In 1931, when A. W. Schorger published The Birds of Dane County, Wisconsin, the American redstart was one of the most common nesting birds in the county. It was, he wrote, “second only to yellow warblers in point of numbers,” and he added that “every woodland contains at least one or more nesting pairs.”
He has discovered that many species of woodland birds are area-sensitive, that the size of a woodland is as important to these species as the kinds, and sizes, of trees and shrubs that grow in it.
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Temple divided his woodland study areas into three size classes: 0-10 hectares, 11-100 hectares, and more than 100 hectares. A hectare is a metric unit of area, and as loyal Americans all we know about the metric system is the size of a two-liter soft-drink bottle, so I should explain that a hectare is equal to 2.47 acres. Of course, as city dwellers, we really don’t have a very clear sense of what an acre is either. To put things in terms we all can understand, a standard Chicago city block, an eighth of a mile on a side, equals exactly 10 acres or roughly four hectares.
We can easily hazard a guess as to why pileated woodpeckers would live only in large woodlands. They are big birds, the size of crows, and presumably need a large amount of space to maintain themselves. But the rest of the big woods species are tiny songbirds, the biggest of them no larger than a house sparrow. They only need a couple of acres as a breeding territory; why does that small territory need to be surrounded by hundreds of acres of additional habitat?
Other edge-bird species have developed defenses against cowbirds. Robins and catbirds, for example, can detect the alien egg in their clutch and remove it from the nest. Yellow warblers will often build a roof over a parasitized nest and build a new nest on top of it. Birds of the forest interior have fewer defenses, because they didn’t have cowbirds to worry about until quite recently.
Dr. Temple thinks that edges produce more insects, and that this food source, combined with the singing perches trees provide, may “seduce” the birds into nesting near the trees.
