Friday, May 12, 2017

Tree Swallows: Iridescence on the Wing

Swallows of all species are beautiful, graceful birds, swooping acrobatically through the air as they catch insects on the wing.  Tree Swallows (Tachycineta bicolor) are one of our most flashy birds, with their iridescent plumage contrasting with clean white fronts.  In most places, swallows migrate south for the winter, only returning to breed in the summer; but here in California's Great Central Valley, Tree Swallows and similar Violet Green Swallows (Tachycineta thalassina) stick around all year.  Still, they are probably most visible in the warmer months.  I enjoy watching swallows every chance I get, and have been happy to see quite a few Tree Swallows out and about recently. 

Tree Swallows nest in tree cavities, as well as nest boxes.  Because they take so readily to nest boxes, researchers have been able to study the ecology of Tree Swallows in detail.  Like other swallows, Tree Swallows feed primarily on flying insects, which they snatch out of the air with great agility.  Swallows are one of nature's great pest control agents!  They will also consume plant matter, especially in the winter when insects are scarce.  Because of the large amount of insects they consume, Swallows are exposed to high levels of pesticides that bioaccumulate in the ecosystem; these harmful chemicals become concentrated in the bodies of the birds. 

Nests of grass and other soft plant material are placed in cavities, usually excavated by woodpeckers in trees.  Introduced species like the aggressive European Starling compete with our native cavity nesters for suitable nesting sites, and typically succeed in driving out the native species.  Standing dead trees, or snags, provide valuable habitat for Tree Swallows. 

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