Forest fragmentation and the creation of forest edges on breeding sites in the temperate zone have been implicated as possible causes of this decline. Areas in Illinois where continuous forest once stood are now dominated by a mosaic of forested and nonforested habitats. Cowbirds lay their eggs in the nests of other species, and are entirely dependent on the hosts for raising the cowbird young. Parasitized nests frequently fledge few or no host young. With a few exceptions, the dynamics of neotropical migrant populations are poorly understood, especially for forest-interior species.
Most studies rely on crude measures of nesting success and return rates of adult birds.
Until these critical gaps are filled, we will not know the levels of nest predation and parasitism that populations can tolerate. Such detailed, species-specific data are crucial before effective conservation plans can be formulated. Male and female Kentucky warblers. At many sites in Illinois, Kentucky warblers experience moderate but variable levels of both nest predation and cowbird parasitism and seem to be somewhat tolerant to fragmentation. Populations may be quite high in selectively logged tracts of forest, tree plantations, mid-successional clearcuts, and other disturbed forest where understory cover is abundant.