Harsh environmental conditions often impose strong selection on the phenotype of natural populations through impacts on their fitness. For overwintering birds, winter is an important period for survival because the weather conditions in winter is usually severer than in other seasons and birds often suffer more thermoregulation costs while food is in short supply. Thus, the selective effect of weather conditions on phenotype in winter is expected to be strong. In this study, we examined the relationship of overwinter survival of Silver-throated Tits (Aegithalos glaucogularis) with their morphological traits under different levels of winter conditions (winters with and without snowstorms) to explore the differential selective effect of winter weather on their morphology. We found that regardless of whether the winter experienced a snowstorm, the female Silver-throated Tits with a smaller bill surface area or smaller bill depth and a smaller surface area:volume ratio were more likely to survive during the winter, which supported the hypothesis of selection for heat retention. Furthermore, the females with a smaller body length survived better than the larger females, indicating that the lesser food requirements for smaller body sizes may confer advantages during the winter when food availability was reduced. In addition, in agreement with the finding in many short-lived birds that survival rate increases with the increase of age, older (≥2-year-old) female Silver-throated Tits had higher overwinter survival than 1-year-old females. However, we did not find any correlation of morphological traits and age with survival in male Silver-throated Tits. These results illustrate differential selective effects of winter weather on female and male Silver-throated Tits and contribute to the knowledge of sex-specific selection on the phenotype of natural populations.
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Open Access
Research
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Research activities have often been thought to potentially influence avian nesting success by increasing nest predation rates. Although recent studies of species building open nests and cavity nests suggest that research disturbance does not generally induce nest predation, whether it is also the case in species building domednests remains unknown. In birds, domed-nest species exist in about half of the passerine families, and research disturbance to the domed nests may differ from that to the nests of other types for their different nest structures.
We investigated if research activities affected nest predation rate by analyzing the relationships of the daily nest survival rate with the research activities at the egg and nestling stages of a domed-nest species, the Silverthroated Tit (Aegithalos glaucogularis).
Our results showed that nest daily survival rate was significantly affected by the laying date and nest age during the egg stage, and by the hatching date only during the nestling stage. By contrast, there were no significant effects of research activities, in terms of visiting nests and filming nests, on the nest survival of the Silver-throated Tit at both the egg and nestling stages.
Our results coincide with the findings in species building other types of nests that research activities do not always have negative effects on avian nesting success.
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Nest predation is usually the main cause of bird nest failure. 'Predation avoidance hypothesis' of nest-site selection predicts that birds should prefer nest-sites with higher nesting success. We investigated the relationship between nest height and nest fate in Silver-throated Tits (Aegithalos glaucogularis) and asked whether the seasonal change of nest height was adaptive by simultaneously analyzing the seasonal variation of predation rates of the nests.
We monitored nests of the Silver-throated Tit across seven breeding seasons in the Dongzhai National Nature Reserve in Henan Province of China. We compared the difference of nesting success among nests of different heights and analyzed the effect of nesting date on nest height and nest predation rates.
The overall nesting success rate was 24.1% (n = 623). Among the failed nests (n = 472), 59.75% were predated by birds and mammals, followed by snakes (9.11%). The average height of successful nests was significantly lower than that of predated nests. As the season progresses, the height of the nests increased significantly, companied by the decrease in nesting success. Daily survival rate of the nests also decreased with the progression of the season, regardless of breeding stages.
Our findings suggest that nest height is an important factor influencing the nesting success of Silver-throated Tits. However, the seasonal increase of nest height appears inconsistent to the 'predation avoidance hypothesis', because it is associated with higher predation rates. The reason for this maladaptive behavior remains to be explored in future studies.
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