The importance of mate retention and experience on breeding success in Cassin's auklet (Ptychoramphus aleuticus)

We studied the relative effects of mate retention and breedingexperience on reproductive success in Cassin's auklet (Ptychoramphusaleuticus) on Southeast Farallon Island, California, USA. Breedingsuccess of banded birds was monitored from 1985 to 1990 andanalyzed using linear and logistic regression. Breeding performanceimproved with experience and mate retention, but their relativeeffects differed. Hatching success improved with both femaleand male experience but declined with advanced experience inmales, perhaps due to reproductive senescence. In males, butnot in females, hatching success showed a quadratic relationshipwith length of the pair-bond when adjusted for experience, indicatinga greater benefit to males for mate retention. Fledging andbreeding success and weight at fledging also increased asymptoticallywith length of the pair-bond for both sexes. There was no correlationbetween mate switching and previous reproductive failure. Anincrease in weight at fledging with experience in females, butnot in males, suggests that females either became more efficientat foraging or invested greater effort in chick rearing withexperience than males.     

Interspecific Mate Choice Following Cross-fostering in a Mixed Colony of Greylag Geese (Anser anser) and Canada Geese (Branta canadensis). A Study on Development and Persistence of Species Preferences1

The role of early experience in mate choice and species preference of geese was studied in an experiment where young greylag geese (Anser anser) were cross-fostered by Canada geese (Branta canadensis). In two successive years, all eggs from 9 nests of wild Canada geese were removed and replaced by greylag goose eggs. The young greylag geese then followed their Canada goose foster parents on their southward migration in autumn, to return with them in spring. Of 35 returning birds, all 16 females paired with greylag goose males (100%) whereas of 19 males 5 paired with Canada goose females (14%) and the remaining 14 with greylag goose females (74%). The pair bonds generally persisted as long as both birds were present, but after loss of a partner, the remaining bird usually re-mated. Even when this happened several times during the lifetime of a male, the new mate was always a Canada goose female, showing that the males were sexually imprinted to this species. The Canada goose females which had mated with the greylag ganders also remated when widowed, but their new mate could be either a Canada goose or a greylag goose.     

Fitness benefits of polyandry for experienced females

Females often mate with several different males, which may promote sperm competition and increase offspring viability. However, the potential benefits of polyandry remain controversial, particularly in birds where recent reviews have suggested that females gain few genetic benefits from extra‐pair mating. In tree swallows (Tachycineta bicolor), we found that females with prior breeding experience had more sires per brood when paired to genetically similar social mates, and, among experienced females, broods with more sires had higher hatching success. Individual females breeding in two consecutive years also produced broods with more sires when they were more genetically similar to their mate. Thus, experienced females were able to avoid the costs of mating with a genetically similar social mate and realize fitness benefits from mating with a relatively large number of males. This is one of the first studies to show that female breeding experience influences polyandry and female fitness in a natural population of vertebrates. Our results suggest that the benefits of polyandry may only be clear when considering both the number of mates females acquire and their ability to modify the outcome of sexual conflict.     

Pairing success relative to male plumage redness and pigment symmetry in the house finch: temporal and geographic constancy

Male house finches (Carpodacus mexicanus) have carotenoid-based ornamentalplumage coloration. In previous research it was shown that fora single population of house finches in a single year, malesthat paired were on average redder in plumage coloration thanmales that did not pair, and males with redder plumage tendedto nest earlier than males with less red plumage. Here we showthat these patterns continued over 6 years and at two widely separatedlocations. We also tested whether the symmetry of carotenoid-based crownpigmentation differed between paired and unpaired males andfound that paired males have, on average, more perfect symmetryof crown pigmentation than males not paired. These observationssupport the idea that expression of carotenoid-based plumagecoloration by males is a persistent and widespread criterionin female mate choice in the house finch.     

Socially transmitted mate preferences in a monogamous bird: a non-genetic mechanism of sexual selection

There is increasing evidence that animals can acquire mate preferences through the use of public information, notably by observing (and copying) the mate preferences of others in the population. If females acquire preferences through social mechanisms, sexual selection could act very rapidly to spread the preference and drive elaboration of the preferred trait(s). Although there are reports of 'mate-choice copying' in polygynous species, there is no clear evidence for this process in monogamous species. Here, we investigated whether adult female zebra finches Taeniopygia guttata can socially acquire sexual preferences for individual males and, in a separate study, for a generalized trait (coloured leg bands) of males. In both studies, test females observed males in two simultaneous conditions: a ('chosen') mixed-sex situation in which a male was paired with a (model) female, and a ('unchosen') same-sex situation in which a male was paired with another male. In the first experiment, after two weeks of females observing males, test females significantly preferred individual males who had been paired with another female (i.e. chosen males). In the second experiment, test females significantly preferred novel males that were wearing the same leg band colour as the apparently chosen males. Our findings are consistent with the conclusion that female zebra finches' mate preferences are altered by public information. Our study implies that mate preferences can spread rapidly through populations by social mechanisms, affecting the strength of sexual selection in a monogamous species.     

Effects of Aging and Mate Retention on Reproductive Success of Captive Female Peregrine Falcons

SYNOPSIS. Separating ecological (extrinsic) factors affectingreproduction from physiological and experiential (intrinsic)factors can be problematic in free-ranging animals. This studyexamined age-related changes in six measures of reproductivesuccess (clutch size, fertility, hatchability, brood size athatching, survivorship of nestlings, and brood size at fledging)for captive female peregrine falcons (Falco peregrinus) whereecological factors (i.e., access to mates, nesting sites, andfood) were not limiting and were similar across years and betweenindividuals. Mean nestling survivorship increased throughoutthe lifespan of the female while all other measures of reproductionpeaked at about seven years of age and decreased thereafter.Birds with prior breeding experience had higher productivitythan inexperienced birds of the same age. Productivity increasedwith increasing experience of the pair. Productivity droppedby an average of 53% when females experienced a change of mate,and then subsequently increased over a period of several years.Productivity was not affected when birds were moved to differentbreeding sites. There was no evidence that early improvementsin reproduction were related to chronological aging in the absenceof experiential differences. Birds that began breeding earlierproduced more fledglings during their lifetimes as a resultof higher annual productivity. Lifetime reproduction was notcorrelated with longevity because birds with higher maximalegg production had shorter breeding lifespans. Birds retainingmates produced more fledglings during their lifetime than birdsthat changed mates, but birds that changed mates more than oncedid not have lower lifetime reproduction than birds that hadonly one mate change. These results suggest that 1) age-relatedchanges in reproduction are not necessarily resource-mediated,2) in the absence of resource limitation, experience of thepair is a primary factor determining annual reproductive success,3) benefits of increasing experience may be offset by the onsetof senescence, 4) the cost of present egg production on futurereproductive potential supports a "pleiotropic" theory of aging,and 5) costs associated with mate changes may encourage selectionfor low "divorce" rates (i.e., lifetime monogamy) in this species.     

Black-and-white plumage in male pied flycatchers (Ficedula hypoleuca) reduces the risk of predation from sparrowhawks (Accipiter nisus) during the breeding season

Bright colors in birds might signal that they are undesirableas prey (aposematic), an idea that has been difficult to test.When stuffed pied flycatchers Ficedula hypoleuca are exposedto migrating sparrowhawks Accipiter nisus in spring or in autumn,the hawks attack cryptic females more often than bright males.To achieve better statistical control and to assess whethermale plumage also reduces predation risk in the breeding seasonand in the nesting habitat of the pied flycatcher, I placedpairs of male and female flycatcher mounts in similar positionsnear 22 nests of sparrowhawks. The hawks attacked mainly femalemounts, verifying that the preference is real. The sparrowhawkscaught at least 19 live pied flycatchers; 12 young, 5 adultmales, 1 adult female, and 1 female or young. Hawks that caughtan adult male seemed to prefer attacking female mounts. I discussthree interpretations of these results, suggesting that black-and-whitemale flycatchers may benefit from being a novel and aberrantprey, at least early in the breeding season.     

A potential mate influences reproductive development in female,but not male,pine siskins

The role of photoperiod in avian reproductive timing has been well studied, and we are increasingly recognizing the roles of other environmental cues such as social cues. However, few studies have evaluated the extent to which males and females of the same species respond similarly to the same type of cue. Moreover, previous studies have rarely examined how variation in the quality or nature of a given social cue might modulate its effect. Here, we examine the sensitivity of male and female pine siskins (Spinus pinus) to a potential mate as a stimulatory cue for gonadal recrudescence, and we investigate whether variation in the relationship between a bird and its potential mate modulates the effect of that potential mate. Birds were initially housed without opposite sex birds on a 12L:12D photoperiod with ad libitum food. After gonadal recrudescence had begun males and females were randomly paired with an opposite sex bird or housed alone. An additional group of males was paired with estradiol-implanted females. In males, these social treatments had no effect on testis length, cloacal protuberance length, luteinizing hormone (LH) levels, or testosterone levels. In females, presence of a potential mate had a significant and positive effect on ovary score, defeathering of the brood patch, and LH levels. Among paired birds, the degree of affiliation within a pair corresponded to the extent of reproductive development in females, but not males. Thus, reproductive timing in females appears to be sensitive to both the presence of a potential mate and her relationship with him.     

Do females trade copulations for food? An experimental study on kittiwakes (Rissa tridactyla)

Females of many species copulate more frequently than necessaryto fertilize their eggs despite the potential costs. Severalstudies, particularly on socially monogamous birds, have suggestedthat females obtain immediate material benefits by trading copulationsfor nutrients or other resources. We experimentally tested thishypothesis by manipulating the food resources available to prelayingfemale black-legged kittiwakes (Rissa tridactyla). If femalekittiwakes trade copulations for courtship feeding because theyneed the extra resources, well-fed females (experimental group)should be less willing to copulate compared with females thatare more food limited (control group). Contrary to our predictions,we found that close to the start of laying experimental femalescopulated more frequently with their mate than control females.We also observed that males from the experimental group fedtheir mate at least as often as males from the control group.In experimental pairs, we still observed a positive correlationbetween the rate of copulation and the rate of courtship feeding.Our results thus refute the immediate material benefits hypothesis.Currently available data are consistent with the hypothesisthat prelaying courtship feeding is a form of mating effort.We suggest that the rate of courtship feeding might be a sexuallyselected trait, on which females base decisions about timingand frequency of copulations, but this remains to be tested.     

Mating preference of female zebrafish, Danio rerio, in relation to male dominance

Mating success tends to be skewed toward dominant males, thoughfemale mate preferences may not always correlate with male dominance.In this study, we investigated the mating preferences of femalezebrafish, Danio rerio, in the absence of male–male competition.We paired females sequentially with males of known dominancerank, using a nested, repeated measures design, with egg productionas a measure of female mate preference. We predicted that femaleswould spawn more frequently and produce larger clutches whenpaired with males of higher dominance rank. We found significantdifferences among females in the size of clutches produced andamong males in the size of clutches received, but these differenceswere independent of male dominance rank. Male body size wasnot related to either dominance rank or clutch size received.These results indicate that females vary clutch size in relationto the males with which they are paired but that they do notfavor dominant males. Thus, male competition may normally overridefemale mate preference in zebrafish.     

Females Paired with More Attractive Males Show Reduced Oxidative Damage: Possible Direct Benefits of Mate Choice in Pied Flycatchers

Direct benefits of female mate choice may concern female fertility and fecundity but also physiological status. In birds with biparental care, males may contribute to improve the condition and health of their pair‐mates through help in constructing nests, incubation or incubation feeding and nestling provisioning. They may also reduce harassment of females by non‐pair males. A consequence of these male activities could be expressed in terms of oxidative damage, which may depend on metabolic effort and social stress. Here, we have related male contribution to parental and territorial duties to female oxidative status in the pied flycatcher Ficedula hypoleuca, a species where preferred males present darker dorsal plumage and, in Iberian populations, a large white forehead patch. Darker males were paired with females with high incubation attendance and reduced nestling provisioning rates, which may lead to reduced female exertion. These males owned nest boxes at which there were fewer visits by non‐pair males. Although females paired with dark mates worked less hard, they were able to raise more fledglings. Female oxidative damage measured as malondialdehyde (MDA) level in plasma declined with increasing incubation attendance and male incubation feeding. Moreover, levels of MDA in females declined with both darkness of male dorsal plumage and male forehead patch size when controlling for female forehead patch size and male age. The effect of male plumage darkness was especially strong. Females paired with middle‐aged males (2–3 yr) showed reduced levels of MDA compared with those paired with 1‐yr‐old and more than 3‐yr‐old males. Male age could not explain the effects of male attractiveness. Females paired with attractive males were more successful in reproduction while suffering reduced oxidative damage, possibly mediated by help during incubation and nestling rearing from their pair‐mates. Although correlative, the evidence suggests direct benefits of females paired with more attractive males.     

Mate switching and copulation behaviour in King Penguins

Extra‐pair paternity (EPP) in monogamous birds may result from either extra‐pair copulations (EPCs) or mate switching. In this study of King Penguins in South Georgia, we observed no EPCs at all, an effect of very efficient mate guarding. Onshore males fast and need not divert attention to foraging or to defending nest or territory, as this species has neither. However, we found that mate switching was common. On average 38% (range: 29%–56%; three years pooled) of the birds established pair bonds with at least one initial partner before switching to the partner they bred with (i.e. the “pair mate”). Of the observed copulations of 44 studied females, 22% were with initial partners and 78% with the pair mate. This and the high proportion of mate switching suggest that roughly 10% of the females could have received sperm from males other than the pair mate. The average copulation frequency was 0.026 h^?1, resulting in an estimated 8.2 copulations per clutch (which consists of one egg). That more copulations than necessary for fertilisation occur suggests that males try to protect paternity by sperm competition, and that this is a result of the potential for EPP due to mate switching in King Penguins. All observed copulations except one took place between days 13 and 5, with the peak 7.5 days prior to egg‐laying. The birds found their pair mates (often not the same as in the previous year) on average about 10 days before egg‐laying, and always established themselves at the outskirts of the colony about 8 days before egg‐laying. Thus, most copulations occurred around the time the birds joined the colony. We suggest that it is adaptive to obtain a breeding spot early, because the colony will grow and pairs joining later will protect the offspring. Additionally, we suggest that early copulation outside the colony is adaptive because of the risk of failing to fertilise the egg when copulating among aggressive neighbours inside the dense colony. Based on these two arguments we suggest a “safe place hypothesis” to explain the early copulation peak in King Penguins.     

Mate choice based on static versus dynamic secondary sexual traits in the dark-eyed junco

Some secondary sexual traits (SSTs) such as structural characteristicsare semi-permanent or static, while others, such as courtshipdisplay, are more labile or dynamic. In this paper we reportresults from two experiments designed to test the relative attractivenessto female dark-eyed juncos (Junco hyemalis, Passeriformes, Aves)of a relatively static plumage trait, the amount of white inthe tail, and a relatively dynamic behavioral trait, courtshipintensity. The experiments derived from a study showing that femalejuncos prefer males that court more vigorously. We asked whether femalesalso base their preferences on plumage traits and how they respond whenpresented with a choice between attractive traits that are eitherstatic (plumage) or dynamic (courtship) in nature. In the firstexperiment we presented males to females in paired mate-choicetrials and found that males enhanced with more white in theirtails were more attractive to females than controls with unenhancedtails. Females spent more time with enhanced males and directedmore sexual displays toward them. In the second experiment we testedwhether females preferred males with enhanced tails (a staticSST) or males with enhanced hormone-mediated courtship behavior(a dynamic SST). In this experiment females did not demonstratea consensus preference for either the static or the dynamictrait. Instead, some females preferred the male whose courtshipperformance was enhanced with testosterone, while others preferredthe male with an enhanced tail. We conclude that both kindsof traits are important in junco mate choice, but that somefemales apparently weigh static traits more heavily than dynamicones, while other females use opposite weightings.     

Female pied flycatchers trade between male quality and mating status in mate choice

According to mate choice theory, females should consider both male quality and mating status when choosing a mate. In birds, strong experimental evidence indicates that females prefer males with elaborate traits. No comparable evidence exists to determine whether females take male mating status into account or how they may trade between male quality and male mating status. We studied mate choice of female pied flycatchers (Ficedula hypoleuca) in outdoor aviaries where the effect of territory quality could be eliminated and where we could control which males were mated and which were unmated. We used male plumage colour as our measure of male quality. In the aviaries, focal females could easily compare males and assess their plumage colour and mating status, and resident females were prevented from attacking prospecting females because of separation in different compartments. The study provided evidence for a trade-off in mate choice. Females may compromise by choosing an already mated male if he is more brightly coloured and, presumably, of higher quality than available unmated males. The study did not support the idea that polygyny is based on male deception of females, but the results were consistent with the female aggression hypothesis.     

Influence of helping and breeding experience on reproductive performance in the Seychelles warbler: a translocation experiment

Reproductive success of the cooperative breeding Seychelleswarbler (Acrocephalus sechellensis) increases with age. Thisage effect is not due to differential survival or increasedreproductive effort, but to accumulated helping and breedingexperience. In their first year of breeding, reproductive performanceof inexperienced warblers with neither helping nor breedingexperience was significandy lower than that of warblers of thesame age with either previous helping or breeding experience.Reproductive performance was the same for primiparae with helpingexperience and for birds with breeding experience. Female primiparaewith helping experience or breeding experience built betternests and spent more time incubating than inexperienced females,which led to increased hatching success. Male primiparae withhelping experience or males with breeding experience guardedthe clutch better than inexperienced males, which led to reducedegg predation. Even-aged warblers with different previous experienceswere transferred to unoccupied islands, where birds startedbreeding immediately in high-quality territories. The experimentshowed that birds with helping experience produced their firstfledgling as fast as experienced breeders, and significandyfaster than inexperienced birds. Breeding performance did notimprove further with experience after the first successful breedingattempt. Only birds with previous breeding experience who pairedwith inexperienced birds, were likely to change mate. The otherpair combinations remained stable. Thus, primiparous birds withhelping experience have greater lifetime reproductive successthan inexperienced primiparae of the same age. This experimentshows that helping behavior has not only been selected for inthe context of promoting an individual's indirect fitness, butalso in the context of gaining helping experience which translatesinto improved reproductive success when a helper becomes a breeder.[Behav Ecol 7: 326-333 (1996)]     

Effects of Male Vocal Learning on Female Behavior in the Budgerigar, Melopsittacus undulatus

Parrots are unusual among birds and animals in general in the extent of their ability to learn new vocalizations throughout life and irrespective of season. The budgerigar (Melopsittacus undulatus), a small parrot that is well suited for laboratory studies, has been the subject of numerous studies investigating the neurobiology of vocal learning. To date, few studies have focused on the function of vocal imitation by parrots. Previous work from our research group has shown that vocal imitation in budgerigars is sex‐biased, as males paired with females learn vocalizations from their new mates, but not vice versa. This bias led us to hypothesize that vocal learning has a reproductive function. To test this hypothesis, we conducted two experiments. In the first experiment, we tutored males so that they could produce a call similar to one shared by a group of experimental females. The experimental females were then presented with one of the tutored males and another, equally unfamiliar, male that had not been tutored. We found that the females spent a greater proportion of time in proximity of, and made more affiliative displays toward, the tutored males. In the second experiment, seven males received small bilateral brain lesions that disrupt vocal learning. These males and an equal number of control males were then released into an aviary containing females and reproductive resources. We found that lesioned and control males were equally successful in obtaining social mates, but females mated to lesioned males were more likely to engage in extra‐pair activities. These experiments indicate that a male's ability to imitate a female's call can influence the sexual behavior of the female even though lack of imitation ability does not appear to influence social pairing. We hypothesize that mate choice in budgerigars has multiple stages. Upon meeting a strange male, a female quickly assesses its ability for social acquisition of calls by the presence or absence of a call type similar to its own in its repertoire. As courtship proceeds into pair formation, the female assesses the ability of male to learn more directly by the extent of the male's perfection of imitation.     

Assortative Pairing Based on Contact Call Similarity in Budgerigars, Melopsittacus undulatus

A trait typical of parrots, but rare in other groups of birds, is the acquisition of new learned calls (acquired by copying conspecifics) throughout an individual's lifetime. The significance of this distinctive psittacid trait is not understood. In budgerigars, females preferentially affiliate with unfamiliar males whose contact calls resemble their own during brief dyadic choice trials; also, in forced‐pair situations, contact call similarity of members of pairs increases as a result of a male tendency to imitate his mate's call type. The functions of budgerigar call imitation and preference for pre‐pairing similarity are currently unknown. Moreover, as budgerigar pair formation occurs over a span of days or weeks, it is important to determine whether birds in breeding colonies assort and proceed to breed on the basis of pre‐pairing contact call similarity, and whether high levels of call similarity are maintained after pair formation is complete. To explore these questions, we recorded contact calls of male and female budgerigars before and after they were placed into an aviary equipped for breeding. As predicted, birds paired assortatively based on pre‐pairing call similarity. Once birds had paired, their calls converged further in acoustic structure, as previous work had led us to expect. However, after eggs were laid and the males began to feed their mates, the calls of mated birds diverged, suggesting that there might be some cost to maintenance of shared calls. Male care‐giving correlated with the degree to which his pre‐pairing calls resembled those of his mate, but not with the similarity achieved through convergence. These results suggest that female budgerigars may use a male's pre‐pairing call similarity as a predictor of paternal investment. The questions of why such similarity predicts male care‐giving, and why calls converge following initial pairing activities, require further work.     

Survival costs of within‐ and between‐season mate change in the European blackbird Turdus merula

Many studies of socially monogamous birds discuss the adaptive role of between‐season partner change, but only a handful of them refer to the benefits of pair fidelity in terms of increased survival. Moreover, there are no studies describing the benefits of within‐season mate retention. Our data relating to an urban population of European blackbirds Turdus merula enabled us to test the dependence of survival on pair faithfulness. Because blackbirds divorce within and between seasons, we were able to test the influence of pair faithfulness on their within‐ and between‐season survival and mate fidelity. For this purpose, we used a multievent capture–mark–recapture (MECMR) statistical model, which is based on recapture rates and different pair states (faithful to mate, paired with new partner, or dead). Our study indicated that between‐ and within‐season survival depends on pair states: pair‐bond duration increases survival to the next capture occasion in both sexes. We found that the pair‐bond duration to the current partner increased the chances of being with the same partner during the next breeding occasion, although we failed to find any within‐season pair‐bond influence for females. Our results showed sex differences in mating at the end of the season: females had a much smaller chance of breeding with the current new partner in the next year. This study has demonstrated that within‐ and between‐season survival is dependent on mate retention, and we discuss this in the context of how searching for a new partner could affect the birds’ survival.     

Breeding site fidelity and natal philopatry in the Redshank Tringa totanus

This paper presents the results of a study carried out on breeding Redshank in the Ribble Marshes, Lancashire, England.
Redshank tend to return to the same breeding area year after year. There was no detectable sex bias in return rates. Experienced birds were more site faithful than inexperienced birds, with previously successful birds exhibiting the highest degree of breeding site fidelity. Older, more experienced birds were more successful at hatching eggs than inexperienced birds.
Breeding dispersal was the same both within and between years. Faithful pairs and males nesting with a new mate dispersed significantly shorter distances than females nesting with a new partner. Dispersal distances in female Redshank were affected by breeding success: unsuccessful females, nesting with a new mate, dispersed significantly farther than successful females. A pair's breeding success influenced the following year's mate fidelity. However, other factors such as overwintering survival and date of return may also have influenced mate fidelity.
Redshank were highly faithful to their natal area; a high proportion of birds that survived post-fledging mortality returned to breed in their area of birth. No sex bias in natal dispersal was detected. Approximately 50% of Redshank breed in their first year of life.     

Female ground tits prefer relatives as extra‐pair partners: driven by kin‐selection?

Socially monogamous female birds routinely mate with males outside the pair bond. Three alternative hypotheses consider genetic benefits as the major driver behind the female strategy. The inbreeding avoidance hypothesis predicts that females paired with closely related males should seek copulations with distantly related extra-pair partners to avoid fitness loss from inbreeding depression; the outbreeding avoidance hypothesis predicts the opposite; the kin-selection hypothesis suggests that regardless of social mate relatedness, females should give related males extra-pair fertilization opportunities to gain inclusive fitness if the costs from inbreeding are minor. We test these hypotheses with a facultative cooperative breeder, the ground tit (Parus humilis). Social pairs of ground tits formed randomly with respect to genetic relatedness. In both bi-parental and cooperative groups, a female's engaging in extra-pair mating was independent of relatedness to her social mate; however, females preferred extra-pair sires to which they were more related than to their social mates. Moreover, females had higher relatedness with either their extra-group extra-pair sires in both bi-parental and cooperative groups, or within-group helper sires in cooperative groups, than expected by chance. When more than one potential extra-pair partner was available around a female's nest, she tended to select a relative. There was no indication of fitness reduction from extra-pair mating, which occurred at an intermediate level of inbreeding. These data support the kin-selection hypothesis, although there might be alternative nongenetic reasons associated with the extra-pair mating preference. Our finding offers a new explanation for why female birds pursue extra-pair mating. It also may broaden our understanding of the role of kin-selection in the evolution of cooperative society.