The birds in this genus are infamous for laying their eggs in other birds’ nests. The female cowbird notes when a potential host bird lays its eggs, and when the nest is left momentarily unattended, the cowbird lays its own egg in it. The female cowbird may continue to observe this nest after laying eggs. Some bird species have evolved the ability to detect such parasitic eggs, and may reject them by pushing them out of their nests, but the female cowbird has been observed to attack and destroy the remaining eggs of such birds as a consequence, dissuading further removals. Widespread predatory behaviors in cowbirds could slow the evolution of rejection behaviors and further threaten populations of some of the greater than 100 species of regular cowbird hosts, favoring host acceptance of parasitic eggs in a mafia-like contest between cowbirds and other species.[7]

en.m.wikipedia.org/wiki/Cowbird