What’s going on in this little nest with eggs from two different species of birds? House Finches love to nest in the eaves of our front porch and every year we host a brood, or two. This year, the pair abandoned their first nest of five eggs and moved to eaves on the other side of the porch. When we checked again on the nest before taking it down, three of the House Finch eggs were missing and in their place, a Brown-headed Cowbird had laid one of its own. I guess Mama Cowbird didn’t get the 411 that no one was home to raise her baby.
Avian brood parasitism, or laying of one’s eggs in the nest of another individual, is a reproductive strategy when parasitic birds, such as the Brown-headed Cowbird, foist the cost of rearing their offspring onto another bird.
From wiki –
Brood parasites are organisms that rely on others to raise their young. The strategy appears among birds, insects and fish. The brood parasite manipulates a host, either of the same or of another species, to raise its young as if it were its own, using brood mimicry, for example by having eggs that resemble the host’s (egg mimicry).
Brood parasitism relieves the parasitic parents from the investment of rearing young or building nests for the young, enabling them to spend more time on other activities such as foraging and producing further offspring. Bird parasite species mitigate the risk of egg loss by distributing eggs amongst a number of different hosts. As this behaviour damages the host, it often results in an evolutionary arms race between parasite and host as the pair of species coevolve.
The strength of defenses and counter-adaptation rely on the host/parasitic species’ ability to evolve; some host species have very strong rejection defenses resulting in the parasitic species evolving to have very close mimicry. In other species, hosts do not show rejection defenses and as a result, the parasitic species will show no evolved trait (example: egg mimicry).