits. Owing to the absence of long-term fitness data, we are unable to determine whether the difference in tarsus length is related to a difference in fitness benefits. At this time, we cannot exclude the possibility that females do gain genetic benefits through EPP.

Dispersal potential of spores and asexual propagules in the epixylic hepatic Anastrophyllum hellerianum

Dispersal ability is of great importance for plants, which commonly occupy spatially and temporally limited substrate patches. Mixed reproductive strategies with abundant diaspore production are favoured in a heterogeneous landscape to ensure successful colonisation at different distances. In bryophytes, long-distance dispersal has been thought to take place primarily by spores, while asexual propagules are important in local dispersal and in the maintenance of colonies. In the present study, we investigated the dispersal potential of two equally sized propagules, sexually formed spores and asexually produced gemmae in the dioecious, epixylic hepatic, Anastrophyllum hellerianum, which inhabits spatially and temporally limited substrate patches. We trapped propagules at different distances (0–10 m) and directions from the source colonies in two experiments: one in a natural habitat within a forest and another involving an artificial set-up in an open habitat. Spore dispersal showed only slight distance dependence both in the open and the forest habitats, presumably as a consequence of wind affecting the dispersal pattern. Gemma dispersal was more strongly distance-dependent in the open habitat than in the forest sites. Considerably more gemmae were deposited during rainy than dry periods, possibly because of the effect of rain drops on gemma release. However, weather conditions had no effect on the dispersal patterns of spores or gemmae. In A. hellerianum, the combination of occasional spore production and practically continuous, massive gemma production facilitates dispersal both on local scale and over long distances. Unlike previously assumed, not only spores but also the asexual propagules may contribute to long-distance dispersal, thus allowing considerable gene flow at the landscape level.

Metabolism and development of pelagic larvae of Antarctic gastropods with mixed reproductive strategies

The Antarctic gastropods Marseniopsis mollis (Smith, 1902) and Torellia mirabilis (Smith, 1907) lay eggs in the late austral summer and autumn; these hatch the following late winter and early summer. Field observations indicate that M. mollis lays eggs in the tests of ascidians in January to March. Ascidians with M. mollis broods were collected in April and held in culture. Larvae hatched between mid-October and mid-December, a brooding period of 8 to 11 mo. T. mirabilis egg masses laid in aquaria in April took 6 to 7 mo to hatch. Both species released large veliger larvae (presumably planktotrophic). Natural phytoplankton were provided, but feeding was not confirmed. These larvae were maintained in culture for 25 to 50 d, during which period they were regularly offered a range of substrata, but no settlement behaviour was observed. Metabolism was estimated in individual larvae using microrespirometers of 30 to 45 μl volume, and a couloximeter system to measure water oxygen-content. Average M. mollis veligers contained 14.1 μg ash-free dry mass (AFDM) and consumed 310 pmol O2 h–1. Corresponding values for T. mirabilis veligers were 17.6 μg AFDM and 221 pmol O2 h–1. The periods of protected development here were shorter than those previously reported for Antarctic gastropods, including T. mirabilis, by a factor of ×4, although previous reports were for species hatching as juveniles. The protected periods here were, however, around 5 time longer than for temperate gastropods. The oxygen-consumption rates for M. mollis and T. mirabilis veligers were around ×10 less than those for comparable temperate gastropod and bivalve veligers.

Male extraterritorial forays, age and paternity in the socially monogamous reed bunting (Emberiza schoeniclus)

Genetic studies have shown that extra-pair paternity is widespread among socially monogamous bird species. Yet, the role of males and females and their behavior leading to this mixed reproductive strategy is poorly understood. Here, we analyze paternity in relation to male age and mating behavior in the socially monogamous reed bunting (Emberiza schoeniclus). We report a positive relation between male extraterritorial forays and success in obtaining extra-pair fertilizations. Extraterritorial forays tended to increase in frequency with male age and older males sired a larger number of extra-pair offspring than young males. Identified extra-pair sires were old in nine out of ten cases. The likelihood of being cuckolded was not affected by male age. Although based on correlative data, our results highlight age-dependent explorative male behavior as a key determinant for the understanding of extra-pair mating in the reed bunting. We do, however, emphasize the need for further studies to reveal the role of females in extra-pair copulations and fertilizations.

Extra‐pair paternity in the Lesser Kestrel Falco naumanni: a re‐evaluation using microsatellite markers

Modern molecular techniques based on DNA typing have revolutionized our view of avian mating systems. Birds were considered to be essentially monogamous only a few years ago (Lack 1968). In contrast, Griffith et al . (2002) have recently summarized how molecular techniques have revealed that birds are only rarely monogamous, with extra-pair offspring found in approximately 90% of the species studied to date. It has been known for some time that individuals in supposedly monogamous species often adopt a mixed reproductive strategy (Trivers 1972). Both males and females may seek extra-pair copulations (EPC), which could provide them with some genetic benefits (for a review of benefits and drawbacks of engaging in EPC, see Birkhead & Moller 1992; Wink & Dyrcz 1999). Although there is great interest in the reasons for differences in extra-pair paternity (EPP) rates in birds, few studies have compared the results obtained by different molecular techniques. Among available markers, DNA fingerprinting in the 1990s and, more recently, microsatellites, are the most popular in elucidating genetic relationships in birds (Burke & Bruford 1987, Ellegren 1992), but, to our knowledge, comparisons of the performance of these two techniques using the same sample sets have not been reported. In this study, we reappraised EPP in the Lesser Kestrel Falco naumanni . Our aim was to recalculate levels of EPP in this species using microsatellite markers instead of DNA fingerprinting, and to compare the results obtained with both techniques. Earlier studies of the copulatory behaviour of Lesser Kestrels (Negro et al . 1992) have already shown that some males pursue a mixed reproductive strategy, with a 6.7% incidence of EPC attempts, and DNA fingerprintings of 26 families revealed that 3.4% of the nestlings resulted from extra-pair fertilizations (Negro

Population growth versus population spread of an ant-dispersed neotropical herb with a mixed reproductive strategy

In plants that produce seeds with contrasting genetic background (selfed versus outcrossed), the question arises whether the ecological function of the two types of progeny differ. This paper addresses this issue for the ant-dispersed Calathea micans by introducing a novel application of the Neubert–Caswell model for analysis of wave speed for structured populations. Because dispersal as well as vital rates are structured, the model allows for distinct dispersal kernels for different types of progeny and thus permits comparisons of the sensitivity to changes in demographic and dispersal parameters of in situ population growth rate versus population spread across space. The study site was a lowland, evergreen tropical rain forest at La Selva Biological station, Costa Rica, where the species is commonly found throughout the forest. In C. micans, seeds produced by open flowers (potentially outcrossed) or by closed flowers (selfed) bear oily arils and are dispersed by ants. Five life-history stages were used to characterize the population: seedlings originating from seeds produced by open flowers, seedlings originating from seeds produced by closed flowers, juvenile vegetative plants, reproductive plants without new shoots and reproductive plants with new shoots. Demography varied seasonally. Transitions were estimated from marking and following the fate of plants ( N = 400) in a natural population over a dry and a wet season. The population dynamics was described by a 10 × 10 matrix, with five life-history stages and two habitat states. The habitat states cycle repeatedly, dry–wet–dry–wet. To estimate dispersal kernels for each seed type, individual seeds ( N = 225 and 306 seeds produced by open and closed flowers, respectively) were color-coded and placed in depots, allowing the ants to redistribute them. Five months later, seedlings with an attached seed coat bearing the intact color-coding, were surveyed around the depots. Radial distances and angles were recorded for each seedling ( N = 67 and 81 seedlings arising from open and closed flowers, respectively). The results of the model give an asymptotic growth rate of 1.06 per season and an asymptotic rate of spread of 8.36 cm per season. There is a high correlation ( r = 0.99) between elasticity of growth rate and elasticity of rate of spread of the population. Both rates are most sensitive to changes in stasis of juveniles during the dry season. However, most interesting is the analysis that revealed that population spread is more sensitive than in situ population growth to demographic rates of seedlings arising from open flowers. The analysis suggests a new way of thinking about ecological functions of multiple modes of reproduction.

Mixed reproductive strategy in Tubifex tubifex (Oligochaeta, Tubificidae)?

Despite the wide use of the tubificid oligochaete Tubifex tubifex in eco-toxicological studies, the reproductive strategy adopted by the species, that can reasonably be assumed to be the key to its ecological success, is still not well investigated. This study reports on breeding experiments analysed by allozyme markers (Pgi, Pgm, and Idh), accompanied by a study of the sperm production in the species, with the aim to: (1) clarify the type of uniparental reproduction adopted by the species, (2) test the hypothesis that uniparental reproduction is reversible, (3) investigate the occurrence of biparental reproduction in laboratory cultures. Studies of parent-offspring comparison at polymorphic allozyme loci showed parthenogenetic reproduction and maintenance of asexuality in the parthenogenetic individuals. A cross-breeding test performed with couples made up of randomly assorted individuals, whose genotype combinations were suitable for discriminating between sexual and asexual reproduction, failed to show biparental reproduction in laboratory cultures: T. tubifex always reproduced parthenogenetically. Unexpectedly, spermiogenetic analysis indicated that both kinds of sperm produced by the species (eusperm and parasperm) were differentiated in individuals raised either in cohort cultures or in isolation (first, second, and third parthenogenetic generations), with a similar pattern of sperm production correlated to the sexual stage. Interestingly, there was no avoidance of mating in any of the collective cultures analysed. Concomitance between parthenogenetic reproduction and a "normal" male functionality, which is typical of a sexually reproducing species, could be justified by a mixed reproductive strategy or a pseudogamy process occurring in T. tubifex. However, several aspects of the reproductive behaviour of the species deserve further investigation.

Does promiscuous mating preclude female choice? Female sexual strategies in chimpanzees (Pan troglodytes verus) of the Ta� National Park, C�te d?Ivoire

According to sexual selection theory, females should selectively mate with high-quality males to enhance offspring survival and maximize reproductive success. Yet, chimpanzee (Pan troglodytes) females are known to mate promiscuously. Although there is substantial rationale for a promiscuous mating strategy, there is also a strong expectation that females should be selective, and the question arises as to whether promiscuity precludes female choice. The aims of this study are to: (1) compare wild female chimpanzee sexual strategies throughout estrus, and (2) determine whether females exhibit mate preferences for particular males. Over 2,600 h of data were collected on two habituated chimpanzee (Pan troglodytes verus) communities in the Tai National Park, Cote d’Ivoire. Female mate preferences were measured by quantifying proceptivity and receptivity toward males. Results indicate that all females exhibited proceptivity and resistance to male solicitations, but that there was substantial variation in their magnitudes within and among females. Female proceptivity rates were lower and resistance rates were higher in the periovulatory period (POP) when conception is most likely. Females were more selective during POP, and more promiscuous outside of POP, suggesting that females may follow a mixed reproductive strategy, being selective when conception is likely and more promiscuous when conception is unlikely. Results from this study emphasize the importance of considering the fertility window when determining female mate preferences, and of examining female behavior in POP and non-POP phases separately when evaluating hypotheses for multi-male mating.

Nest cavity orientation in black‐capped chickadees Poecile atricapillus: do the acoustic properties of cavities influence sound reception in the nest and extra‐pair matings?

Birds that nest in cavities may regulate nest microclimate by orienting their nest entrance relative to the sun or prevailing winds. Alternatively, birds may orient their nest entrance relative to conspecific individuals around them, especially if the acoustic properties of cavities permit nesting birds to better hear individuals in front of their nest. We measured the cavity entrance orientation of 132 nests and 234 excavations in a colour‐banded population of black‐capped chickadees Poecile atricapillus for which the reproductive behaviour of nesting females was known. Most chickadees excavated cavities in rotten birch Betula papyrifera, aspen Populus tremuloides and maple Acer saccharum. Nest cavities showed random compass orientation around 360° demonstrating that chickadees do not orient their cavities relative to the sun or prevailing winds. We also presented chickadees with nest boxes arranged in groups of four, oriented at 90° intervals around the same tree. Nests constructed in these nest box quartets also showed random compass orientation. To test the acoustic properties of nest cavities, we conducted a sound transmission experiment using a microphone mounted inside a chickadee nest. Re‐recorded songs demonstrate that chickadee nest cavities have directional acoustic properties; songs recorded with the cavity entrance oriented towards the loudspeaker were louder than songs recorded with the cavity entrance oriented away from the loudspeaker. Thus, female chickadees, who roost inside their nest cavity in the early morning during their fertile period, should be better able to hear males singing the dawn chorus in front of their nest cavity. Using GIS analyses we tested for angular‐angular correlation between actual nest cavity orientation and the azimuth from the nest tree to the territories and nest cavities of nearby males. In general, nest cavity entrances showed no angular‐angular correlation with neighbourhood territory features. However, among birds who followed a mixed reproductive strategy and nested in the soft wood of birch and aspen trees, nest cavity entrances were oriented towards their extra‐pair partners. We conclude that nest cavity orientation in birds may be influenced by both ecological and social factors.

Patterns of extrapair mating in relation to male dominance status and female nest placement in black-capped chickadees

In sexually promiscuous animals, females may benefit by nesting close to the edge of their partner's territory to facilitate extrapair copulations. In the present study, we describe the extrapair mating system of black-capped chickadees, Poecile atricapillus, and test whether nest locations are influenced by conspecific attraction to extrapair partners. We conducted a spatial analysis of female mating strategies by using microsatellite paternity analysis in conjunction with geographic information system (GIS) analysis of nest and territory locations. Extrapair offspring comprised 52 of 351 offspring (14.8%) and were present in 19 of 57 broods (33.3%). Females paired to males with low dominance status in the previous winter's flock hierarchy were more likely to engage in a mixed reproductive strategy than were females paired to males with high dominance status. Females had extrapair copulations and extrapair fertilizations with high-ranking males more often than with low-ranking males. Not all extrapair copulations resulted in extrapair fertilizations. Females constructed their nests within 16.8 ± 1.0 m of the edge of their partner's territory, significantly closer to the edge of their nearest neighbor's territory than to the center of their own partner's territory. Extrapair males usually shared territory boundaries with cuckolded males. Females paired to low-ranking males constructed nests near the territory edges of neighboring high-ranking males. However, females did not have extrapair copulations with the neighbor nearest to their nest or even with the high-ranking neighbor nearest to their nest. We conclude that conspecific attraction to neighbors may influence nesting location in black-capped chickadees; however, it does not operate by facilitating extrapair copulations.

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

Female European blackbirds, Turdus merula have been shown to engage in mixed reproductive strategies and females do not seem to gain any material benefit from their extra-pair copulation behaviour. To test the hypothesis that such behaviour arises out of constraint on their choice of social mate, I investigate if and how these females are constrained in their choice at mate acquisition. I recorded patterns of territorial distribution and interactions between resident birds both before pairing and during the breeding season. Patterns of observed behaviour agreed with earlier descriptions of winter and breeding territoriality and provided clues to the underlying mechanism of mate acquisition. I conclude that the constraint on breeding success imposed by high nest predation promotes intrasexual competition for high quality nesting habitat between females prior to pair formation. Female distribution promotes intrasexual competition between males for territorial area overlapping females or for habitat likely to be occupied by females. Mate choice during pair formation is constrained by the outcome of this intrasexual competition and, typically, overlapping territorial males and females pair into social monogamy on a shared breeding territory. Although this ultimately leads to assortitive mating by competitive ability, low quality females paired to low quality males may improve upon the quality of their genetic mate by engaging in extra-pair copulations. These data suggest that the process of mate acquisition in urban European blackbirds constrains the choice of social mate and so generates sexual selection pressures to drive mixed reproductive strategies during the breeding season.