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.

Extrapair fertilizations (EPFs) occur widely in socially monogamous birds and result in mixed parentage in the brood. The response of an individual to these EPFs of its social mate remains poorly investigated in terms of parental care for the mixed brood. We addressed this question in a cooperatively‐breeding corvid, the azure‐winged magpie Cyanopica cyana. Parentage analysis indicated that 45% of females and 37% of males engaged in EPFs. There were 49% of cooperative groups and 36% of bi‐parental nests with extrapair paternity (EPP) offspring, and 22% of cooperative groups and 19% of bi‐parental nests with extrapair maternity (EPM) offspring. Based on the identity of offspring, we classified adults into four types: EPP offspring fathers/mothers, EPM offspring fathers/mothers, cuckolded males/females, and faithful males/females. A comparison of provisioning rates among all four types of breeders showed that 1) EPM offspring fathers had the highest provisioning rate; 2) cuckolded males did not reduce parental care, compared to faithful males and EPP offspring fathers; and 3) females of different types did not differ in their provisioning rates. Our findings suggest that a combination of frequent opportunities, low costs of cuckoldry, and the benefits of establishing a cooperative neighbourhood can explain why EPFs occur frequently in the Tibetan population of azure‐winged magpie.

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