Breeding synchrony and paternity in the barn swallow (Hirundo rustica)

The barn swallow (Hirundo rustica) is a socially monogamous passerine which usually breeds in colonies where extra-pair copulations are frequent. Males intensively guard their mates during the female fertile period. Since males are more likely to be available for extra-pair copulations when their mate is not fertile, synchrony in timing of breeding may affect paternity of individual males. In this study, we analysed the change in mate-guarding rate by males in relation to the fertility condition of the female, and the relationships between breeding synchrony and density with paternity in first broods of 52 male barn swallows. Paternity (proportion of nestlings fathered in own brood) was assessed by typing of three highly polymorphic microsatellite loci. Mate guarding by males peaked during the fertile period of their mates. Paternity increased as breeding synchrony in the colony increased. Paternity of barn swallows is positively associated with the degree of exaggeration of male tail ornaments. The relationship between male ornamentation and paternity was partly mediated by an effect of ornament size on breeding synchrony. We suggest that females might delay breeding with low-quality males to enhance their opportunities for being fertilised by high-quality extra-pair males. Received: 16 January 1998 / Accepted after revision: 25 October 1998     

Do male plumage and song characteristics influence female off-territory forays and paternity in the hooded warbler?

Extra-pair fertilizations are common in many socially monogamous species, and paternity studies have indicated that females may use male vocal performance and plumage ornaments as cues to assess male quality. Female off-territory forays may represent a key component of female choice and male extra-pair mating success, and female foray behaviour is expected to be strongly influenced by indictors of male quality. In this study, we examined how male song and ornamentation affect how often females left their territories, which males they visited and extra-pair paternity in a socially monogamous passerine, the hooded warbler (Wilsonia citrina). We radiotracked 17 females during the fertile period and quantified male vocal performance (song output and rate) and plumage characteristics (size of the black melanin hood and colour of the black hood, yellow cheeks and breast areas). We obtained blood samples and determined paternity at 35 nests including those of 14 females that we radiotracked. Eleven (65%) of the 17 females forayed off-territory, whilst fertile and female foray rate was positively correlated with the number of extra-pair young in the nest. Females that left their territories more frequently were paired with males that sang at a low rate. In addition, extra-pair mates had higher song rates than the social mates they cuckolded (5.3 songs/min vs. 4.4 songs/min). Female off-territory forays or extra-pair paternity were not significantly related to male plumage characteristics. Our results indicate that a high song rate influences both the foray behaviour of a male’s social mate and the likelihood that he will sire extra-pair offspring with neighbouring females.     

Dalliances and doubtful dads: what determines extra-pair paternity in socially monogamous wandering albatrosses?

Genetic techniques have revealed surprisingly high rates of extra-pair paternity (EPP) in socially monogamous albatrosses. We sought to establish social and genetic influences on EPP in wandering albatrosses (Diomedea exulans) at Marion Island, where EPP rates were 14–24?% in three successive seasons. EPP probably resulted from both female solicited extra-pair behaviours and male forced copulations. EPP was not linked to breeding experience nor with poor reproductive performance, despite a tendency for pairs to consistently produce either EPP or within-pair paternity (WPP) chicks. Mate guarding may inhibit extra-pair behaviour; however, parental arrival date and presence in the colony prior to laying did not correlate with EPP. There was little support for genetic advantages to producing EPP chicks, but the population is characterised by low genetic variability, which may result in mate incompatibility. Mates of pairs that failed and pairs producing EPP young tended to be more similar genetically to their partners than mates producing WPP young, suggesting that EPP may counter mate incompatibility. EPP and WPP chicks grow equally well, so cuckolded males did not reduce investment in EPP chicks. The lack of discriminatory behaviour by cuckolded males together with low genetic diversity in the population may allow continued high levels of EPP. In albatrosses, pair bonds are typically long lasting and the costs of forming new pairings may discourage mate swapping. Females may undertake extra-pair copulations as an adaptive alternative to mate swapping because the costs of extra-pair behaviour are small.     

Sex allocation and paternity in a cooperatively breeding passerine: evidence for the male attractiveness hypothesis?

Sex allocation theory predicts that female birds with high-quality mates will benefit from producing more sons, since sons will inherit their father’s superior traits and enjoy a great reproductive success, whereas females with low-quality mates will benefit from producing more daughters, since the variance in reproductive success among daughters is typically lower. The male attractiveness hypothesis may apply to extra-pair paternity (EPP) because socially monogamous females routinely mate with higher quality males outside the pair bond. We test these predictions using the Tibetan ground tit (Pseudopodoces humilis), a sexually monomorphic, socially monogamous, facultatively cooperative breeder. There was greater variation in actual reproductive success among males than females due to EPP. An excess of sons was detected for bi-parental (i.e., non-cooperative) broods wherein EPP was mainly sired by bi-parental males. The pattern was attributed to male-biased sex ratios produced for both EPP and within-pair offspring within the same broods. The reason for the latter case might be a random allocation of more offspring to sons by the potentially EPP-exposed females that have an inability to control fertilization by specific males. In cooperative broods where EPP mostly resulted from within-group helpers of presumed low-quality, as indicated by their failure in acquiring a social mate, there was a non-significant tendency for EPP offspring to be daughters and for within-pair offspring in the same broods to be unbiased. These results support the EPP-related male attractiveness hypothesis especially in terms of the overproduction of sons. Offspring produced through quasi-parasitism was unbiased towards either sex, suggesting a weak female choiceness with respect to the quality of host males.     

Social and ecological factors affecting paternity allocation in American robins with overlapping broods

Factors that affect extra-pair mating in birds are likely to vary across the breeding season. Changing densities of active nests may alter the opportunities for extra-pair mating, and parental duties may alter a male’s opportunity to guard his mate from extra-pair mating. The latter affects species with multiple broods, where males care for fledglings from first nests while females initiate second nests. We studied a population of multi-brooded American robins (Turdus migratorius) to assess how seasonal changes in nesting density and changes in mate-guarding opportunity influenced paternity patterns over successive breeding attempts. Extra-pair paternity (EPP) occurred in 71.9% of broods and accounted for 48.1% of young. High nesting densities in the study population may explain the high overall rate of EPP, but seasonal variation in breeding density did not explain patterns of EPP among nests. Contrary to the predictions of the mate-guarding hypothesis, EPP did not increase in the second nests that followed successful first nests, and the percentage of extra-pair young in second nests did not decline as the overlap between successive nests increased. The fact that EPP was actually lower when the interval between clutches was shorter suggests that the sooner the males can assume sole care of first broods and allow their mates to renest (indicative of superior paternal quality), the more paternity they realize in the next nest. These results suggest that mate-guarding opportunity does not influence paternity in this population of American robins and that female robins may allocate paternity based on their assessment of male parental performance at first nests.     

Extra-pair paternity and tail ornamentation in the barn swallow Hirundo rustica

Females of socially monogamous species may copulate with attractive non-mates to obtain access to the genes of such males, and a preference for attractive copulation partners may result in sexual selection. Extra-pair copulations are common in the socially monogamous barn swallow Hirundorustica, and a 2-year study of paternity using multi-locus DNA fingerprinting demonstrated that 33% of 63 broods and 28% of 261 offspring were sired by extra-pair males. The frequency of extra-pair offspring within broods was highly skewed with the majority of all broods having either no extra-pair offspring or only extra-pair offspring. Individual pairs were consistent in their frequency of extra-pair paternity among broods, and the repeatability of extra-pair paternity of multiple broods of the same female was statistically significant. The proportion of extra-pair offspring was negatively related to the tail length of the male attending the nest. Behavioural observations showed that extra-pair fertilizations were more likely in broods raised by females that had been observed to engage in extra-pair copulations. The frequency of extra-pair offspring was unrelated to the intensity of two male paternity guards, mate guarding and the rate of intra-pair copulations. In an analysis of extra-pair paternity and male parental care in different broods of the same male, male barn swallows fed their offspring relatively less frequently if the brood contained more extra-pair offspring. Therefore, female barn swallows pursue extra-pair copulations with attractive males, which may result in sexual selection, even though extra-pair paternity is costly for females due to the reduction of paternal care by their social mates. Received: 24 January 1997 / Accepted after revision: 2 August 1997     

Extra-pair paternity in the monogamous alpine marmot (Marmota marmota): the roles of social setting and female mate choice

Extra-pair paternity (EPP) can be influenced by both social setting and female mate choice. If evidence suggests that females try to obtain extra-pair copulations (EPCs) in order to gain genetic benefits when mated to a homozygous and/or to a related male, females may not be able to choose freely among extra-pair mates (EPMs) as the social mate may constrain female access to EPMs. In this study, we investigated, first, how EPP depended on social setting and specifically on the number of subordinate males in the family group in a highly social and monogamous mammal, the alpine marmot. Second, we investigated how EPP depended on female mate choice for genetic benefits measured as male mate-heterozygosity and within-pair relatedness. Our results reveal, first, that EPP depended on the social setting, increasing with the number of subordinate males. Second, EPPs were related to relatedness between mates. Third, EPMs were found to be more heterozygous than within-pair males. Thus, social setting may constrain female choice by limiting opportunities for EPC. However, after accounting for social confounding factors, female choice for genetic benefits may be a mechanism driving EPP in monogamous species.     

Yolk androgens in great tit eggs are related to male attractiveness,breeding density and territory quality

Females can adaptively adjust phenotype of their offspring via deposition of various compounds into eggs, including androgens and other hormones. Here, I investigated how egg yolk androgens (testosterone and androstenedione) related to environmental conditions and parental traits in the great tit (Parus major) across three breeding seasons. Male and female traits studied included age, condition and multiple feather ornaments, both carotenoid- and melanin-based (carotenoid and UV chroma of yellow breast feathers, area of black breast band and white cheek immaculateness). Yolk mass increased with laying temperature, laying date and area of male black breast band. Concentration of androgens increased with breeding density, territory quality and carotenoid chroma of male yellow breast feathers and was higher in mates of 1 year old as compared to older males. Yolk androgens were not related to any of the female traits analysed. These patterns were thus consistent with (1) social and environmental effects on yolk mass and composition and (2) both positive and negative differential allocation strategies of resource allocation in females. Overall, male traits were the most important predictors of egg yolk characteristics in this socially monogamous songbird.     

Interactive effects of male and female age on extra-pair paternity in a socially monogamous seabird

Females sometimes obtain older sires for their offspring through extra-pair interactions, but how female age influences paternity is largely unexplored and interactive effects across the age span of both sexes have not been analyzed. To test whether female choice of sire age varies with female age in the blue-footed booby (Sula nebouxii), we examined associations between ages of both partners and the probability of extra-pair paternity (EPP) in 350 broods of parents up to 22 years old in a single breeding season. Extra-pair paternity enables a female to select an alternative sire for her offspring and could function to avoid or achieve particular combinations of parental ages. A male age?×?female age interaction revealed that in young females (≤4 years), EPP decreased with increasing age of the social partner, whereas in old females (≥8 years), it increased. Moreover, sires of extra-pair (EP) chicks of young females paired to young males were on average 6.33 years older than the females’ social partners. Since female boobies control copulatory access, this pattern could imply that young females choose old sires for their proven genetic quality and that old females avoid very old males because matings with them may risk infertility or genetic defects in offspring. Taking female age into account and observing across the whole age span may be necessary for understanding female age-based mate choice.     

Multiple paternity and offspring quality in tree swallows

There is mounting evidence in a variety of taxa that females increase offspring quality by mating with multiple males, often resulting in multiple paternity. In birds, however, few studies have explicitly examined the benefits of mating with several different males; instead, the focus has been on whether or not extra-pair mating occurs, and its adaptive significance remains controversial. We examined the hypothesis that offspring quality, particularly immune response (phytohaemagglutinin assay) and growth, increases with the number of sires in broods of socially monogamous tree swallows (Tachycineta bicolor). We found one of the highest known levels of multiple paternity in birds (84% of nests with two or more extra-pair young had at least two extra-pair sires). Among nests with extra-pair young, the number and diversity of sires continued to increase linearly with the number of extra-pair young, so there was no evidence that some males monopolized paternity at high levels of extra-pair fertilization. Indeed, the number of sires was actually greater than expected in large broods, suggesting that some females might be seeking more mates. We found no effect of the number of sires on nestling immune response or growth. In mixed paternity broods, the immune response of extra-pair young did not differ from that of their within-pair half-siblings. However, among all broods, nestlings had a stronger immune response in nests with at least one extra-pair nestling than in nests with all within-pair nestlings. These results are not consistent with a good genes benefit of extra-pair mating, but they do suggest that there are environmental effects associated with extra-pair mating that increase nestling immune response. These environmental effects could produce indirect genetic effects on sexual selection if they are heritable. The extraordinarily high number of sires in this species highlights a relatively unexplored source of sexual selection in birds.     

Sexual selection in a socially monogamous bird: male color predicts paternity success in the mountain bluebird, <Emphasis Type="Italic">Sialia currucoides</Emphasis>

Ornamental traits are thought to evolve because they give individuals an advantage in securing multiple mates. Thus, the presence of ornamentation among males in many monogamous bird species presents something of a conundrum. Under certain conditions, extra-pair paternity can increase the variance in reproductive success among males, thus increasing the potential for sexual selection to act. We addressed this possibility in the mountain bluebird (Sialia currucoides), a socially monogamous songbird in which males possess brilliant ultraviolet (UV)-blue plumage. Specifically, we asked whether a male’s success at siring offspring within his own nest and within the nests of other males was related to his coloration. In pairwise comparisons, males that sired extra-pair offspring were not more colorful than the males that they cuckolded. However, males that sired at least one extra-pair offspring were, on average, brighter and more UV-blue than males that did not sire extra-pair offspring. Brighter, more UV-blue males sired more offspring both with their own mate and tended to sire more offspring with extra-pair mates and thus sired more offspring overall. Our results support the hypothesis that the brilliant UV-blue ornamental plumage of male mountain bluebirds evolved at least in part because it provides males with an advantage in fertilizing the eggs of multiple females.     

No evidence for acoustic mate-guarding in duetting buff-breasted wrens (Thryothorus leucotis)

There are few empirical tests of the acoustic mate-guarding hypothesis for the function of duetting in birds. This hypothesis states that when females are fertile, males initiate many songs or answer most of their mates solo songs to form duets and repel rival males seeking extra-pair copulations. We tested the hypothesis by comparing song initiation and answer rates of males and females in socially monogamous buff-breasted wrens (Throthorus leucotis) during pre-fertile and fertile periods. During pre-fertile periods, males often sang for short periods before being answered by their mates, yet first duets were formed earlier relative to dawn and more duets were given during the dawn chorus on pre-fertile than fertile mornings. Males initiated more songs during pre-fertile than fertile periods, whereas there was no difference between stages in female song initiation rates. The proportion of songs answered by individuals of both sexes did not differ between breeding stages. Other mate-guarding behaviours, such as frequent copulation and maintaining close proximity to mates when fertile, did not appear to be important in this species, as no copulations were observed and there was no difference in the time pairs spent in close proximity when females were fertile or not. Parentage analysis revealed that only 3% of 31 broods had young that were likely the result of extra-pair paternity. These findings do not support the acoustic mate-guarding hypothesis, and suggest that the low rate of extra-pair paternity in buff-breasted wrens was maintained without the use of acoustic or traditional paternity guards.Communicated by R. Gibson     

Different tactics,one goal: initial reproductive investments of males and females in a small Arctic seabird

Despite a great number of studies on extra-pair paternity in birds, the actual roles of males and females in extra-pair contacts is poorly understood, as detailed behavioural studies comparing the reproductive performance of the two sexes prior to egg laying are relatively scarce. Here, we investigated mating behaviour (copulations and aggressive interactions), time budget and body condition (size-adjusted body mass and baseline corticosterone level) in the little auk (Alle alle), a monogamous and highly colonial, Arctic seabird. We performed the study in a large breeding colony of the little auk in Hornsund (Spitsbergen). We found that the males frequently attempted extra-pair copulations (EPCs), although these contacts were almost always unsuccessful, mostly because of the females’ rejection behaviour. These results clearly indicate that genetic monogamy is maintained through female control. Nevertheless, males tried to protect their paternity by staying in close proximity to their females and aggressively intervening when their mates became involved in EPCs. Compared to females, males also spent more time in the colony guarding nest sites. Despite the apparent sex differences in the time budget and frequency of aggressive interactions, body condition was similar in the two sexes, indicating comparable parental investments during the mating period.     

Positive association between social and extra-pair mating in a polygynous songbird, the dickcissel (Spiza americana)

In polygynous species, males appear to gain additional offspring by pairing with multiple females simultaneously. However, this may not be true if some females copulate outside of the social pair bond. Polygynous males could experience lower paternity because of trade-offs among gaining multiple social mates, guarding fertility with these mates, and pursuing extra-pair matings. Alternatively, polygynous males could simultaneously gain extra social mates and have high paternity, either because of female preferences or because of male competitive attributes. We tested four predictions stemming from these hypotheses in a facultatively polygynous songbird, the dickcissel (Spiza americana). Unlike most previous studies, we found that males with higher social mating success (harem size) also tended to have higher within-pair paternity and that the number of extra-pair young a male sired increased significantly with his social mating success. Females that paired with mated males were not more likely to produce extra-pair young. In contrast, extra-pair paternity was significantly lower in the nests of females whose nesting activity overlapped that of another female on the same territory. This pattern of mating was robust to differences in breeding density. Indeed, breeding density had no effect on either extra-pair mating or on the association between polygyny and paternity. Finally, nest survival increased with harem size. This result, combined with the positive association between polygyny and paternity, contributed to significantly higher realized reproductive success by polygynous male dickcissels.     

Condition-dependent control of paternity by female purple martins: implications for coloniality

Proposed causal links between extra-pair copulation (EPC) and colony formation in socially monogamous birds hinge on the question of which sex controls fertilizations. We examined in colonial purple martins Progne subis (1) whether EPCs were forced or accepted by females, and (2) the degree to which apparently receptive females were able to obtain EPCs against their mates’ paternity defenses. Paternity analyses of multilocus DNA fingerprinting confirmed previous findings of a marked relationship between age class and extra-pair fertilizations (EPFs), with young males losing paternity of 43% (n = 53) of their putative offspring compared to 4% (n = 85) by old males. All assignable extra-pair offspring were sired by old males, with one male obtaining most EPFs each year. Contrary to the hypothesis that EPCs are forced, EPF frequency within age class did not increase with seasonal increases in the number of males per fertile female. Whereas the male control hypothesis predicted that the male age class that mate-guarded more would be cuckolded less, the reverse was true: young males guarded significantly more intensely. The male age class difference in cuckoldry could not be explained by the possibility that young and inexperienced females (which are usually paired to young males) were more vulnerable to forced copulation because EPFs were unrelated to female age. These findings suggest that females (1) pair with old males and avoid EPCs, or (2) pursue a mixed mating strategy of pairing with young males and accepting EPCs from old males. The receptivity to EPCs by females paired to young males put them in conflict with their mates. Two factors determined the paternity achieved by young males: (1) the relative size of the male to the female, with young males achieving much higher paternity when they were larger than their mates, and (2) the intensity of mate-guarding. Both variables together explained 77% of the variance in paternity and are each aspects of male-female conflict. Given female receptivity to EPCs, mate-guarding can be viewed as male interference with female mating strategies. We conclude that EPCs are rarely or never forced, but the opportunity for females paired to young males to obtain EPCs is relative to the ability of their mates to prevent them from encountering other males. Evidence of mixed mating strategies by females, combined with other features of the martin mating system, is consistent with the female-driven “hidden lek hypothesis” of colony formation which predicts that males are drawn to colonies when females seek extra-pair copulations. Received: 23 March 1995/Accepted after revision: 14 January 1996     

Mating opportunities,paternity, and sexual conflict: paternal care in northern and southern temperate house wrens

Males are generally predicted to care less for their young when they have more additional mating opportunities, lower paternity, or when their mates care more. We tested these predictions using male provisioning as a proxy for paternal care in two temperate populations of house wrens (Troglodytes aedon) with divergent life histories. Males in the migratory, occasionally socially polygynous New York, USA (northern) population provisioned less when more local females were fertile. A similar relationship was only weakly supported in the resident, socially monogamous Buenos Aires Province, Argentina (southern) population, possibly due to the higher density of house wrens there. A relationship between male provisioning and level of paternity within the brood was supported in both populations, but in opposite directions: while males in the southern population provisioned less at broods containing more extra-pair young, males in the northern population provisioned such broods more, contradicting predictions. Males provisioned less when their mates provisioned more in both populations, in agreement with sexual conflict theory. Additionally, the populations both exhibited a positive relationship between male provisioning and nestling age, but differed in the direction of the relationships of male provisioning with date and brood size. Our results suggest that even within a species, life history differences may be accompanied by differences in the determinants of behavior such as paternal care.     

Extra-pair paternity and the opportunity for sexual selection in a socially monogamous bird (Dendroica petechia)

We used multi-locus DNA fingerprinting to characterise the genetic mating system of the socially monogamous yellow warbler (Dendroica petechia). Over 2 years there were no instances of brood parasitism, but 59% of families (n = 90) contained extrapair sired young and 37% of offspring (n = 355) were of extra-pair paternity. Most hypotheses for extra-pair mating in monogamous species assume a paternity benefit to extra-pair sires, and focus on the benefit(s) to females. However, the assumption of male benefit has been little tested. Among yellow warblers, known extra-pair sires were just as likely to be cuckolded as any male in the population, and there was at least one reciprocal exchange of extra-pair paternity. Nevertheless, among known extra-pair sires, the paternity gains from extra-pair paternity were, on average, greater than the losses in their own families. These results show there is a paternity benefit to certain males. However, the benefit is not absolute but relative and therefore more difficult to measure. The results also suggest that patterns of extra-pair fertilisation are not determined by female choice alone. Most confirmed extra-pair mates were territorial neighbours, but some resided as far as three territories apart, and greater spatial separation was implied in other cases. Thus, the opportunity for extra-pair mating is great. We estimate that as a result of extra-pair fertilisations, variance in male mating success is increased somewhere between 3-fold and 15-fold over that which would result from within-pair reproduction alone. These findings affirm the potential importance of extra-pair reproduction for sexual selection in monogamous species and they support earlier suggestions that extra-territorial forays by male yellow warblers are for the purpose of extra-pair mating.     

Safe betting: males help dull females only when they raise high-quality offspring

In evolutionary biology, whether parents should enhance or reduce parental care according to mate ornamentation is a subject of great debate. However, the evolution of female ornaments can shed light on this question. In theory, female ornamentation should be traded off against fecundity and thus cannot be wholly informative to males without a direct indication of fecundity. Hence, direct cues of offspring quality should affect the relationship between male investment and female ornamentation. Under this hypothesis, we manipulated two direct cues of offspring quality (egg size and color) after first egg laying in the blue-footed booby and registered male incubation patterns. In this species, foot color is a dynamic signal of current condition and in females is traded off with egg size. We found that males spent more time incubating when paired with dull females but only in nests with large eggs. Males also spent less time incubating small dull eggs. Results indicate that egg size, a direct cue of reproductive value, affected the relationship between male effort and female ornamentation. Males may be willing to help females that have invested in offspring at the expense of ornamentation, which suggests compensation when females are in low condition. Another possibility is that males relax their effort when paired with highly ornamented and fecund females because they have high parenting abilities. Our findings suggest that the information conveyed by female ornaments may depend on direct cues of fecundity. Results also highlight that parental decisions are complex, modulated by a combination of information sources.     

Polygyny and extra-pair paternity enhance the opportunity for sexual selection in blue tits

Polygyny and extra-pair paternity are generally thought to enhance sexual selection. However, the extent to which these phenomena increase variance in male reproductive success will depend on the covariance between success at these two strategies. We analysed these patterns over four breeding seasons in facultatively polygynous blue tits Cyanistes caeruleus. We found that both polygyny and extra-pair paternity increased variance in male reproductive success and that standardised variance in annual number of genetic fledglings was 2.6 times higher than standardised variance in apparent success when assuming strict monogamy. Nevertheless, male success at securing within-pair paternity was unrelated to success at gaining extra-pair paternity and, when considering the positive effect of age on extra-pair success and attracting a second female, polygynous males were no more likely to sire extra-pair fledglings. Overall, polygynous males fledged more genetic offspring than monogamous males, but first-year polygynous males lost a greater share of within-pair paternity. A literature review suggests that this adverse effect of polygyny on within-pair paternity is frequent among birds, inconsistent with the prediction that females engage in extra-pair copulation with successful males to obtain good genes. Furthermore, a male's share of paternity was repeatable between years, and among females of polygynous males within years, such that a compatibility function of extra-pair copulations was likewise unsupported. Instead, we suggest that the observed patterns are most consistent with a fertility insurance role for extra-pair copulations, which does not exclude the greater opportunity for sexual selection through differential ability of males to gain paternity.     

Extra-pair paternity in tree swallows: why do females mate with more than one male?

Recent studies of monogamous tree swallows (Tachycineta bicolor) suggest that females may receive some type of genetic benefit from extra-pair fertilizations. In this study we attempted to determine what type of genetic benefits might be gained by females. We compared numerous morphological and behavioral traits (Table 1) of every male nesting on one grid of nest-boxes (n = 23) to determine what male traits were correlated with male success at gaining extra-pair fertilizations. DNA fingerprinting revealed an increase in the level of extra-pair paternity from the previous year (50% of broods contained extra-pair young in 1990 vs. 87% of broods in 1991), but no significant correlates of paternity. We found six extra-pair fathers at seven nests (20 nests had extra-pair young). The traits of these extra-pair males did not differ from those of the males they cuckolded. We discuss several reasons for this lack of difference, but argue that our results are not inconsistent with females choosing extra-pair males to enhance the genotypic quality of their offspring. Despite a complete search of the nest-box grid for extra-pair fathers, we were able to explain the paternity of just 21% (13/63) of all extra-pair young. This suggests that extra-pair fathers were either residents off our study grid or non-territorial floaters. Tree swallows are quite mobile and spend only part of the day at their nest prior to laying. In addition, we rarely see swallows visiting other grids of nest-boxes. Therefore, we suggest that most extra-pair copulations occur at some unknown location, possibly at a feeding or roosting area where females may be able to choose from many more potential extra-pair fathers than at their nest-site.