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Black and White Warbler

by Laura Brandon, Lesser Slave Lake Bird Observatory

Although many of us are just starting our summer vacations, birds are already on the move during the third week of fall migration monitoring. In total, we’ve banded 826 birds from 34 species so far this season, the majority of which have been Tennessee warblers and yellow warblers. Myrtle warblers, black-and-white warblers, and western tanagers have also begun their long journeys south. While most birds that end up in our nets in the fall are often newly fledged youngsters, many of the adults we’re banding are looking a little ruffled or motely lately. This signifies the end of breeding season and the beginning of the seasonal moult for many species.

Female Western Tanager

From the tiniest hummingbird to the tallest sandhill crane, all birds must systematically replace their feathers at least once a year in order to survive, which is called moult. Over time, a bird’s feathers naturally wear out due to physical use and bleaching from the sun. Feathers are made of keratin, the same protein that our hair and nails are made of. Birds can’t repair feathers if they become damaged, instead, they must be entirely regrown. Moults can either be complete, where the bird replaces every single one of its feathers over the same moult period, or partial, in which the bird replaces only some of its feathers at a time (like flight or body feathers, for example). Chickadees, flycatchers, thrushes, and vireos all undergo one complete moult every year. Tanagers and warblers will undergo one complete moult in the summer after their young have fledged, then an additional partial moult before the next breeding season, which gives them their brilliant breeding plumages in the spring.

Molting Myrtle Warbler

Feathers must be kept in pristine condition to allow for fast and efficient flight, so songbirds need to make sure their feathers are in tiptop shape before embarking on their migratory journeys. Most songbirds will complete their moults on or near their breeding grounds and being their migration shortly after they have a new set of feathers. Others may travel a short distance to another location with more food and resources before they begin their moult.

Although a moulting bird can still fly and insulate itself from cold or wet weather, moulting can be a dangerous. Birds that are replacing their flight feathers may be more vulnerable to predators. It takes a lot of time and energy to grow new feathers, so many birds will lay low to avoid detection while they’re busy moulting. Although the breeding season is finishing up and many birders hang up their binoculars between spring and fall migration, birds are always doing something interesting and moulting is yet another fascinating behaviour to observe in the field