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Female cowbirds prefer wooded edges and small woodlots for finding host nests and can lay up to 77 eggs a season.^17,18 Cowbird numbers have recently increased greatly as a result of several factors including forest fragmentation.¹⁹ Because of this high egg production and recent increase in population size, cowbird populations now have the potential to be a serious threat to the survival of many bird species.²⁰

Many edge tolerant bird species have developed ways of responding to nest parasitism. Some species recognize cowbird eggs and remove them; others abandon parasitized nests and build another.²¹ Many edge tolerant bird species are short-distance migrants or permanent residents and can nest several times a year. Since cowbirds rarely parasitize late season nests, this improves their possibility of raising a successful brood.

In contrast many forest-interior birds have not developed behaviors to cope with nest parasitism and frequently accept the cowbird eggs as their own. Additionally, forest-interior bird species are generally long-distance migrants and usually nest only once or twice per year.^19,22 Nest parasitism of their nests often results in the failure of many forest-interior birds to raise any of their young successfully.²³

While there is some variation in the importance and impact of predation and parasitism on forest birds, it is widely suspected that these factors are responsible for the elimination of forest-interior species from forest edges or small woodlots. Frequently, all that remains in these small woodlots are edge tolerant species that can better compensate for nest losses from predation and parasitism.