Quantifying the effect of feather abrasion on wing and tail lengths measurements

Abstract

Wing and tail length measurements are important tools in ornithology. Amongst linear measurements, wing length has been considered to be the best indicator for body size for taxonomy and ecomorphology, as well as for studies about the impact of climate change on morphology. As feathers are dead tissue, abrasion will lead to a reduction in wing and tail length within a moult cycle. The aim of our study was to analyse the effect of feather abrasion on wing and tail length in the Stonechat Saxicola torquata. Generalized additive models revealed that wing lengths and tail lengths decrease significantly between the termination of feather growth and the next moult. The decrease in wing length was faster with increasing feather age. The decrease in tail length was nearly linear through time. Multiple measurements of recaptured individuals revealed a similar decrease in wing length to that observed in analyses based on single measurements of multiple individuals. An analysis of the length of the third outermost primary revealed the same pattern. Hence, the decrease in wing and tail length over time was caused by within-bird changes and not by mortality, emigration or immigration associated with wing and tail length. We found that feather abrasion was more pronounced in females compared to males at least during the breeding season, but there were no strong indications that feather abrasion was more pronounced in birds before their first complete moult compared to older individuals. A review of previous studies showed that a reduction in wing length of about 0. 2-0. 5 %/month is a common phenomenon. Our study shows that feather abrasion must be taken into account when analysing time-series of wing and tail length measurements to avoid spurious conclusions. © 2012 Dt. Ornithologen-Gesellschaft e.V.