The question of why socially monogamous females engage in extra-pair behaviour is long-standing in evolutionary biology. Recent theoretical work has moved away from the indirect-benefits hypothesis to explain female extra-pair behaviours, instead favouring suggestions that they are the result of pleiotropic effects. That is, a trait under strong positive selection in either or both sexes are genetically linked to another, often unrelated, trait. For example, where genes beneficial to female fecundity (contributing to within-pair solicitation of her social partner) are linked with extra-pair behaviour (soliciting copulations from extra-pair males). Here, we test two predictions from this hypothesis: We test the prediction that female divorce, measured from the number of social mates within a given year, is linked with (1) the number of extra-pair males and (2) the proportion of her offspring that are extra-pair. Our results suggest that females who frequently divorce social partners are more likely to produce extra-pair offspring than those who maintain social monogamy. However, by contrast, those females do not also have a higher proportion of extra-pair offspring. The number of broods initiated was also positively correlated with extra-pair males, probably through increased opportunity for extra-pair males to sire offspring over a longer breeding season. Our results provide an empirical example of a behavioural trait, beneficial to female fecundity, that is linked with extra-pair behaviour. These empirical results support the intrasexual pleiotropy hypothesis as a driver of female extra-pair behaviour.

As a result of a warming global climate, understanding how organisms adjust their behaviour to environmental thermal conditions has become an increasingly important question in animal biology. Temperature-driven adjustments in parental care are potentially important due to their repercussions on offspring size, quality and survival. In 2015 and 2016 we monitored 70 zebra finch (Taeniopygia castanotis) breeding attempts in the wild. We recorded the frequency of parental visits to the nest together with mean maximum ambient temperature experienced between day 7 and 14 of the nestling period. We found that for each increase of 1 °C in the daytime temperature there was a 1% reduction in the hourly rate of parental visits. Our data suggest that nestlings may receive less food under thermally challenging conditions, which is consistent with recent studies that demonstrate offspring are smaller when reared during periods of high temperature. Understanding the behavioural drivers that may contribute to the production of smaller offspring in the heat could prove useful to forecast long-term consequences for fitness triggered by climate change.

Bird song is crucial for attracting mates and defending territories, but different types of song or different singing behaviours may be involved in acquiring or maintaining each resource. Furthermore, male songbirds may adjust when and where they sing throughout the breeding season, depending on their breeding stage. However, such relationships remain untested in several avian taxa. Here, we studied male Bermuda White-eyed Vireos (Vireo griseus bermudianus), a passerine with two distinct song types (discrete and rambling), to test the mate attraction, territory defence, and nesting stage hypotheses. We compare song rate and song perch height among different stages of the breeding season and during the non-breeding season. We show that male vireos produce both song types during the breeding and non-breeding seasons, suggesting dual roles in mate choice and territorial defence. Singing rate did not differ between the two seasons, but, within the breeding season, males without nesting duties had significantly higher song rates than males with nesting duties. Song rate was lowest during the nestling stage, which coincided with the highest rate of nest predation. Song perch height was higher during the breeding season versus non-breeding season, among males without nesting duties compared to males with nesting duties, and when males produced discrete versus rambling songs. Our findings suggest that male vireos increase their conspicuousness to prospecting females by increasing singing rate and song perch height, and that they sing during the breeding and non-breeding seasons to defend year-round territories. Collectively, our study supports the mate attraction and territory defence hypotheses of bird song and suggests that Bermuda White-eyed Vireos adjust their singing rate in response to nest predation risk.