Settlement patterns of female barn swallows Hirundo rustica across different group sizes: access to colorful males or favored nests?

In most avian species, individuals are faced with two critical decisions at the start of a breeding season: choosing a breeding site and a mate. An analysis of these decisions in light of population-level patterns, such as group size variation in social breeders, can illuminate the causes and patterns of habitat selection behavior. Group sizes are variable in barn swallows; however, no clear and consistent benefits of group breeding have been found in this species, and it is puzzling as to why individuals breed socially. Previous analyses demonstrated that individuals aggregate to gain access to nests that were constructed during previous seasons; however extra-pair matings are also prevalent in this species, raising questions about the mate-selection strategies of females across different group sizes. In this paper, I address the question of how females make their first site-selection decisions in terms of two features known to be causally related to seasonal reproductive success: (1) colorful males or (2) old nests. Using experimental and observational data, I tested, but found no support for, the hypotheses that propose female settlement decisions are a function of (1) the prevalence of colorful males or (2) the increased opportunity for extra-pair matings at group sites. Instead, it is apparent that female settlement patterns are strongly tied to the availability of old nests at a site. Extra-pair fertilizations are equally common across all group sizes in this population, suggesting that females do not face a trade-off between old nests and the possibility of extra-pair mating decisions when making settlement decisions.     

Male tail streamer length predicts fertilization success in the North American barn swallow (Hirundo rustica erythrogaster)

In socially monogamous species, extra-pair paternity has the potential to increase the variance in male reproductive success, thereby affecting the opportunity for sexual selection on male extravagant ornamentation. In the European barn swallow (Hirundo rustica rustica), the tail streamer length is a sexually selected male ornament and an honest indicator of viability. The North American barn swallow (Hirundo rustica erythrogaster) also shows sexual dimorphism in tail streamer length, but whether this trait holds the same signalling function in this subspecies is a controversial issue, and the available literature is presently scarce. Here, we present data on paternity in the North American barn swallow, including a complete sampling of extra-pair sires in four colonies. We analysed how extra-pair paternity affected the variance in male fertilization success and examined whether male tail streamer (i.e. the outermost tail feather) length correlated with fertilization success (n=86 males). Extra-pair paternity constituted 31% of all offspring and significantly increased the variance in male fertilization success. The number of offspring sired by extra-pair males accounted for almost half of the total variance in male fertilization success. Males with naturally long tail streamers had a higher fertilization success than males with shorter tail streamers, and this pattern was mainly caused by a higher extra-pair success for males with long tail streamers. Males with long tail streamers also paired with early breeding females in prime body condition. These results are consistent with the idea that there is directional sexual selection on male tail streamer length, possibly mediated through male extra-pair mating success or the timing of breeding onset.     

Experimental assessment of ecological and phenotypic factors affecting male mating success and polyandry in northern watersnakes, Nerodia sipedon

To resolve conflicting field observations regarding the action of sexual selection, we used breeding experiments and paternity analysis of the 927 resulting offspring to assess how male size, condition, tail length, genetic similarity to the female, and variation in operational sex ratio (OSR) affected male reproductive success and the incidence of polyandry in northern watersnakes (Nerodia sipedon). Only size affected male mating success. Large males were more successful, but only when male size varied substantially and competition among males was intense (i.e., male-biased OSR). The conditional nature of the size advantage may explain why studies of free-living watersnakes have produced inconsistent results regarding the relationship between male size and mating success. Size differences between males did not affect the proportion of offspring each male sired within multiply sired litters. We found positive size-assortative mating, but only when the OSR was female biased, suggesting that smaller males had improved access to females when competition among males was reduced, but that competition with larger males still restricted mating opportunities of small males to less preferred, smaller females. Most litters (58%) were multiply sired and larger females were more likely to produce multiply sired litters, similar to free-living watersnakes. There was no association between the incidence of multiple paternity and OSR, however, suggesting that polyandry is not simply a function of opportunity, with females passively waiting for males to court them.     

Search costs influence the spatial distribution,but not the level,of extra-pair mating in tree swallows

Costs of searching for a mate are an important component of models of sexual selection, yet they have rarely been examined in wild populations of vertebrates. In this paper, we report an experiment in which we handicapped female tree swallows by clipping some flight feathers. This manipulation increased the costs of flight and searching for extra-pair mates. Despite these costs, handicapped females had the same level of extra-pair mating (percentage of extra-pair young, percentage of broods with extra-pair young, and the number of extra-pair sires per brood) as control females. However, handicapped females were more likely to have young sired by extra-pair males that lived closer to her nest than control females. This change in the distribution of extra-pair mating was most likely due to female choice rather than male coercion, and it suggests that extra-pair mating has significant benefits to females. One important implication of our study is that ecological and social factors that influence search costs could affect the spatial distribution of extra-pair sires and, consequently, the intensity of sexual selection. These effects may have been overlooked in previous studies that did not identify extra-pair sires.     

Maternal characteristics and the production and recruitment of sons in the eastern kingbird (<Emphasis Type="BoldItalic">Tyrannus tyrannus</Emphasis>)

Sex allocation theory predicts that if variance in reproductive success differs between the sexes, females who are able to produce high-quality young should bias offspring sex ratio towards the sex with the higher potential reproductive success. We tested the hypothesis that high-quality (i.e., heavy) female eastern kingbirds (Tyrannus tyrannus) that bred early in the breeding season would produce male-biased clutches. A significant opportunity for sexual selection also exists in this socially monogamous but cryptically polygamous species, and we predicted that successful extra-pair (EP) sires would be associated with an excess of male offspring. Although population brood sex ratio did not differ from parity, it increased significantly with female body mass and declined with female breeding date, but was independent of the morphology and display (song) behavior (correlates of reproductive success) of social males and EP sires. Male offspring were significantly heavier than female offspring at fledging. Moreover, the probability that male offspring were resighted in subsequent years declined with breeding date, and was greater in replacement clutches, but lower when clutch size was large. Probability of resighting female offspring varied annually, but was independent of all other variables. Given that variance in reproductive success of male kingbirds is much greater than that of females, and that male offspring are more expensive to produce and have a higher probability of recruitment if fledged early in the season, our results support predictions of sex allocation theory: high-quality (heavy) females breeding when conditions were optimal for male recruitment produced an excess of sons.     

Varying levels of female promiscuity in four <Emphasis Type="Italic">Apodemus</Emphasis> mice species

Sexual selection in most vertebrates is based on the evolution of fitness optimization strategies such as multiple-male mating (MMM). Several ecological correlates of MMM have been identified in bird and fish populations; however, only few studies have documented the effects of environmental change on promiscuity in mammals. In this study, the 127 pregnant females from four central European and ecologically diverse species of field mice (genus Apodemus) were studied to assess the role of ecological factors that may have shaped the evolution of particular mating systems. MMM was found in all analyzed species: in Apodemus uralensis and Apodemus flavicollis, up to two males could be identified as the fathers of a particular litter, while three males sired 9.1% of analyzed litters of Apodemus sylvaticus and 20.6% of Apodemus agrarius. Furthermore, there were obvious differences between species in relative testes size and the proportion of multiple sired litters during those seasons when the opportunity for multiple mating was high. The species with the smallest testes and the least promiscuous was A. uralensis (only 43.5% of multiple sired litters), while the species with the biggest testes and the most promiscuous was A. agrarius (69.2%). MMM was significantly associated with higher litter size in A. flavicollis, and the probability of MMM strongly increased with season in A. agrarius and with abundance in A. uralensis. These results indicate that ecological factors are associated with MMM rates in Apodemus field mice and more research is needed to fully understand the evolution of mating strategies at different levels of biological resolution.     

Sperm competition and sexual selection: a meta-analysis of paternity studies of birds

Sperm competition (the competition among the sperm of different males for fertilization of the eggs of a female) has been suggested to be an important component of sexual selection, but no general assessment has been made of this proposition. We used a meta-analytic approach to assess the extensive literature on paternity (the proportion of offspring in a focal nest sired by an attending male) in birds based on allozyme and molecular techniques. The relative variance in male mating success was on average increased by a factor of 4.6 over the variance in apparent male success. Males with more extravagant secondary sexual characters had higher paternity in their own nests than less adorned males. There was a weak effect of male age being positively associated with paternity in own nests. Male body size measured as the length of wing and tarsus was weakly positively associated with paternity in own nests. Male survival prospect was positively associated with paternity in own nests. Polygynous males generally had decreased paternity of their broods compared to monogamous males. Paternity of the resident male decreased with increasing population density and breeding asynchrony. The intensity of paternity guards such as within-pair copulation rate and mate guarding were not significantly related to extra-pair paternity. Sperm competition was thus an important component of sexual selection by increasing the variance in male mating success, and by being associated with the expression of secondary sexual characters, in particular in dense and asynchronously breeding populations of birds. Received: 12 February 1998 / Accepted after revision: 31 May 1998     

High paternal investment in unrelated young: extra-pair paternity and male parental care in house martins

The response of males to reduced paternity has important consequences for the evolution and maintenance of a mixed reproductive strategy. Paternity is predicted to affect directly the level of male parental care in some cases but not in others. The response of males to reduced paternity will be influenced by their ability to assess their paternity, the predictability of cuckoldry and the costs and benefits of parental care. Although male house martins (Delichon urbica) provide among the highest levels of male parental care known in passerines (incubation, brooding and feeding nestlings), there was no evidence that cuckolded males substantially reduced their level of parental care, and, as a result, all young fledged successfully. Thus, extra-pair fertilizations enhanced the reproductive success of some males because they were able to parasitize the parental care of cuckolded males. We discuss several conditions which may favor extensive male parental care even when the male's paternity is very low.     

Context-dependent genetic benefits of extra-pair mate choice in a socially monogamous passerine

Extra-pair paternity is common in socially monogamous passerines; however, despite considerable research attention, consistent differences in fitness between within-pair offspring (WPO) and extra-pair offspring (EPO) have not been demonstrated. Recent evidence indicates that differences between maternal half-siblings may depend on environmental conditions, but it is unclear whether the influence of paternal genetic contribution should be most apparent under comparatively poor or favourable conditions. We compared phenotypic characteristics of WPO and EPO in 30 mixed-paternity broods of the tree swallow (Tachycineta bicolor) in relation to experimentally increased nest temperature (n = 13 heated nests; 17 control nests) and natural abundance of haematophagous parasites (Protocalliphora spp.). This allowed us to test the hypothesis that genetic benefits of extra-pair mating are environment dependent. EPO grew their ninth primary feathers faster than WPO regardless of nest temperature or parasite load and had significantly longer ninth primary feathers at fledging when parasite abundance was low, and when they were positioned early in the hatching sequence relative to WPO. In contrast, WPO under similar conditions did not differ from EPO in any phenotypic trait measured. These results indicate that the fitness benefits of extra-pair mating are likely to be context dependent, and that genetic effects on some phenotypic traits may be more apparent when conditions are relatively favourable.     

Sex-specific nest defense in house sparrows (Passer domesticus) varies with badge size of males

According to indicator models of sexual selection, females can benefit from choosing males with above average epigamic traits, but empirical evidence for such benefits is scarce. Here, we report results from an experiment with 29 pairs of house sparrows (Passer domesticus) where the intensity of nest defense against a mounted mustelid predator was related to the size of the black throat and breast patch (“badge”) in males. Using principal components analysis (PCA), original response variables of both sexes were reduced to two factors: “Approach” to the predator and “Distant warning”. “Approach”, the more risky behavior, increased from small- through medium- to large-badged males and decreased in their females. Since large-badged males have a higher certainty of paternity (i.e. greater benefits from defense) and may be older and more experienced (i.e. incur lower costs), the most likely explanation for male defense intensity increasing with badge size is an improving benefit/cost ratio. The resulting optimal response of their females and evolutionarily stable participation in joint parental care is illustrated by a graphical model. It shows that females would, indeed, benefit directly from choosing large-badged males. This, however, is no proof of a direct evolutionary tie between badge size and paternal behavior, as assumed by indicator models of sexual selection. It may simply represent a spurious relationship, originating from the correlation of badge size and defense with confidence of paternity. Received: 22 September 1997 / Accepted after revision: 3 November 1997