The past two days in Toronto have been warmer than normal, leading to a large snow melt and the promise of spring (and I know it’s a tease, but it’s a nice break from what has felt like an especially cold winter). With spring comes the return of migratory birds, so this research seems relevant.
First, some background. Feathers, unlike our bones, lack an ability for self-repair. They can be attacked by feather-loving bacteria1 and suffer from UVB damage (and sunblock doesn’t really help out). So birds moult, or replace feathers, up to twice a year. According to my Sibley Guide to Bird Life and Behaviour, most North American birds replace feathers in a complete moult in the late summer or early fall. Some birds undergo a second moult in the spring, before the breeding season.
Birders out there have experienced these two moults when faced with confusing fall warblers: males no longer are in their colourful, easy-to-identify plumage (which appeared during the spring moult) and we humans suffer under the burden of having to learn, what could easily be described in some cases, as a whole new bird.
Why there is a difference in moults is the subject of this research by Thomas P. Weber, Johan Borgudd, Anders Hedenstram, Kent Persson & Garan Sandberg. Comparing the mechanical strength of flight feathers from two species of similar birds that have different moulting strategies, researchers found that the bird species that moults twice2 have comparatively weaker (more prone to fatigue) feathers than the species that moults once. Since the authors suggest that “the species with feathers that fatigue faster moults twice annually and not once” it would seem the willow warblers moult out of a form of necessity: they would suffer from the consequences of weakened feathers as they migrated to or from their breeding territory.
Now, if this feather weakness is a similar case for birds of the Americas that moult twice, it’s interesting to note that Wood Warblers (Family Parulidae) also change the colour of their plumage with the two moults. Because of this breeding plumage / winter plumage dichotomy, it would seem that feather weakness isn’t the only reason why a bird would moult twice. The interesting question would be if the adaptation to two moults a year “allowed” wood warblers to have a colourful breeding plumage–the ability for males to “change” their appearance for the breeding season and visually demonstrate just how fit they are.