Why Do Mockingbirds Accept Invaders' Eggs?

http://youtu.be/qvx5JD3VyxI In the form of brood parasites, the bird world has enough irresponsible moms to start a reality TV show: cowbirds, for instance, lay their eggs in other species' nests, stab most of the hosts' eggs to death, and then leave their offspring to be raised by the host parents. The standing explanation for this involves most host birds being not that sharp on the uptake (watch a tiny warbler fussing over a cuckoo chick ten times its size (above) and you'd think that too). But maybe, a new study suggests, it's sometimes to the host's benefit to let imposter eggs stay in their nests. The researchers chose mockingbirds as their hosts and cowbirds as their parasites, because mockingbirds usually fight like crazy to keep cowbirds of their nests but get strangely quiescent once the invaders have laid their eggs, a behavior that piqued the researchers' interest. Once all the ...