Influences of feather wear and age on wing lengths of Blue Tits Cyanistes caeruleus

Selective effect of winter weather on a songbird's morphology depends on individual sex and winter condition

Knowledge of the effect of harsh weather on the phenotypic traits of organisms is essential for understanding the environmental influence on phenotype evolution and holds implications for predicting how species respond to current climate change. For many birds, harsh weather in winter often imposes a strong selective effect on their survival, and only the individuals with certain phenotypes may survive. However, whether the selective effect on phenotype varies with winter weather conditions has been poorly investigated. Here, we explored the selective effect of winter weather on black-throated tit's (Aegithalos concinnus) morphological traits under winters with and without severe snowstorms. We found that for males, the sizes of their bills, heads and wings significantly affected their overwinter survival, but the effects varied with winter conditions. In relatively benign winters, males with smaller bill depths, smaller bill surface areas, and greater head lengths survived better; whereas, in winters with severe snowstorms, a reverse pattern was found. This phenomenon was likely driven by selection pressures from heat retention and foraging requirements, with their relative importance depending on winter conditions. Additionally, wing length was positively correlated with male survival and the relationship was stronger in harsher winters, which was probably due to longer wings' higher flight efficiency in adverse weather. By contrast, we found no correlation between morphological traits and survival in females. These results suggest a sex-specific and condition-dependent selective effect of environment on bird phenotypes, implying complicated interactions between different selection pressures and phenotype evolution.

Integrity of and damage to wings, feather vanes and serrations in barn owls

The silent flight of owls is well known. It has served as role model for the designs of new airplane wings and ventilators. One of the structural features that underlies silent flight is the serrated leading edge of the wing that is mainly formed by the tenth primary flight feather (P10). We examined here how much the wings, the P10 feather and the serrations in different populations of barn owls reflect the intact situation. First, when the P10 feather molts, no or fewer serrations are present. Furthermore, damage to feathers and serrations may occur. Damage may be due to several factors like broken feather tips, lost rami, barbules, or broken tips of serrations. The latter may cause a narrowing of the outer vane of the P10 feather. We quantitatively assessed damage by counting the number of wings with missing or broken primary feathers, the number of wings with a narrowed outer vane of the P10 feather, and the number of serrations with reduced length. Considerable damage occurred on wings and feathers on both the macroscopic and microscopic levels. The observed damage most likely influences flight performance. More damage occurred in Galapagos barn owls than in North American and European barn owls. The Galapagos population may be more vulnerable than the other populations because it may at least temporarily be in a bad nutritional state and, thus, postpone molt.