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.

Moult Terminology: Envisioning an Evolutionary ApproachThe study of avian moult has been inhibited not only by its complexity but by convoluted and often conflicting terminologies that have combined to cloud the subject. Over time, two nomenclatures have emerged with differing bases of definition. The ’life-cycle’ system is used widely in European literature (e.g., Cramp 1988, Jenni & Winkler 2020) and defines moult terms based on timing relative to current-day life-history events, primarily breeding. Conversely, the Humphrey-Parkes or ’H-P’ system (Humphrey & Parkes 1959, as modified by Howell et al. 2003), is used more widely in the New World and Australia (e.g., Higgins & Davies 1996; Pyle 2008, 2022a; Howell 2010), and defines terms based on the principle of how moults have evolved along avian lineages (Pyle 2022b).Recently, Kiat (2022) proposed that moult terminology be simplified, and advocated use of the life-cycle approach as a more understandable system to those not familiar with moult terminology. We appreciate and endorse Kiat’s (2022) plea for a simpler moult terminology, and we empathize with his frustration, but we disagree with his primary conclusion. We argue that if one intends moult to be a subject for study, then a standardized terminology based on the best scientific evidence is preferable to a system that, while perhaps more widely understood among ornithologists and the general public for passerines in boreal regions, is often imprecise or ambiguous and is difficult to apply to the majority of avian taxa on a global basis (Higgins & Davies 1996, Howell et al. 2004, Johnson and Wolfe 2018, Kiat 2022, Pyle 2022b).The debate over the best moult and plumage terminology to use has persisted since the H-P system was proposed by Humphrey and Parkes (1959) over 60 years ago (e.g., Howell et al. 2003, 2004; Jenni and Winkler 2004, 2020; Kiat 2022; Pyle 2022b; and references therein). At this point, certain opinions on which system should be favoured are calcified, and we do not wish to belabour the details yet again. In our experience, however, newer students grasp the H-P system quickly, whereas older ornithologists—including ourselves—who first learned life-cycle moult terminology, often have initial difficulty envisioning the H-P system’s evolutionary approach. Our goal with this perspective is to propose and illustrate how moults may have evolved in birds, with the hope that it will assist future ornithologists to envision and appreciate the H-P system.Adhering to the evolutionary H-P approach, we propose considering the prebasic moult, and perhaps the preformative moult, as ancestral to all modern bird lineages, having evolved from reptiles (Howell & Pyle 2015, Pyle 2022b; see also Kiat et al. 2020, 2021; Figs. 1, S1). The complete or near-complete prebasic moult occurs in all current-day avian taxa and, rather than simply a replacement of feathers, appears to be part of an endogenous restoration of body tissues (Voitkevich 1966, Murphy 1996, Kuenzel 2003) that may be ancestral to all vertebrates (King 1972). Considering the prebasic moult as homologous among modern birds is thus a parsimonious hypothesis, providing a robust nomenclatural framework that can be applied to all bird moults and plumages. The preformative moult also appears to be present in most if not all modern bird lineages, and may have evolved in reptiles as body size developed quickly in the first year of life; if not, it evolved early in avian evolution as it is found in most or all basal lineages (Figs. 1, S1). Assuming homology among all modern bird taxa, the prebasic and preformative moults can provide the framework for defining all subsequently evolved moults within the H-P system (Figs. 1, S1). Additional inserted moults, including prealternate and presupplemental moults, can evolve (both appearing and disappearing) along bird lineages such that, unlike prebasic and preformative moults, they should not be considered homologous across all birds (Howell et al. 2003, Johnson and Wolfe 2018, Pyle 2022b).Once an evolutionary basis is appreciated, moult strategies become substantially easier to compare across all species and latitudes. For example, the four underlying strategies identified by Howell et al. (2003), which are defined by the number of moults that occur within the first and later moult cycles, can be provisionally placed in an evolutionary context (Fig. 1; see Supporting information file and Fig. S1 for details). Although it is quite possible that inserted moults may have disappeared without trace along some bird lineages, envisioning how these four strategies may have evolved for current taxa has the potential to help inform a greater appreciation for the adaptive causes of inserted moults (Figure S1).We contend that the evolutionary (H-P) system is more applicable on a global basis for studying avian moult than is the life-cycle system, and we thus encourage those who use the life-cycle system to also attempt visualizing an evolutionary approach to moult terminology, rather than trying to simply synonymize H-P terms with life-cycle terms. We suggest first determining the prebasic moult cycle, then whether or not inserted moults occur in the first and/or later cycles, and using an evolutionary approach to infer the correct designation of each inserted moult. Once envisioned, learned, and appreciated, the evolutionary approach proposed by Humphrey & Parkes (1959), as modified by Howell et al. (2003), is scientifically more precise, allows the recognition of all inserted moults, and is easier to apply consistently, to all taxa and by all parties interested in the study of moult in birds. We provide more detail on taking an evolutionary approach in the Supporting information file.

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.

The Nanling Mountains are an important mountain range and watershed in southcentral China. Because of the abundance in relictual plant species and geological stability throughout the Cenozoic, the mountains are considered a “museum” of subtropical biological diversity. With respect to birds, however, the roles of the Nanling Mountains in impeding the dispersal of the subtropical birds and, as a result, shaping their population and community structures have received little consideration. To examine these roles, we compiled and analyzed two datasets. (1) To test the mountains’ influence on gene flow, we undertook a population phylogeographic study comparing mitochondrial COI and Cytb DNA sequences of 5 sylvioid resident bird species of the mountains (Huet’s Fulvetta Alcippe hueti, Red-billed Leiothrix Leiothrix lutea, Greater Necklaced Laughingthrush Pterorhinus pectoralis, Indochinese Yuhina Staphida torqueola, and Mountain Bulbul Ixos mcclellandii). (2) To examine differentiated community development over the history of modern birds, we examined distributional data of all species of the Nanling region using public species occurrence records. For part (1), we sampled 327 individuals from 36 sites and conducted correlation analysis of genetic and geographic distances, taking into account the landscape of the mountains. We found that the mountains do not seriously impede gene flow among populations but influenced species differently. For part (2), comparative analysis of 446 species in 81 families indicated that family membership influenced the community composition of birds in Nanling region. Variation in family distributions is attributable to both environmental and evolutionary factors. Overall, we found that the Nanling Mountains are not currently a substantial barrier to gene flow among the species we studied but act as a corridor and refuge for these birds. However, analyses on higher ranked community data suggest the mountains acted as a barrier in older times, corresponding to the known diversification events in southeast Asian avifauna.

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.