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"Don't confuse the fact of predation with the effect of predation." These are important words.
There are some ecological truths that must be understood if one is to wisely manage wild birds. By this, I mean putting up nest boxes for Eastern bluebirds.
It is important to know and remember that a habitat — a backyard or an acre or two — will support only a certain number of bluebirds. When the limit is reached, and the drain on the available natural food supply is pushed beyond limits, nature, in the form of predation, takes over.
By predation, I mean natural predation such as snakes, raccoons, tree swallows, jays, house wrens, hawks; owls, chipmunks (yes, chipmunks!), squirrels (both gray and flying), and parasites in the form of flies that lay their eggs on nestlings. On hatching, the larvae feed on the nestlings on hatching and eventually kill them. Microscopic parasitic mites also prey on nestlings and often kill them.
It is important to remember that cats, house sparrows and European starlings are not natural predators. These predators, introduced by man, have had a negative impact on bluebird populations, and it was because of this that an urgent message went out in the mid-1970s to put up nest boxes in suitable habitat and monitor them.
Monitoring, in this case, meant removing nests, eggs and even young of house sparrows. Starlings can be kept from nest boxes by making the entrance hole exactly 1 1/2 inches in diameter.
Bluebirds, being adaptable survivors, responded, and the population of Sialia sialis has made an excellent comeback. No matter how much we love bluebirds for their beauty and seeming gentleness (an illusion — bluebirds are very
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