A Possible Benefit of Nest Parasitism

Shiny Cowbird / Photo by Lip Kee Yap

Brown-headed Cowbirds are well-known among birders for parasitizing the nests of other songbirds, particularly sensitive species like Kirtland's Warbler. Nest parasitism, in this sense, means laying eggs in the nests of other songbirds for the host birds to incubate and feed. Such nest parasitism puts pressure on the host species, but scientists studying Chalk-browed Mockingbirds (Mimus saturninus) and Shiny Cowbirds (Molothrus bonariensis) in South America have found an unexpected benefit.

Shiny cowbirds regularly visit mockingbird nests and attack and puncture any eggs they find there -- damaged eggs are later removed by the mockingbird host. During these visits cowbirds will often lay their own eggs in the nest for mockingbirds to hatch and bring up alongside their own chicks.

Whilst mockingbirds will mob an attacking cowbird, once an alien egg has been laid in their nest they will usually accept it -- even though it looks very different from their own eggs.

The researchers recorded video of 130 cowbird visits to see what happened to the eggs in mockingbird nests over three breeding seasons. They experimentally manipulated clutch compositions to compare host egg survival in clutches with different numbers of cowbird eggs. They found that mockingbird eggs were more likely to survive a puncture attack when more cowbird eggs were present in the nest.

Computer simulations showed that this is likely to be a widespread phenomenon, and that, paradoxically, the greater the local density of parasites, the stronger the benefit hosts get from the presence of parasite eggs.

It would be interesting to see if this applies to parasite-host relationships elsewhere as well.