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     

Enhancement of reproductive success through mate choice in a social rock-wallaby, Petrogale assimilis (Macropodidae) as revealed by microsatellite markers

The reason why a female who is socially paired to one particular male seeks extra-pair copulations (EPCs) with others has important implications in life history models and to the study of behaviour. The Allied rock-wallaby, Petrogale assimilis, lives in spatially isolated colonies in tropical north Queensland, Australia. Extensive observations of a colony at Black Rock showed that intense behavioural bonding occurs between pairs of adult males and females; about two-thirds of males paired with one female, the remainder paired with two females simultaneously. Single-locus microsatellite profiling determined the paternity of 63 offspring from 21 females for which long-term behavioural data were available. One-third of the young were fathered by males which were not paired socially with the mother. The mating system was heterogeneous: (1) all offspring of 11 females were fathered by the mother's partner, (2) all young of 5 females were fathered by extra-pair males, and (3) only some of the young of 5 females were fathered by their regular consort. Analysis of individual longitudinal demographic records showed that females whose young were always fathered by their consort had higher reproductive success than those whose young were always fathered as a result of (EPCs). However, females with some offspring fathered by their regular consort and others via EPCs had the highest probability of raising young to independence. These females were significantly more likely to have an offspring fathered as a result of an EPC if their previous young had failed to survive to pouch emergence. These results are consistent with the hypothesis that females choose mates for their genetic quality. Comparison of the males with which these females sought EPCs and the regular consorts suggested that arm length rather than body weight or testes size was used as the index of genetic quality. Results from a second colony of rock-wallabies in which the reproductive rate was accelerated were also consistent with the genetic-quality hypothesis. These results imply that by choosing better-quality fathers irrespective of social pairing, females are able to maximise their overall lifetime reproductive success, and presumably, those of their offspring. Received: 8 June 1997 / Accepted after revision: 28 February 1998     

Small testes size despite high extra-pair paternity in the pair-living nocturnal primate Phaner furcifer

Sperm competition will be the inevitable consequence of polyandrous mating behavior if two or more males inseminate a single female. It has been demonstrated for a wide variety of animals that males adapt to this situation behaviorally, physiologically and morphologically, e.g. by evolving relatively large testes size to produce more sperm. All pair-living primates investigated so far were found to have relatively small testes, suggesting a monandrous mating system. We investigated the relationship between extra-pair paternity (EPP) rate as a measure of sperm competition intensity and relative testes size in a pair-living primate, the fork-marked lemur (Phaner furcifer). Paternity exclusion analyses for seven offspring using six polymorphic DNA-microsatellite markers suggested a high EPP rate. Female nocturnal travel distances were longer during the mating season, suggesting that females take an active role in achieving extra-pair copulations (EPCs). Surprisingly, fork-marked lemur testes size was relatively small compared to 23 other lemuroid primates, a result that is in contrast to predictions of sperm competition theory. Neither possible behavioral and morphological adaptations to an alternative paternity guard (i.e. mate guarding), nor sampling biases, phylogenetic constraints, and population density effects explain the absence of large testes in a species with high EPP, a phenomenon also known from birds with moderate to low EPP rates. We conclude that more data are needed on the frequency of EPCs, the timing of in-pair and extra-pair copulations, as well as the role of female choice, to explain why males of some species apparently do not adapt to sperm competition.Communicated by S. Alberts     

Occurrence of extra-pair copulation in the tree swallow (Tachycineta bicolor)

Summary A new method to mark sperm transfer events between birds, using microspheres inserted into males' cloacae, was employed to assess the frequency of extra-pair copulation (EPC) in a population of tree swallows (Tachycineta bicolor) in Ontario, Canada, during the summer of 1988. We inserted 25 males with microspheres; spheres from 8 (32%) of these males were found in extra-pair females, from which we infer that males pursue a mixed reproductive strategy of monogamy coupled with seeking EPCs. The cloacae of 10 of 41 (24.4%) females were found to contain microspheres from non-mates. EPCs occur more frequently between neighboring birds than between non-neighbors. Only two within-pair copulations were detected with this method, suggesting that it underestimates the occurrence of all copulations, extra-pair or within-pair. Copulation watches revealed that within-pair copulations occur very frequently. We propose that frequent pair copulations are used by males to ensure their paternity in their mate's offspring. Offprint requests to: R.J. Robertson     

Extra-pair copulations in a lek: the secondary mating system of monogamous razorbills

Summary I studied the mating strategies of razorbills Alca torda, a monogamous colonial seabird, during 1987–1989 in Wales, U.K. The outstanding feature of the study population's mating system was the performance of most copulations in mating arenas outside the colony. Females visited the arenas and sometimes accepted extra-pair copulations (EPCs), and males aggressively competed for EPCs. During the fertilizable period of most females, the arenas resembled leks because they contained no resources except mates, and males contributed only sperm to extra-pair females. Typical of leks, the operational sex ratio was strongly male-biased, and male mating success was highly skewed. To examine the potential for sexual selection to operate in this system, I correlated male EPC success with variables that could be associated with male-male competition and/or female choice. The frequencies of three behaviors (interference of copulation attempts, fights won against other males, and EPC attempts), accounted for 62% of the variance in EPC success and were strongly intercorrelated, with interference explaining most of the variance in a multivariate test. EPC success was not correlated with body size, age, paired status or either of two estimates of the time spent in the arena. The hypothesis that male EPC success was determined by active female choice was tested by examining the relative success (EPCs per attempt) of males. Relative success was not correlated with any of the three behaviors associated with absolute success, or with age, body size or attendance, suggesting that male-male competition, mainly in the form of copulation interference, is the principal correlate of EPC success. These findings in a monogamous species illustrate that EPC is a secondary mating system of razorbills in which sexual selection operates. The existence of this system outside the colony raises the possibility that lekking for EPCs may also occur within the nesting territories of other monogamous, colonial species but is hidden by competition for nests and breeding partners.     

Breeding synchrony and extra-pair mating in red-winged blackbirds

Using data from a 6-year paternity study of red-winged blackbirds, I tested the hypotheses that increased nesting synchrony should either promote extra-pair mating by increasing the advantage of extra-pair mating to females, or decrease extra-pair mating by constraining males from seeking extra-pair copulations. Contrary to these hypotheses, the occurrence of extra-pair paternity did not vary with nesting synchrony over the breeding season, or vary with the number of synchronous nests within territories or within marshes, or with nesting order on territories. However, for nearly all nests with extra-pair young, there were fewer females synchronous with that nest on the cuckolder's territory than on the territory of the cuckolded male. This “advantage” of a synchrony difference was less pronounced for older males that cuckolded younger males, particularly when the two males were not neighbors. Collectively, these results suggest that breeding synchrony affects extra-pair mating by affecting mate guarding, but that breeding synchrony alone can not be used to predict which females are more likely to engage in extra-pair mating, nor with which extra-pair males they will mate. Understanding why extra-pair mating by older males is less affected by breeding synchrony may explain much about both the proximate and ultimate causes of extra-pair mating in red-winged blackbirds. Received: 7 June 1996 / Accepted after revision: 25 November 1996     

Female control of offspring paternity in a western population of red-winged blackbirds (Agelaius phoeniceus)

Extra-pair copulations, which occur when individuals that have formed social relationships to breed copulate outside their pairbond, now are recognized as an important component of reproductive success in many species. In situations where both males and females benefit from extra-pair copulations without incurring much risk, an inevitable conflict arises between pairbonded mates. In this study I investigated the conflict of interest between male and female reproductive strategies in a western population of red-winged blackbirds (Agelaius phoeniceus). Female red wings in this population initiate extra-pair copulations, which resulted in a 35% rate of extra-pair fertilization. Females initiated the majority (78%) of extra-pair copulations away from their nesting territory where pairbonded individuals typically copulate, and females that engaged in extra-pair copulations spent a significantly greater amount of time off the marsh during peak fertilization compared to females that did not. In addition, females that nested in areas with a large number of potential extra-pair partners produced significantly more extra-pair fertilized young compared to females that nested on marshes with few male neighbors. Males’ strategies to protect paternity were limited primarily to patrolling territory boundaries and to opportunistically preventing extra-pair copulations off the marsh when they were visible. In this population females appear to use behavioral means to control nestling paternity, which in turn directly affected their mate’s reproductive success, and males were restricted to using strategies that were largely ineffective at preventing the threat of extra-pair paternity. Received: 23 December 1994/Accepted after revision: 17 December 1995     

The pursuit of extra-pair copulations by monogamous female razorbills: how do females benefit?

Summary Monogamous female razorbills Alca torda actively sought extra-pair copulations (EPCs) in mating arenas outside their nesting colony. Females showed marked variation in promiscuity, defined as the number of EPCs they accepted (0–7), and in receptivity, defined as the percentage of EPC opportunities accepted (0%–100%). The opportunity of females to encounter males for EPCs was measured by (a) the time spent in the mating arena, and (b) the guarding effectiveness of their mates. Females appeared to encounter males and accept EPCs independently of opportunity, which suggests that variation in promiscuity and receptivity is caused by differences in the degree to which individual females might benefit from EPCs. I tested predictions of four hypotheses which propose benefits which females could obtain from EPCs: good genes, genetic diversity of offspring, insurance against infertility of mate, and acquisition of a new mate. No evidence was found to support two predictions made exclusively by the good genes hypothesis: (a) paired males did not achieve greater EPC success than unpaired males, and (b) males who were cuckolded did not obtain fewer EPCs than males whose mates avoided EPCs, suggesting that a male's attractiveness to other females does not ensure his own mate's fidelity. A prediction of both the insurance and good genes hypotheses was supported: females (who in this species retain their mates between years) showed similar receptivity to EPCs between years. This finding contradicts the genetic diversity hypothesis which predicts that in species that lay one egg per year, females who refuse EPCs in one year will accept them in the next. The good genes and genetic diversity hypotheses both predict that females who accept EPCs will behave as if to maximize their chances of being fertilized by extra-pair males. Results contradicted this prediction: females who accepted EPCs were more, rather than less receptive to their mates' copulation attempts than were females who did not accept EPCs. This finding is compatible with the insurance hypothesis which only requires females to store the sperm of extra-pair males, without necessarily being fertilized by it. Insurance against male infertility was the only hypothesis whose predictions were not contradicted by any of the results.     

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     

Sex-specific reproductive behaviours and paternity in free-ranging Barbary macaques (Macaca sylvanus)

In a wide variety of species, male reproductive success is determined by contest for access to females. Among multi-male primate groups, however, factors in addition to male competitive ability may also influence paternity outcome, although their exact nature and force is still largely unclear. Here, we have investigated in a group of free-ranging Barbary macaques whether paternity is determined on the pre- or postcopulatory level and how male competitive ability and female direct mate choice during the female fertile phase are related to male reproductive success. Behavioural observations were combined with faecal hormone analysis for timing of the fertile phase (13 cycles, 8 females) and genetic paternity analysis (n = 12). During the fertile phase, complete monopolisation of females did not occur. Females were consorted for only 49% of observation time, and all females had ejaculatory copulations with several males. Thus, in all cases, paternity was determined on the postcopulatory level. More than 80% of infants were sired by high-ranking males, and this reproductive skew was related to both, male competitive ability and female direct mate choice as high-ranking males spent more time in consort with females than low-ranking males, and females solicited copulations mainly from dominant males. As most ejaculatory copulations were female-initiated, female direct mate choice appeared to have the highest impact on male reproductive success. However, female preference was not directly translated into paternity, as fathers were not preferred over non-fathers in terms of solicitation, consortship and mating behaviour. Collectively, our data show that in the Barbary macaque, both sexes significantly influence male mating success, but that sperm of several males generally compete within the female reproductive tract and that therefore paternity is determined by mechanisms operating at the postcopulatory level.     

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.     

Female control of extra-pair fertilization in tree swallows

Summary In a Canadian population of tree swallows, Tachycineta bicolor, DNA fingerprinting has previously shown that half of all broods contain many offspring resulting from extra-pair copulations (EPCs), whereas the other half contain only legitimate offspring. This bimodal pattern of extra-pair paternity might be due to variation in the effectiveness of male paternity guards, variation in female ability to resist EPCs, and/or variation in female pursuit of EPCs. Here we report experimental evidence for female control of copulations and fertilizations and the occurrence of two alternative copulation strategies among females in this population. Ten paired male tree swallows were removed on the day their mates laid the first egg. Replacement males took over the nestbox within 0.5–23 h and attempted to copulate with the widowed female. Assuming that eggs were fertilized approximately 24 h prior to laying, the first two eggs were fertilized before the male was removed, while the third and subsequent eggs could potentially be fertilized by the replacement male. Fingerprinting revealed that the first two eggs were sired by the resident males in five nests and by extra-pair males in the remaining five nests. The widows that had been faithful to their initially chosen mate rejected copulation attempts by the replacement male until most of the eggs had been laid. Consequently, nearly all eggs laid by these females were sired by the original male. The widows that had been unfaithful prior to male removal copulated sooner with the replacement male than females that were faithful to their mate. However, these replacement males also had a very low fertilization success; most eggs were sired by males that were not associated with the nest. This is consistent with the situation in non-experimental nests where unfaithful females copulate with their mate at the same rate as faithful females, yet unfaithful females have a majority of offspring sired by extra-pair males. We conclude that fertilization patterns to a large extent are determined by the female through active selection and rejection of copulation partners, though our results also allow some speculation that females have control over sperm competition. Female copulation tactics are probably determined some currently unknown fitness benefits of having the offspring sired by particular males.Correspondence to: Raleigh J. Robertson     

Multiple sexual pigments,assortative social pairing,and genetic paternity in the yellow warbler (Setophaga petechia)

In socially monogamous species, extra-pair paternity may increase the reproductive success of highly ornamented males, mediating the evolution of sexual ornaments. However, ornaments may also attract social mates, and a tradeoff between extra-pair paternity (EPP) and within-pair paternity (WPP) may complicate mating strategies. Further, in many socially monogamous species, females are also ornamented, and the relationship between female ornamentation and patterns of EPP has been neglected. We investigated the patterns of genetic paternity with respect to carotenoid- and melanin-based pigmentation in yellow warblers (Setophaga petechia) of both sexes. We asked whether males face tradeoffs between EPP and WPP, how paternity patterns relate to carotenoid- versus melanin-based pigmentation, and whether less EPP occurs in broods when males and females are assortatively paired. Males faced a tradeoff between EPP and WPP. Moreover, non-additive relationships existed between paternity patterns and the two pigment types in both sexes. Males with high melanin coverage but dull carotenoid pigmentation achieved EPP but lost WPP, whereas males with high levels of both pigment types had high WPP but gained little EPP. A parallel pattern occurred in females. Warblers paired assortatively by pigmentation and EPP was less common in broods when the sexes were assortatively paired by carotenoid pigmentation. Results suggest that the most colorful birds obtain high quality social mates and advance reproductive success through WPP, show that correlations can arise between female ornamentation and patterns of EPP, and also uniquely suggest that social pairing patterns may influence extra-pair mating strategies.     

Non-aggressive mate guarding by the blue-footed booby: a balance of female and male control

Thirteen pairs of blue-footed boobies (Sula nebouxii) were observed on their colony. Pairs courted frequently and, on average, copulated 24 times during the 30 days before laying, with 38% of those copulations occurring in the last 5 days (presumed fertile period). Males and females increased attendance at the nest site as laying approached. Seven females performed an average seven extra-pair copulations, with 1–2 paired male neighbors, but these were less concentrated in the presumed fertile period than within-pair copulations, and the last two copulations of all 7 females were with their social mates. Rates of female extra-pair copulations were six times lower when their social mate was present, and during the presumed fertile period, no female performed an extra-pair copulation in the presence of her mate. Males did not respond to infidelity of social mates with aggression, prompt copulation, retaliatory copulation, or increase in copulation. Seven of 13 males performed an average of five extra-pair copulations, with 1–3 paired female neighbors, before their own mates began egg-laying. The males' extra-pair copulations represented only 4% of their total copulations during their own mates' presumed fertile periods. Females, the larger sex, apparently control sexual access and copulate with extra males to achieve extra-pair fertilization. Males pursue a mixed strategy: they copulate with extra females, mostly outside their own mate's presumed fertile period, and they copulate increasingly with their social mate as laying approaches, probably assuring some paternity by mate guarding, involving attendance and courtship. Behavior of males and females is also consistent with other hypotheses for extensive joint nest site attendance: pairbonding, copulation access, and territory acquisition. Received: 14 November 1997 / Accepted after revision: 16 May 1998     

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     

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.     

Extra-pair paternity,sperm competition and the evolution of testis size in birds

Sperm competition should select for increased sperm production if the probability of fertilization by a specific male is proportional to the relative number of sperm inseminated. A review of the literature generally supports the predicted positive association between sperm production or allocation and various measures of the presumed intensity of sperm competition. However, it is not clear how increased sperm competition is related to extra-pair paternity, and it remains unknown whether certainty of paternity should be associated with relative testis size. Based on a large sample of bird species with information on extra-pair paternity gathered from the literature, we demonstrate that testis mass is related positively to the level of extra-pair paternity, after controlling for body size and phylogeny. Although large testes may be necessary to avoid sperm depletion in species in which males frequently engage in multi-pair copulations, we argue that selection has favoured increased testis mass in situations of more intense sperm competition in order to retaliate against copulations by rival males. The fact the males are not always successful in retaliating against rival ejaculates further suggests that females may largely control the allocation of paternity in birds and that increased sperm production by males may simply be a male strategy to make the best of a bad situation.     

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     

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.     

Do female black-capped chickadees prefer high-ranking males as extra-pair partners?

Previous studies have shown that some female black-capped chickadees (Poecile atricapillus) solicit copulations from males that rank higher in winter flocks than their social mates, and extra-pair paternity in nests occurs commonly enough to be considered a potential female mating tactic. This study uses blood samples collected in 1992–1995 from 58 families of black-capped chickadees to test whether females with extra-pair offspring have chosen extra-pair sires higher in social rank than their mates. Paternity was assessed with multilocus DNA fingerprinting in 1992–1994 nests and with microsatellite and single-locus minisatellite DNA typing in 1995 nests. Seventeen of 58 nests (29.3%) contained young genetically mismatched with their social father. In 11 of 15 cases where the identity of the extra-pair male was known, the extra-pair male was dominant to the social father. Using data from 29 nests located in 1994 and 1995 for which we had the most data on relative ranks of males, high-ranking males had greater realized reproductive success than low-ranking males as a result of extra-pair fertilizations. There was no significant difference between the number of nests containing extra-pair young of females mated to low-ranked versus high-ranked males. Two nests in 1995 contained young either genetically mismatched with both social parents (intraspecific brood parasitism) or, in one nest, genetically mismatched with the social mother but not the social father (quasi-parasitism). The implications of female strategies acquiring genetic benefits through extra-pair copulations are discussed. Received: 7 July 1997 / Accepted after revision: 14 March 1998