A second factor affects warbler reproduction even if suitable nesting habitat is available. The brown-headed cowbird is a blackbird species native to North America, but not found in northern Michigan prior to European settlement. The cowbird invaded warbler nesting grounds around the beginning of the 20th century after logging cleared the original forests. Kirtland's warbler did not evolve with the cowbird, and so never developed natural defenses against its parasitism. The warbler parents do not recognize that a cowbird chick in its nest is not its own, and so will naively care for the cowbirds.
Cowbird chicks hatch a day before any warbler eggs, and are much larger than warbler chicks.
Consequently, they get most of the food brought to the nest by warbler parents. Biologists believed that Kirtland's warbler was likely to become extinct if that rate of parasitism was allowed to continue. Events occurring during migration or at the bird's winter habitat also may be harmful. Biologists know little about the warbler's Caribbean home. Droughts and logging on the islands may have contributed to the decline of the warbler.