|Female Brown-headed Cowbird|
They lay their eggs in the nests of other birds. The birds whose nest is used by the cowbird is tricked into raising the young cowbirds.
The Brown-headed Cowbird has over 220 hosts. The other cowbird species have fewer known hosts, but all the species are generalists when it comes to choosing a host. This means that the eggs may look very different from the hosts' eggs.
Mafia-type behaviour [change]
It seems that Brown-headed Cowbirds periodically check on their eggs and young after they have deposited them. Removal of the parasitic egg may trigger a retaliatory reaction termed 'mafia behaviour'. Cowbirds may penalise hosts which remove the cowbird egg: they often destroy the eggs or nestlings of those hosts.
- "Central to the persistence of mafia-type behavior in brood parasites and acceptance in hosts is the fact that hosts produce significantly more offspring by accepting parasitism rather than ejecting parasitic eggs".
- Mermoz, Myriam E. and Francisco Oralis, Juan 2004. Phylogenetic analysis of life-history adaptations in parasitic cowbirds. Behavioral Ecology 15 109-119 
- Jeffrey P. Hoover; Scott K. Robinson (2007). "Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs". Proceedings of the National Academy of Sciences. http://www.pnas.org/content/104/11/4479.full.pdf. Retrieved 26 August 2009.
- Friedmann, Herbert; LLoyd E. Kiff and Stephen I. Rothstein (1977). "A further contribution to knowledge of the host relations of the parasitic cowbirds.". The Smithsonian. Smithsonian Institution Press. http://si-pddr.si.edu/dspace/bitstream/10088/5409/2/SCtZ-0235-Lo_res.pdf. Retrieved 26 August 2009.