Extra-pair Young Research Articles

Female house wrens (Troglodytes aedon) increase the size, but not immunocompetence, of their offspring through extra‐pair mating

House wrens are typically socially monogamous, but frequently engage in extra-pair matings leading to multisired broods. Because females do not appear to acquire direct material benefits from their extra-pair mates, we tested the hypothesis that female house wrens derive indirect genetic benefits, such as enhanced immunocompetence (cutaneous immune activity, humoral immunity, and plasma bactericidal activity) and condition (size and haematoserological traits) for their offspring, by mating polyandrously. We predicted that extra-pair young (EPY) should show greater immune responsiveness and better body condition than their within-pair maternal half-siblings (WPY). Contrary to our prediction, WPY had higher cutaneous immune activity than their EPY brood-mates in two of three years, and EPY and WPY did not differ in measures of innate and humoral immunity. WPY also had higher albumin to gamma-globulin ratios than EPY; however, they were not in better condition based on other measures. EPY had consistently longer tarsi (a measure of long-bone size) than their WPY half-siblings, suggesting that females engage in extra-pair copulations with larger males. The benefits of large structural size in the study population is unknown, but based on evidence from other passerines, we suggest that structural size may be an important fitness-related trait in house wrens. We conclude that our results are not consistent with the hypothesis that females gain immune-related benefits for their offspring by engaging in extra-pair matings. Further study of the fitness consequences of differences in tarsus length is needed to determine whether females acquire size-related benefits for their offspring from extra-pair mates.

Incubation feeding by male Scarlet Tanagers: a mate removal experiment

Incubation feeding, where males feed their mates, is a common behavior in birds and may improve female condition, nest attentiveness, and nesting success. We used behavioral observations and a temporary mate removal experiment to test the female nutrition hypothesis for incubation feeding by male Scarlet Tanagers (Piranga olivacea). All males (N = 20) were observed incubation feeding and fed females both at the nest ( ¯ × 1.36 trips/h) and away from the nest ( ¯ × 20.1 trips/h). Male feeding rate off-nest was negatively correlated with the duration of female foraging bouts and positively correlated with the total time females spent incubating per hour. Eggs were predated at seven of 19 (37%) nests, but nest survival during incubation was not related to either female incubation behavior or male feeding rate. During temporary removal experiments (N = 12), female Scarlet Tanagers remained on the nest significantly longer and did not have longer foraging bouts. An unexpected outcome of the removal experiments was a dramatic change in female vocal behavior. All 12 experimental females gave chik-burr calls during the male-removal experiments ( ¯ × bout length = 11.7 min), but during normal observation periods only six of 20 females at the incubation stage gave chik-burr calls ( ¯ × bout length = 0.7 min, N = 20). Our results suggest that female tanagers likely gain nutritional benefits from incubation feeding, but male feeding may not improve immediate reproductive success. Nine of 54 (17%) nestlings in five of 17 broods (29%) were extra-pair young (EPY), indicating that males could potentially benefit from incubation feeding via mate retention and fidelity as well as, or instead of, through immediate gains in reproductive success. Our study indicates that females benefit from incubation feeding and do not simply passively accept food from their mates, but instead may influence male feeding rates through direct (e.g., mate following and vocalizing) and indirect (the threat of mate abandonment or cuckoldry) means.

Reed bunting females increase fitness through extra-pair mating with genetically dissimilar males

Females of many socially monogamous species accept or even actively seek copulations outside the social pair bond. As females cannot increase the number of offspring with promiscuous behaviour, the question arises why they engage in extra-pair mating. We used microsatellite data to determine paternity, heterozygosity and genetic relatedness in the reed bunting (Emberiza schoeniclus), a species with high levels of extra-pair paternity (EPP). We found that extra-pair young (EPY) were more heterozygous than within-pair young (WPY). The high heterozygosity of the EPY resulted from a low genetic similarity between females and their extra-pair mates. EPY were heavier and larger when compared with their maternal half-siblings shortly before they left the nest. Recapture data indicated a higher fledgling survival of EPY compared with WPY. Our data suggest that reed bunting females increase the viability of their offspring and thus fitness through extra-pair mating with genetically dissimilar males.

MHC-associated mating strategies and the importance of overall genetic diversity in an obligate pair-living primate

Mate choice is one of the most important evolutionary mechanisms. Females can improve their fitness by selectively mating with certain males. We studied possible genetic benefits in the obligate pair-living fat-tailed dwarf lemur (Cheirogaleus medius) which maintains life-long pair bonds but has an extremely high rate of extra-pair paternity. Possible mechanisms of female mate choice were investigated by analyzing overall genetic variability (neutral microsatellite marker) as well as a marker of adaptive significance (major histocompatibility complex, MHC-DRB exon 2). As in human medical studies, MHC-alleles were grouped to MHC-supertypes based on similarities in their functional important antigen binding sites. The study indicated that females preferred males both as social and as genetic fathers for their offspring having a higher number of MHC-alleles and MHC-supertypes, a lower overlap with female’s MHC-supertypes as well as a higher genome wide heterozygosity than randomly assigned males. Mutual relatedness had no influence on mate choice. Females engaged in extra-pair mating shared a significant higher number of MHC-supertypes with their social partner than faithful females. As no genetic differences between extra-pair young (EPY) and intra-pair young (IPY) were found, females might engage in extra-pair mating to ‘correct’ for genetic incompatibility. Thus, we found evidence that mate choice is predicted in the first place by the ‘good-genes-as-heterozygosity hypothesis’ whereas the occurrence of extra-pair matings supports the ‘dissassortative mating hypothesis’. To the best of our knowledge this study represents the first investigation of the potential roles of MHC-genes and overall genetic diversity in mate choice and extra-pair partner selection in a natural, free-living population of non-human primates.

Is extrapair mating random? On the probability distribution of extrapair young in avian broods

A dichotomy in female extrapair copulation (EPC) behavior, with some females seeking EPC and others not, is inferred if the observed distribution of extrapair young (EPY) over broods differs from a random process on the level of individual offspring (binomial, hypergeometrical, or Poisson). A review of the literature shows such null models are virtually always rejected, with often large effect sizes. We formulate an alternative null model, which assumes that 1) the number of EPC has a random (Poisson) distribution across females (broods) and that 2) the probability for an offspring to be of extrapair origin is zero without any EPC and increases with the number of EPC. Our brood-level model can accommodate the bimodality of both zero and medium rates of EPY typically found in empirical data, and fitting our model to EPY production of 7 passerine bird species shows evidence of a nonrandom distribution of EPY in only 2 species. We therefore argue that 1) dichotomy in extrapair mate choice cannot be inferred only from a significant deviation in the observed distribution of EPY from a random process on the level of offspring and that 2) additional empirical work on testing the contrasting critical predictions from the classic and our alternative null models is required. Key words: extrapair copulation (EPC), likelihood, mate choice, null model, sexual selection. [Behav Ecol]

Are extra‐pair young better than within‐pair young? A comparison of survival and dominance in alpine marmot

1. In socially monogamous species, females may seek extra-pair copulation to gain genetic benefits. In order to test this 'genetic quality' hypothesis, one must compare the performance of extra-pair young (EPY) and within-pair young (WPY). Such tests, however, are scarce and results published so far are inconclusive. 2. Here, we test the 'genetic quality' hypothesis using multistate capture-recapture models to compare age-specific survival and access to dominance between EPY and WPY in the alpine marmot Marmota marmota, a socially monogamous mammal showing extra-pair paternities. 3. When compared with WPY, survival of EPY was higher by 15%, 10% and 30%, for juveniles, yearlings and 2-year-old individuals, respectively. Survival at older ages did not differ. 4. Survival corresponded to true survival for yearlings and juveniles as dispersal does not occur before 2 years of age in marmots. For older individuals, survival estimates included a mixture of survival and dispersal. The 30% increase of the 2-year-old EPY survival might reflect delayed dispersal rather than high survival of EPY as compared with WPY. 5. WPY and EPY had the same probability (0.28) to access dominance at 2 years of age, but EPY were more successful at older ages than WPY (0.46 vs. 0.10). 6. Both survival and reproductive performance were higher in EPY than in WPY. The fitness advantages of adopting such a mixed mating tactic are thus likely to be high for marmot females. We suggest that obtaining genetic benefits is the main evolutionary force driving extra-pair paternity in alpine marmots.

Extra-pair paternity in alpine marmots, Marmota marmota: genetic quality and genetic diversity effects

Assuming that a male’s genetic characteristics affect those of his offspring, extra-pair copulation has been hypothesized to increase heterozygosity of the progeny—the “genetic compatibility” hypothesis—and the genetic diversity within litters—the “genetic diversity” hypothesis. We tested these two hypotheses in the alpine marmot (Marmota marmota), a socially monogamous mammal showing a high rate of extra-pair paternity (EPP). In a first step, we tested the assumption that a male’s genetic characteristics (heterozygosity and genetic similarity to the female) affect those of his offspring. Genetic similarity between parents influenced offspring heterozygosity, offspring genetic similarity to their mother, and litter genetic diversity. The father’s heterozygosity also influenced litter genetic diversity but did not affect offspring heterozygosity. Hence, heterozygosity seems not to be heritable in the alpine marmot. In a second step, we compared genetic characteristics of extra-pair young (EPY) and within-pair young (WPY). EPY were less genetically similar to their mother but not more heterozygous than WPY. EPY siblings were also less genetically similar than their WPY half siblings. Finally, the presence of EPY promoted genetic diversity within the litter. Thus, our data support both the “genetic compatibility” and the “genetic diversity” hypotheses. We discuss further investigations needed to determine the primary causes of EPP in this species.

Genetic similarity, extrapair paternity, and offspring quality in Savannah sparrows (Passerculus sandwichensis)

The occurrence of extrapair paternity (EPP) in birds is often attributed to the action of good-genes sexual selection whereby females ‘‘trade up’’ on male genetic quality by allocating fertilizations to males with better genes than those possessed by their social mate. To date, most studies of EPP in birds focus on absolute measures of male quality as a criterion for female choice, although multiple mating by females in other taxa is more commonly ascribed to benefits associated with the individual optimization of offspring genotypes. Here, we examine whether the genetic similarity of social mates predicts female mating patterns in a population of Savannah sparrows (Passerculus sandwichensis) where as many as 70% of adults produce extrapair young (EPY). We consider the influence of genetic similarity across all stages of a female’s decision-making process, from pair formation through the decision to produce EPY, to the allocation of fertilizations to specific extrapair sires. In a 4-year study of 175 males, 206 females, and 506 offspring, females were more likely to produce EPY when paired to genetically similar males, but they did not appear to be influenced by the size, age, mass, individual heterozygosity, and genetic diversity of their social mates. In paired comparisons, females were almost twice as likely to decrease their genetic similarity to males when producing EPY as they were to increase it. Nonetheless, females did not select especially dissimilar males when mating outside the pair-bond nor did they pair disassortatively with respect to genetic similarity. Relative measures of male quality may influence mating patterns in birds, but only at some points in a female’s decision-making process. Key words: EPP, genetic compatibility, microsatellites, relatedness, sexual selection. [Behav Ecol]

Paternity covaries with laying and hatching order in the collared flycatcher Ficedula albicollis

Females in most bird species engage in extra-pair copulations. Although this behaviour is widespread, benefits for females of doing so are less understood. The most widely accepted hypothesis is that they improve their previous choice of social partner and gain genetic benefits for their offspring. Some evidence for this comes from studies that find that extra-pair young (EPY) have greater fitness than their half-sibs. However, this might be also caused by maternal, non-genetic effect, a possibility that remains largely untested. Here we test whether EPY are laid in larger eggs or eggs laid early in the laying sequence in the collared flycatcher (Ficedula albicollis). The size of eggs bearing EPY and within-pair young (WPY) did not differ, however, EPY were laid in early eggs and consequently hatched earlier than WPY. As hatching asynchrony is a strong determinant of offspring size and survival in many species, including collared flycatcher, our results suggest that a caution is needed when paternal genetic effects are to be inferred from comparison of naturally occurring half-sibs.

Manipulation of male quality in wild tits: effects on paternity loss

Extrapair paternity (EPP) has proved to be widespread and highly variable among birds and other taxa, including sociallymonogamous species. A multitude of hypotheses have been put forward to explain this variation, but its occurrence is not fullyunderstood. Male age, social dominance rank, song and breeding density or synchrony have been among the suggestedcorrelates of EPP, but results so far are inconclusive. We interspecifically cross-fostered blue tits (Parus caeruleus) and great tits(Parus major) in the wild, thus manipulating males to exhibit reduced social dominance rank, sing aberrant songs, andconsequently be perceived as low-quality males as compared to controls. This allowed us to test if male quality had an influenceon loss of paternity. We found no statistically significant differences between cross-fostered and control males of either species,neither with respect to levels of cuckoldry nor proportions of extrapair young (EPY) in the broods. Paternity levels werecomparable to other studies on the same species. No effect of density could be detected on levels of EPP either, while an ageeffect seemed to be present at least in the blue tit, EPY being almost absent in broods of older blue tit males. We conclude thatthe effects of male quality on paternity loss are minor, if any, in these populations. Key words: cross-fostering, early learning,extrapair paternity, microsatellite, sexual imprinting, social rearing conditions. [Behav Ecol]

Extrapair paternity and offspring immunocompetence in the reed bunting, Emberiza schoeniclus

Female promiscuity is widespread in birds, as well as in other taxa. It is often assumed that in bird species with a socially monogamous mating system, females engage in extrapair copulations to obtain indirect (genetic) benefits. ‘Good genes’ and ‘compatible genes’ models predict that extrapair young (EPY) should be of higher quality than within-pair young (WPY), but the empirical evidence for this effect is limited. A recent study, however, indicated an enhanced cellular immunity in EPY in a passerine bird, the bluethroat, Luscinia svecica. To assess the generality of that finding, we replicated the study using the reed bunting, a passerine with an extrapair mating system similar to that of the bluethroat. Using the same immune assay protocol, we found no indication of enhanced cellular immunity in EPY compared with WPY. Female reed buntings thus do not seem to engage in extrapair copulations to achieve the same type of genetic benefit as in bluethroats. Furthermore, EPY in mixed-paternity broods did not grow faster and were not in better body condition close to fledging than WPY. We conclude that our results do not support the hypotheses assuming indirect benefits to female extrapair copulations. It appears that the adaptive value for females of pursuing extrapair fertilizations varies, even between species with similar extrapair mating systems.

Microsatellite Markers for Paternity Testing in the Willow Warbler Phylloscopus Trochilus: High Frequency of Extra-Pair Young in an Island Population

Four microsatellite markers from the willow warbler Phylloscopus trochilus: were isolated and characterised. The combined exclusion probability of the three most polymorphic markers (Phtr1, Phtr2 and Phtr3) was 0.96. These markers were used to test for the occurrence of extra-pair young (EPY) in twelve willow warbler families sampled from the island of Gotland in 1992 and 1993. The mean frequency of extra-pair young was found to be 28% and the mean frequency of families with extra-pair young 58%. These results contrast with an earlier study in a mainland Swedish population, where no EPY were found. The island population in this study has a higher breeding density than the mainland population, rendering support for density dependent factors affecting the frequency of EPY. A cross-species study showed that the willow warbler microsatellite markers can also be used in other European passerines as well as distantly related species, such as the Australian magpie Gymnorhina tibicen.

Pair identity — An important factor concerning variation in extra-pair paternity in the coal tit (Parus ater)

AbstractIn the majority of socially monogamous bird species, extra-pair paternity (EPP) has been shown to be common, while its frequency varies considerably among as well as within species. Our knowledge of factors affecting this variation, however, still remains comparatively poor. A thorough analysis of EPP patterns on an individual level may lead to a better understanding of the general diversity in EPP. Here, we analyse intra- and inter-annual as well as individual variation in the occurrence of EPP in first and second broods of coal tits (Parus ater) in three successive years. Based on a sample of 483 broods with 3559 offspring genotyped, we show that EPP was extraordinarily frequent in the study population, surpassing the values observed in other species of the genus Parus by far. On the population level, the rates of EPP were found to be remarkably similar between years, while there was pronounced intra-annual variation such that EPP rates increased significantly from first to second broods. Considering consecutive broods of individual females and males, the general occurrence of EPP was 'inconsistent' (i.e. EPP often affected only one of two broods), but the proportion of extra-pair young (EPY) per brood showed significant repeatability for both sexes in case of mate retention. When mate change occurred, repeatability collapsed, indicating that pair identity was more important in determining the repeatability of EPP than female or male identity alone. This was further supported by the fact that, besides female and male age, also their interaction (reflecting pair age combination) significantly predicted the proportion of EPY, though not in all breeding periods surveyed. Hence, the identity of the breeding pair (reflecting possible interactions of male and female characteristics) should be explicitly considered and accounted for in future studies investigating patterns of EPP on an individual level. Finally, we show that, depending on the boundary conditions, in principle, all coal tits experience EPP: When analysing three or more broods of the same individual, there were no completely 'faithful' females and also nearly no males, which were not cuckolded at least once.

Within-Season Prevalence of Extrapair Young in Broods of Double-Brooded and Mate-Faithful Eastern Phoebes (Sayornis Phoebe) in Indiana

Abstract We examined prevalence of extrapair young (EPY) between first and second broods of a multibrooded passerine, Eastern Phoebe (Sayornis phoebe). During the 1998–2000 field seasons, we conducted behavioral observations to assign social parentage to offspring in nests and then conducted paternity analysis on a subset of social families where (1) the male and female maintained their pair bond for all broods within the breeding season, and (2) adults successfully fledged their first brood and subsequently attempted a second brood within the same breeding season. We used three hypervariable microsatellite loci to document the presence and frequency of EPY in first and second brood offspring from 87 social family units where all individuals had been sampled (n = 174 broods, n = 81 social males, n = 82 social females, n = 769 offspring). Although the majority of pairs were both socially and genetically monogamous within a breeding season, 17.2% (15 of 87 families) of all families had EPY in at least one brood and we found two cases of intraspecific brood parasitism. Overall, 9.2% of all broods (16 of 174 broods) contained EPY and 5.1% of all nestlings that hatched were EPY (39 of 769 nestlings). Second broods were more likely to contain EPY than first broods (4.6% first broods with EPY [4 of 87], 13.8% second broods with EPY [12 of 87]). The total number of EPY was greater in second broods than in first broods (1.0% EPY [4 of 411 nestlings] in first broods, 9.8% EPY [35 of 358 nestlings] in second broods).

Within-Season Prevalence of Extrapair Young in Broods of Double-Brooded and Mate-Faithful Eastern Phoebes (Sayornis phoebe) in Indiana

Within-Season Prevalence of Extrapair Young in Broods of Double-Brooded and Mate-Faithful Eastern Phoebes (Sayornis phoebe) in Indiana

WITHIN-SEASON PREVALENCE OF EXTRAPAIR YOUNG IN BROODS OF DOUBLE-BROODED AND MATE-FAITHFUL EASTERN PHOEBES (SAYORNIS PHOEBE) IN INDIANA

Abstract We examined prevalence of extrapair young (EPY) between first and second broods of a multibrooded passerine, Eastern Phoebe (Sayornis phoebe). During the 1998–2000 field seasons, we conducted behavioral observations to assign social parentage to offspring in nests and then conducted paternity analysis on a subset of social families where (1) the male and female maintained their pair bond for all broods within the breeding season, and (2) adults successfully fledged their first brood and subsequently attempted a second brood within the same breeding season. We used three hypervariable microsatellite loci to document the presence and frequency of EPY in first and second brood offspring from 87 social family units where all individuals had been sampled (n = 174 broods, n = 81 social males, n = 82 social females, n = 769 offspring). Although the majority of pairs were both socially and genetically monogamous within a breeding season, 17.2% (15 of 87 families) of all families had EPY in at least one b...

Extrapair paternity in the red bishop (Euplectes orix): is there evidence for the good-genes hypothesis?

AbstractWe investigated the pattern and frequency of extrapair paternity in a colony of the red bishop, a sexually dimorphic and highly polygynous passerine. In particular, we tested whether there was evidence in support of the good-genes hypothesis, stating that females seek extrapair copulations with high-quality males to enhance the genetic quality of their offspring. Paternity analysis of 432 nestlings from 187 broods using non-radioactive multilocus DNA-fingerprinting revealed that 17.6% of nestlings were extrapair young (EPY) and 30.5% of all broods investigated contained at least one EPY. In a pairwise comparison, extrapair fathers were found to have significantly longer tarsi than the within-pair fathers they cuckolded. Furthermore, extrapair fathers were more often territory owners in the previous season,

High levels of extra-pair paternity in an isolated, low-density, island population of tree swallows (Tachycineta bicolor).

Molecular genetic studies have suggested that apparently nonbreeding males ('floaters') may account for a significant proportion of extra-pair paternity (EPP) in avian populations. Attempts to determine the influence of breeding density on EPP are therefore confounded by the presence of a subpopulation of floaters whose numbers are difficult to estimate. To study EPP in a tree swallow (Tachycineta bicolor) population with few floaters, we chose a nestbox grid on an island with an excess of available breeding sites and very few floaters. We obtained DNA samples from 13 complete families and performed DNA profiling on them using four microsatellite loci. For comparison, we also obtained a sample of 58 extra-pair young (EPY) from a mainland population typed at five microsatellite loci. Paternity assignments among resident males in both populations were made using the microsatellite profiles and a likelihood-based statistical method. Of the 67 island nestlings typed, we found 37 (55%) nestlings from 11 (85%) different nests that were EPY. The proportion of nestlings that were EPY and the proportion of broods containing EPY did not differ significantly between island and mainland populations studied previously. There was no significant difference between island and mainland populations in the proportion of extra-pair paternities assigned among neighbouring resident males. Male breeding density does not appear to affect the ability of female tree swallows to obtain extra-pair fertilizations, at least over the range of densities studied so far. The rate of EPP has remained remarkably consistent over many years, studies and populations implying an important role of active female choice in determining EPP.

Frequencies of multiple paternity in three Acrocephalus species (Aves Sylviidae) with different mating systems (A. palustris, A. arundinaceus, A. paludicola)

Genetic parentage was determined by DNA-fingerprinting with multilocus probes and microsatellite PCR in three Acrocephalus species with different mating systems: the socially monogamous marsh warbler (A. palustris), the polygynous great reed warbler (A. arundinaceus) and the promiscuous aquatic warbler (A. paludicola). The amount of extra-pair paternity (EPP)/multiple paternity (MP) was then related to ecological parameters (increasing productivity) and parental care (decreasing male participation) of Acrocephalus species. Marsh warblers (33 broods, 131 young plus putative parents) were studied over 3 years in a German population. No case of social polygyny but four extrapair young (EPY) were detected in three socially monogamous broods. The low frequency of extra-pair fertilization (EPF, 9.1% of broods, 3.1% of offspring) may be best attributed to intense male mate-guarding and restrictions on mate assessment due to an extremely short breeding window. The rate of social polygyny in a German great reed warbler population varied between 8 and 35% of males during a 5-year period. In 48 broods (27 monogamous and 21 polygynous males) with 194 young, 5 nests (3 of polygynous, 2 of monogamous males) with 19 EPY were recorded: EPF-rates are 10.4% of broods and 9.8% of offspring. In none of the nests were neighbouring males the genetic fathers, but in 2 nests, “bachelor” males from distant territories had sired the offspring. The EPP-rate in this species seems to be related to an incomplete correlation between male and territory characteristics leaving some females to correct their choice. Aquatic warblers were studied during three years in a Polish population. In each of the years, MP was recorded in more than half of a total of 75 broods (mean 76%). The proportion out of 340 young sired by one to five “additional” males varied between 25 and 57% (mean 39.1%). Food resources are neither critical nor economically defendable in the species' habitat and paternity assurance behaviours of males and female choice are yet not fully understood.

Extra-Pair Fertilizations in the Sedge Warbler

Parentage of 201 young (from 44 broods) in a population of Sedge Warblers Acrocephalus schoenobaenus in South Central Sweden, 1990-1992, was determined by using multilocus DNA fingerprinting. The frequency of extra-pair young (EPY) was 7.5% and they occurred in 23% of the broods. For 11 out of 15 extra-pair young we could identify the true father; in all of the cases he was a close neighbour. Among ten broods with extra-pair young, nine contained only one extra-pair sire and the remaining brood two extra-pair sires. The frequency of EPF varied among years (1.8-11.8%). The seasonal timing of broods with and without extra-pair young did not differ, and the occurrence of extra-pair fertilizations (EPF) was not related to the length of the pair male's mate-guarding period. EPF were not related to breeding synchrony (estimated as the mean number of fertile females per day during a female's fertile period). Pairs whose nests contained extra-pair young had more territories within 100 m of their nest than pairs without extra-pair young. Females that engaged in EPF had fewer attractive (i.e. singing) males to choose among the day before and at the day they formed their pair bend. Moreover, in all but one case the extra-pair male was not singing (i.e. not available as pair mate) the day the EPF-female settled. Males that fertilized extra-pair young tended to arrive earlier and to have higher pairing success than both males that were cuckolded and other males. Hence, female Sedge Warblers engaged in extra-pair fertilizations with attractive male neighbours. These results are consistent with the hypotheses that females participating in EPF are seeking genetic benefits to their offspring, but we cannot exclude the alternative explanation that attractive males are more efficient in forcing females to accept EPF.