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Stopover sites are critical for refueling and resting by migrating birds. Clarifying the habitat requirements of migratory birds during stopover is important for understanding migration ecology and for conservation management. Habitat use by migratory birds at stopover sites, however, has been inadequately studied, and individual variation in habitat use among species is largely unexplored. We tracked the movement of migrating Whimbrels, Numenius phaeopus, using Global Positioning System–Global System for Mobile Communication tags at Chongming Dongtan, an important stopover site in the South Yellow Sea, China, in spring 2016 and in spring and autumn 2017. Multinomial logistic regression and multimodel inference were used to detect the effects of the individual bird, the diel factor (day vs. night), and tide height on the habitat use by Whimbrels during the stopover. The activity intensity of Whimbrels was lower during the night than during the day, while the maximum distance that tagged Whimbrels moved was similar between day and night. The saltmarsh and mudflat were intensively used by all of the individuals in all three seasons: > 50% and 20% of all records were obtained from the saltmarsh and mudflat, respectively. Habitat use significantly differed among individuals; the farmland and woodland were used by some individuals in spring 2016, while the restoration wetland near the intertidal area was used by some individuals in 2017. In general, the saltmarsh, farmland, and woodland were more frequently used in the daytime, while the mudflat was more frequently used at night. As tide height increased, the use of the mudflat decreased while the use of the saltmarsh increased. The results suggest that individual-based bio-tracking can provide detailed data on habitat use both during the day and at night. Differences in habitat use among individuals and periods highlight the importance of diverse habitats for bird conservation.

Sex ratio is a fundamental concept in evolutional biology, and theory predicts that parents should invest in sons and daughters according to the fitness returns they expect from them. The fitness returns may depend on the timing of breeding and on parental conditions leading to sex ratios that depend on breeding date and/or parental quality. Here, we investigate the offspring sex ratio in a small shorebird, the Kentish Plover Charadrius alexandrinus, in a large breeding population in Eastern China, and test whether the parents adjust their offspring’s sex in response to hatch date, brood age and their own body condition. Using 1264 chicks from 676 broods that were molecularly sexed, we show that hatchling sex ratio was not significantly different from unity. Hatchling sex ratios were not related to hatch date or to the body condition of parents. In addition, we sexed 138 eggs that were confiscated from illegal egg collectors and found that the mortality of female and male embryos was not significantly different. The latter result is important by suggesting that neither primary sex ratio (i.e., at conception) nor secondary sex ratio (i.e., at hatching) is biased. Taken together, the even offspring sex ratio in Chinese Kentish Plovers is consistent with recent analyses of six plover populations that found even sex ratios at hatching. Future works should investigate whether the even sex ratio persists into adulthood, or it may shift toward more males (or females) due to sex-biased mortalities of juveniles and/or adults.

The Chinese Penduline Tit (Remiz consobrinus) is a small passerine that breeds in Northeast Asia. Despite its common appearance and its high encounter rate, the breeding biology of this species has remained largely unknown. In this paper, we describe for the first time the breeding biology and parental care system of the Chinese Penduline Tit in the coastal area of Northeast China. As a migratory species of birds, it arrives at the Liaohe Delta in late April, the males establish territories in early May, and breeding pairs build their nests from May to July. Most nest building occurs from mid-May to early June. The females produce small white eggs (egg mass, 1.0 ± 0.1 g) and a clutch size of 6.8 ± 0.6 eggs. The parental care system is complicated in Chinese Penduline Tits: uniparental care is dominant (80%) in this population, with a majority (71%) of female-only care and 9% male care for all brooding nests, together with 4% biparental care and 16% biparental desertion. Chicks hatch after 13.9 ± 1.0 days of incubation and fledge after 22.1 ± 1.0 days. The hatching success and nestling survival are 86.7 and 80.6%, respectively. Furthermore, breeding failure is 23.4%, which is commonly caused by predation and nests blown away by strong winds. Compared to other species of the Remiz genus, the Chinese Penduline Tit shows much similarity to the Eurasian Penduline Tit (Remiz pendulinus) but large differences compared to the White-crowned Penduline Tit (Remiz coronatus) with respect to breeding habitat, breeding density, nest site selection and parental care system. Moreover, the good nest attributes, relatively high nestling survival and low risk of nest failure may contribute to the prevalence of the uniparental care system in this Chinese Penduline Tit population.

Incubation is an energetically demanding phase of avian reproduction. Single-sex incubators are often faced with a direct conflict between incubation and foraging. We report an unusual incubation behavior of the Sichuan Partridge (Arborophila rufipectus), with long recesses of females and embryonic hypothermia, at Laojunshan National Nature Reserve, southwest China. The Sichuan Partridge is a globally endangered species that inhabits subtropical evergreen and deciduous broadleaf forests at 1100–2250 m a.s.l. Female Sichuan Partridges incubated eggs alone, and typically took one long incubation recess of 4.5 ± 1.2 h (mean ± SD) day−1. They left the nests at 7:36 a.m. (±50 min) and returned at 12:06 p.m. (±70 min). Nest attentiveness was 81.2 ± 5.2% of the entire incubation period. Females prolonged the recess duration and decreased nest attentiveness significantly in response to high disturbance risk and bad weather. Although eggs experienced ~4.2 h below 26 °C (the physiological zero temperature) for each recess, the total hatching rate was 88.4%, suggesting that embryonic hypothermia had minimal negative effect on the hatching rate. For Sichuan Partridge, the ability to withstand embryonic hypothermia may be an adaptation to cold montane environments.

Choice of mating partners may affect fitness. Both phenotypic and genetic traits have been shown to play roles in the mating processes of animals. We investigated the roles of phenotypic and genetic characteristics in the patterns of social mating in the Silver-throated Tit (Aegithalos glaucogularis), a sexually monochromatic species that exhibits sexual size dimorphism, rarely sings, does not occupy territories, and has a relative low level of extrapair paternity. To explore the role of phenotype traits, we tested for assortative mating based on the sizes of seven morphological traits. To explore the role of genetic traits, we tested for assortative mating with respect to genetic heterozygosity (the heterozygous mate hypothesis) and whether birds mated with genetically dissimilar individuals (the compatible mate hypothesis). We found significant correlations between paired individuals for bill length and body length, indicating possible assortative mating based on these two traits. In contrast, genetic heterozygosity was not correlated between paired individuals, and the mean relatedness of the mates was not significantly different from that of randomly mated individuals, which do not support the idea that Silver-throated Tits assortatively mate with heterozygous individuals or choose genetically dissimilar mates. Also, individual heterozygosity was not reflected in the measured morphological traits, as no correlation was detected. Neither the individual heterozygosity nor the relatedness between mates was correlated with reproductive performance measures, including clutch size, brood size, and number of fledglings. However, we found that clutch size increased with female body length, which could explain the benefit to males of mating with larger females. Taken together, while our current data failed to provide evidence for an effect of genetic characteristics on the social mating pattern of Silver-throated Tits, the results suggest that phenotypic traits are likely associated with their mating pattern.

The explanation for extra-pair paternity (EPP) variation continues to attract research interest in the area of avian breeding systems. Comparisons among closely related species are recommended to explore factors that affect EPP rate variations. We utilised microsatellite markers to investigate extra-pair paternity in two sympatric populations of congeneric Black-throated Tits, Aegithalos concinnus, and Silver-throated Tits, A. glaucogularis. The two species share a similar breeding period at our study site and have helpers during breeding at some nests. The results suggest that the two species had low levels of extra-pair paternity: 5.7 % of the offspring in the Black-throated Tits and 6.8 % of the offspring in the Silver-throated Tits. The proportion of broods with extra-pair paternity in the Silver-throated Tits (33.3 %) was higher overall than the Black-throated Tits (17.2 %). This difference is consistent with the prediction by their different male attendance of incubation, but failed to be explained by major ecological factors. On the one hand, we conducted the study at the same site during the same time, which therefore limited the potential effect of different environmental conditions and habitat on their EPP incidence. On the other hand, other factors such as the inter-nest distances failed to explain the observed pattern. Current results also could not rule out an effect of evolutionary history on the EPP patterns of these two related species. Our research on their cooperative breeding system found a low proportion of cooperative breeding nests with offspring sired by helpers (Black-throated Tit, 8.3 % and Silver-throated Tit, 25.0 %), indicating that a direct reproductive benefit is not a likely major reason for the existence of the helpers in these two species.

Although knowledge of breeding biology is fundamental for investigating the variation and evolution of avian reproductive strategies, research on Asian Aegithalos tits remains limited. The breeding biology of sympatric populations of Black-throated Bushtits (Aegithalos concinnus) and Silver-throated Bushtits (A. glaucogularis) was studied in the Dongzhai National Nature Reserve in central China from 2007 to 2010. Both species produced one brood annually. Clutch sizes, brood sizes and nesting success were not different between species. Parents of both species participated in all breeding activities, except for the male Silver-throated Bushtits, which were not involved in incubation. Cooperative breeding behaviours were observed in the studied populations of both species. Most helpers were found at the provisioning stage, during which time 1–2 helpers were present at 20% of Black-throated Bushtit nests (n = 50) and 30% of Silver-throated Bushtit nests (n = 40). All marked and confirmed helpers of both species were males. Compared with other Aegithalos species and populations, differences in either the occurrence of helpers or the proportion of nests with helpers indicate that the breeding strategies may vary with species and populations, and therefore, more efforts focusing on their basic biology and ecology are required.