The cowbirds are only one reason for diminishing numbers of birds. It adds stress to other female birds and bird behavior is unpredictable.
...Rather than spending any time building a nest for her eggs, (click here) the female cowbird puts all of her energy into producing eggs and does so at the rate of up to three dozen in a single summer. When she is ready to lay her eggs, she begins to look for nests that other birds have constructed (or have already constructed). When the host bird is not at the nest, the female cowbird slips in and pushes one of the eggs out of the nest and replaces it with her own, referred to as "nest parasitism". The baby cowbird is often raised by the unsuspecting host bird, who becomes a foster parent through no fault of her own.
Should the host bird destroy the cowbird's egg, the cowbird will often retaliate by destroying the entire nest in what many people have referred to as "mafia behavior."...
Food sources for birds may be finite in any specific community. Imagine robins hatching six eggs a season opposed to twelve eggs by the Cowbirds, who is going to survive more often in any season?
Cowbirds are not the first thing I look for when bird numbers trend down in numbers, there are food sources and habitat changes to examine, but, if there is a population of Cowbirds, that could be the problem.
