The explanation for extra-pair mating in female birds remains poorly understood and contentious [1-7]. Several leading hypotheses propose that females benefit indirectly by enhancing the genetic quality of their offspring, through good genes or genetic compatibility effects [1, 8, 9]. Supporting this idea, recent studies have identified a range of fitness-related traits for which extra-pair offspring (EPO) are superior to their within-pair (WP) half-siblings [10-21]. However, such performance differences may result from nongenetic maternal effects if EPO are positioned earlier in the laying order and benefit from the advantages of earlier hatching [22, 23]. Here we show that EPO are larger, heavier, and more likely to fledge than their WP half-siblings in a population of blue tits, Cyanistes caeruleus. However, extra-pair paternity declined markedly with laying order, resulting in EPO generally hatching earlier. After correcting for variation in hatch time, none of the observed disparities between EPO and their WP half-siblings remained significant. These findings indicate that phenotypic comparisons between maternal half-siblings must consider potential hatching-order effects and suggest that the evidence for genetic benefits from extra-pair copulation may be less compelling than currently accepted. Moreover, the overrepresentation of EPO early in the laying order may help explain female extra-pair mating.

In many animals, males contribute substantially to caring for their young but also have the opportunity to enhance their reproductive success by attracting additional mates or by seeking copulations with females that are already paired to other males. Sometimes, the opportunity to gain these additional matings coincides with periods when males are providing parental care. At such times, males might be expected to allocate time and effort to these alternative behaviours in a way that maximizes their overall reproductive success. But do they? Here, we examine the recent evidence for a tradeoff between parental effort and additional mating effort and highlight some of the factors that might influence how this conflict is resolved. We conclude that, in spite of the paucity of comprehensive studies, this tradeoff has a potentially important and often overlooked influence on parental behaviour in a range of taxa.

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Summary 1.We examined the influence of helpers on reproductive success in the apostlebird using data gathered over three breeding seasons (116 nests, 34 breeding groups). Initially, annual productivity and individual components of reproductive success were examined in relation to group size (two to 17 birds). The effect of helping per se was then examined in a reduced data set by comparing (a) the predictive power of group size and actual helper numbers, and/or (b) the productivity of groups that changed in size between nests or seasons. 2. Group size was positively and linearly related to annual fledgling production and the number of offspring that survived to the following season. This was attributed to the presence of helpers, as the number of birds that fed nestlings, but not group size, was significantly related to annual fledging success. Neither group size nor helper number influenced fledgling survival over the subsequent winter. 3. The probability of re-nesting after a successful first brood increased with group size. Among double-brooded groups, the interval between nests was inversely related to group size. 4. Clutch size increased linearly with group size among groups with two to five birds, but was not related to group size among larger groups. A matched comparison of groups that changed size between years further suggests that clutch size was facultatively adjusted to group size, a phenomenon rarely reported in species with non-breeding helpers. 5. Hatching success was not related to group size or the number of birds participating in incubation. 6. The number of birds provisioning the brood was a better predictor of feeding rate per chick than group size, and was significantly and inversely related to the incidence of nestling starvation. However, there was no relationship between group size and fledgling body mass. 7. Fledging success per brood increased linearly among groups with three to five members. The only case of pair-breeding was unsuccessful, suggesting that this species is one of only a few birds that may be obligately cooperative. 8. We found no evidence that helpers influenced the likelihood of nest predation, or the probability of group members surviving to the following season.

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Females of some bird species have a high degree of control over the sex ratio of their offspring at laying. Although several mechanisms have been put forward to explain how females might control the sex of their eggs, virtually nothing is known. As females are the heterogametic sex in birds, adjustment of the clutch sex ratio could arise either by pre- or post-ovulation control mechanisms. The Seychelles warbler (Acrocephalus sechellensis) exhibits extreme adaptive egg sex ratio bias. Typically, warblers produce only single-egg clutches, but by translocating pairs to vacant habitat of very high quality, most females were induced to produce two-egg clutches. Overall, females skewed clutch sex ratios strongly towards daughters (86.6%). This bias was evident in the first egg, but critically, also in the second eggs laid a day apart, even when all absent, unhatched, or unsexed second eggs were assumed to be male. Although a bias in the first egg may arise through either pre- or post-ovulation mechanisms, the skew observed in second eggs could only arise through pre-ovulation control. Post-ovulation adjustment may also contribute to skewed hatchling sex ratios, but as sex-biased release of gametes is likely to be a more efficient process of control, pre-ovulation mechanisms may be the sole means of adjustment in this species. High fitness differentials between sons and daughters, as apparent in the Seychelles warblers, may be necessary for primary sex ratio adjustment to evolve.

The pace-of-life syndrome (POLS) hypothesis for animal personality proposes that variation among individuals in life-history strategies is associated with consistent differences in behavior. We tested predictions of this hypothesis in the superb fairy-wren, Malurus cyaneus, by investigating long-term individual differences in risk-related behaviors (latency to enter a novel artificial environment, exploration, activity and response to mirror image stimulation) and survival. We found consistent differences between individuals in these behaviors (adjusted repeatability of exploration of artificial novel environment = 0.37). Individual differences were consistent over several years and bi-variate analyses showed a significant among-individual correlation ("behavioral syndrome") between exploration behavior at two life stages (young adult and old adult). Docility at the nestling stage predicted exploration behavior of juveniles. Behavioral traits measured in a risky context were correlated with one another, forming a behavioral syndrome of coping strategies ranging from "proactive" to "reactive." Nestlings that were more active and exploratory in isolation were less docile during handling, while adults that entered the artificial environment fast were more exploratory, active, and aggressive in the artificial environment. Exploration behavior increased within individuals as they aged and when they were in poorer condition, consistent with expectations of more risk-prone behavior with lower residual reproductive value (RRV) (reduced "asset protection"). Risk-related behavior predicted the probability of apparent survival: more exploratory individuals were less likely to be present in the population 12 months later. Our findings suggest that, consistent with the predictions of the POLS hypothesis, individual variation in survival is associated with consistent individual differences in risk-related behavior that are maintained long-term and span developmental boundaries.

Following the discovery that extra-pair fertilizations are common in many birds, it has been predicted that male participation in parental care may be in£uenced by their opportunity for extra-pair copulations. However, such a trade-o¡ between male contribution to parental care and the availability of fertile females has not been con¢rmed. Here we use a novel remote monitoring technique to show that participation in incubation by male fairy martins, Hirundo ariel, declines with the increasing availability of fertile females in the breeding colonies. Furthermore, male contribution to incubation is most responsive to change in the availability of fertile females in the early morning, when most copulations occur, and also if their clutches are smaller than average. Both of these patterns support the presence of a trade-o¡ between parental and extra-pair copulation e¡ort. We suggest that this trade-o¡ may be widespread among the 90% of bird species where males contribute to parental care.