In addition, parasitism levels are often higher in regions with highly fragmented forests than in largely forested landscapes. Portrait of a Parasite Cowbirds have been called songbird chickens because they can lay more eggs than any other wild bird. Typically, female cowbirds find potential host nests by perching during early morning hours in a location with high visibility such as a dead tree in a forest opening, observing behavior of host females that are building nests, and following the unsuspecting birds directly to their nests. Once the nest location is known, the cowbird returns during the egg-laying period of the host female and deposits her own egg in the nest. Consequently, cowbird young hatch earlier than the host young, affording the parasite a distinct advantage in competing for food with its younger foster siblings.
Cowbirds tend to parasitize birds smaller in size than themselves.
The monstrous cowbird nestling not only can cause host young to starve by monopolizing the food supply brought by parent birds but also literally may crowd host young right out of the nest. All of these attributes of the parasite can add up to severe effects on the host family. Impacts of parasitism are different for different host species, but usually result in loss of at least one, and often all, of the 3-5 host young. Although each cowbird female only lays one egg in a nest, a high abundance of cowbirds in an area often will lead to many cowbirds parasitizing the same limited supply of host nests. The presence of more than one cowbird in a nest almost inevitably leads to death of all host young.