The effects of annual cycle stages and life-history traits on feather abrasion.

Tissue wear in animals occurs consistently and can impair their functionality. In birds, abrasion - which reduces the feathered surface area - is caused by various factors. However, little is known about the underlying mechanisms, patterns and immediate consequences of feather abrasion. Given the lack of reliable and widely accepted methods for quantifying feather abrasion, we present the Minimum Convex Abrasion Area (MCAA), a novel approach for its measurement. This method facilitates the estimation of feather abrasion in both museum specimens and living birds, enabling an exploration of the factors influencing abrasion across different phases of the avian annual- and life-cycle. We analysed 283 feathers from 21 passerine species, revealing that the highest rate of feather abrasion occurs during the breeding season, with a significant reduction in feather surface area compared to other periods of the annual cycle. Surprisingly, migration periods, despite their high metabolic demands and long-distance movement, do not significantly contribute to feather abrasion. The timing of feather moult plays a pivotal role in influencing abrasion rates during the breeding season, with species that undergo moult nearer to the breeding season, during winter (pre-breeding moult), showing significantly lower abrasion rates than those moulting before the autumn migration (post-breeding moult), likely a result of the weakening of the feather structure over time. These findings highlight the importance of moult timing for feather abrasion rate and maintaining plumage integrity during the reproduction season. The application of the proposed abrasion measurement method is expected to facilitate better understanding of plumage function and properties, as well as bird moult strategies, thereby enhancing our comprehension of avian ecology, biomechanics, life-history traits and sexual selection, as well as their evolution.