Brown- headed cowbirds are parasites. Not in the normal sense though. They are called brood parasites because they lay their eggs in other birds nests.
Pictured above is eastern phoebe nest, with five phoebe eggs and one brown-headed cowbird egg.
After the eggs hatch, the cowbird chicks often outcompete the host chicks for resources, from the host parents, because they are bigger.
|© Al Mueller. A brown-headed cowbird chick being fed by an adult chipping sparrow.|
I ask myself, how the heck don't these birds realize that their babies are bigger then themselves? Well, some birds are able to get rid of the parasitic eggs before they hatch. For example, gray catbirds reject their nest 95% of the time if it has been parasitized (Lorenzana 2001) and brown thrashers physically eject these eggs from their nest (Ortega, 1998).
To make things worse for these small songbirds, the cowbirds exhibit "mafia" behavior. This means that they return to the nests where they lay their eggs to check up on them. If they see that their egg has been removed, they destroy the entire nest. This forces the host birds to build a new nest and then the cowbirds lay their eggs in that new nest (Hoover and Robinson, 2007).
Hoover, Jeffrey P. &. Robinson, Scott K. (2007). "Retaliatory mafia behavior by a parasitic cowbird favors host acceptance of parasitic eggs". PNAS 104(11): 4479–4483
Lorenzana, J. C. (2001). "Fitness costs and benefits of cowbird egg ejection by gray catbirds". Behavioral Ecology 12 (3): 325–329.
Ortega, C.P. (1998) Cowbirds and Other Brood Parasites. University of Arizona Press, Tucson,