Migratory divides are contact zones between breeding populations with divergent migratory strategies during the nonbreeding season. These locations provide an opportunity to evaluate the role of seasonal migration in the maintenance of reproductive isolation, particularly the relationship between population structure and features associated with distinct migratory strategies. We combine light-level geolocators, genomic sequencing, and stable isotopes to investigate the timing of migration and migratory routes of individuals breeding on either side of a migratory divide coinciding with genomic differentiation across a hybrid zone between barn swallow (Hirundo rustica) subspecies in China. Individuals west of the hybrid zone, with H. r. rustica ancestry, had comparatively enriched stable-carbon and hydrogen isotope values and overwintered in eastern Africa, whereas birds east of the hybrid zone, with H. r. gutturalis ancestry, had depleted isotope values and migrated to southern India. The two subspecies took divergent migratory routes around the high-altitude Karakoram Range and arrived on the breeding grounds over 3 weeks apart. These results indicate that assortative mating by timing of arrival and/or selection against hybrids with intermediate migratory traits may maintain reproductive isolation between the subspecies, and that inhospitable geographic features may have contributed to the diversification of Asian avifauna by influencing migratory patterns.

It is common in birds that the sizes of nestlings vary greatly when multiple young are produced in one nest. However, the methods used by parents to establish size hierarchy among nestlings and their effect on parental provisioning pattern may differ between species. In the Azure‐winged Magpie Cyanopica cyanus, we explored how and why parents controlled the sizes of nestlings. Asynchronous hatching was the main cause of size hierarchy within the brood, although the laying of larger eggs later in the laying sequence reduced this effect. Parents with asynchronous broods produced more eggs and fledged more nestlings than those with synchronous broods but their brood provisioning rates, food delivery per feeding bout and feeding efficiency did not differ. We performed a cross‐fostering experiment to synchronize some asynchronous broods. Provisioning rates of asynchronous broods were lower than those of synchronized broods, but the daily growth rates and fledging body mass of their nestlings were not different. Our findings indicate that parents of asynchronous broods can achieve higher reproductive success than those of synchronous broods based on the same parental care, and the same reproductive success as those of synchronized broods based on less parental care. It appears that parent birds can better trade off reproductive success and parental care by establishing a size hierarchy among nestlings.

Parent birds show a continuous spectrum of breeding strategies, ranging from a low-fecundity and high-survival pattern to a high-fecundity, low-survival pattern. Investigations of parental breeding strategies under variable environmental conditions can illustrate how parents trade-off the benefits and costs of these two extreme strategies. White-collared Blackbirds Turdus albocinctus can breed twice a year on the Tibetan Plateau. We show that both life-history traits and parental feeding behaviour differ between these two breeding attempts. In the first attempt, the birds produced small clutches and fledged a small number of nestlings of high body condition. In the second attempt, they produced larger clutches and fledged more nestlings of lower body condition. Males made greater contributions to brood provisioning compared with females in the first attempt but there was no sex difference in brood provisioning in the second attempt. In the first attempt, producing smaller clutches can shorten the nestling period, and the increased male contribution to brood provisioning can protect the energy reserves of females. Thus, females can begin a second attempt sooner and produce larger clutches. During the second nesting attempt, when conditions are warmer and wetter, parents rely on a broader array of food types (both invertebrates and plant material, primarily berries) than during the first attempt, which includes only animal food such as arthropods and annelids. We suggest that this difference in breeding strategies between nesting attempts and sexes is in part influenced by marked seasonal variation in food availability.

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Territoriality is an important male behavioral trait used in mate selection of butterflies, but the factors determining the outcome of territorial contests remain controversial. We studied the foraging and territorial behaviors of males in the Parnassius imperator butterfly. Adult males were selective about nectar sources, but they did not defend nectar flowers. By contrast, they fought over areas of bare rocks because bare rocks were used as lek locations to attract and court receptive females. The ownership of a lek, rather than sex pheromones or wing pigments, played the key role in attracting females. In an owner replacement experiment, the owner of a territory always defeated intruders, indicating that perching order, not relative body size, determined the outcome of territorial contests. Territories are small areas of bare rocks that lack food resources, so intruders do not challenge the ownership of a resident in a location where they can easily find vacant territories. Males that first perch in a territory have an advantage compared to intruders in that they can forage on flowers in the vicinity of the territory. This may help explain why satiated owners always defeated fatigued intruders. Our findings suggest that early perching on bare rock allows male butterflies of P. imperator to occupy a territory, attract receptive females and exclude later-arriving competitors.