BIRDWISE: Examples of avian opportunism
Much is said, and justifiably so, about how habitat loss and alteration has been the primary cause of population decline in many species of birds (and other animals) over the past two centuries or more.
Indeed, the conversion of forests to farmland and urban landscapes in the eastern U.S. led to precipitous declines in species adapted to the forest interior. Similarly, you can imagine how much grassland bird populations have declined, as 99 percent of our native tall-grass prairie is now under massive monoculture cultivation for corn and other crops.
Fortunately, many species are resilient and manage to hang on in their modified habitats. But invariably their reproductive success is reduced as a consequence of increased predation, pollution and other factors that go along with man’s intrusion into the world they have been adapted to for millennia.
A few of the more opportunistic species have managed to take advantage of habitat change, sometimes with benefit to their populations. For example, what did chimney swifts do before there were chimneys? Chimneys seem to be much preferred over natural cavities in trees for these birds. They have thrived in chimneys, at least until people have added protective guards.
Avian generalists, such as chipping sparrows, cardinals and robins, have acclimated readily to backyard habitats. The dense shrubbery often planted around homes provides excellent cover.
One of my favorite examples of birds taking advantage of man-made habitats is the small group of hole-nesting species that have found the steep banks of soil in gravel pit operations to be to their liking. The accompanying photo shows the nesting holes of bank swallows in a gravel operation on County 18, near Dorset, this year. If it weren’t for artificial cliffs like these, it is likely we wouldn’t have any bank swallows in Hubbard County.
This highly colonial species, identifiable by the solid brown band across the breast, is a smaller cousin of the tree swallow, which often nests in bluebird houses. It is found worldwide and its native habitat is deep-cut stream banks, which are hard to come by here. These birds excavate their own nesting cavities and require the proper density of soil to prevent caving and the proper slope (nearly vertical) to discourage predators. These conditions vary from year to year in gravel operations, so the location of colonies will vary accordingly.
Another swallow that occasionally nests in smaller numbers among the bank swallows is the rough-winged swallow. They are even more opportunistic, as they will nest in man-made cavities, such as drainage holes in retaining walls or exhaust pipes in abandoned trucks.
The third species that often nests in these banks is the belted kingfisher. Being a larger species, their holes are easy to recognize because of their much wider entrance. Kingfishers also excavate their own holes and, even though they are strictly fish-eaters, they nest wherever there is a suitable bank – sometimes a great distance from the nearest water.
Check out your nearest gravel operation to locate the nesting burrows of any or all of these species.
Marshall Howe is a retired biologist with the U.S. Fish and Wildlife Service. He specialized in bird population studies. Howe has been a Park Rapids resident since 2010.