Avian Brood Parasitism: Variation in Host Repsonse

Obligate brood parasites forego the cost of parental care by laying their eggs in the nest of other species, often to the detriment of that species. About 1% of all bird species are obligate brood parasites, but the most well-known and commonly studied are the many cuckoo and cowbird species. Host species have been observed to endure the cost of parental care to raise the parasitic young, even though these young are not theirs and are therefore not contributing to their fitness. The host-parasite relationship has been studied for many years in an effort to determine why a host would raise the parasitic young. Through these studies, it is has become clear that hosts use a variety of defensive strategies to avoid caring for parasitic young and to minimize the costs associated with being parasitized. Three major forms of host defenses have been observed: frontline, egg rejection and chick rejection. Not all hosts use each of these strategies, leading to the question of what causes this variation in a host's response to parasitism. By comparing the occurrence of brood parasitism, the use of the different defense mechanisms and the outcomes of these differing scenarios, I was able to gain an understanding of the various responses to parasitism and why they differ among hosts. A host's response to parasitism depends on a combination of what species they are being parasitized by (or threatened to be parasitized by), the costs associated with each defense strategy weighed against its potential benefit, and if that host is currently in an evolutionary arms race with the parasitic species.