The Bane of Brood Parasites

December 17, 2007 by  
Filed under Features

When we hear the word “parasites”, most of us would assume it is referring to an organism that feeds off another. In brood parasites, in the avian world, it works a little differently. To put in laymen’s terms, it is when one bird species lays their eggs in a different species‘ nest, so that the parasite species do not have to take care of their young. Over the years, host bird species became wise to the brood parasites, but as a parasite does not give up that easily, the brood parasites have come up with various devious plans to fool the hosts.

Brood parasitism can be found in insect species as well as fish species. But in avian species there are two different types of brood parasites. Non-obligate brood parasites will lay their eggs in host nests of the same species, such as the African Weaver and the Bank Swallow. Obligate brood parasites will lay their eggs in other species nestsm and examples of obligate brood parasites include the Bronzed Cowbird, European Cuckoos and African Honeyguides.

Host species started to notice the difference between their eggs and the parasite eggs, and began to eject them from their nests. To overcome this problem, parasite species started to mimic the host parents’ eggs, so that the difference between the host egg and parasite egg would be almost impossible to tell apart. Some parasitic species have become so advanced that they are able to mimic a variety of species eggs, such as the Brown Headed Cowbird, giving the parasite a choice of hosts. One of the most successful brood parasites is the Common Cuckoo that is able to mimic the eggs of the Meadow Pipit, Brambling, Streaked Laughing Thrush, Great Reed Warbler and Meadow Bunting to such perfection that scientists need to look at genetic markers to tell them apart.

In most cases, the parasite hatchling is much larger than the hosts’ hatchling. The parasite chick will often kill the host chick or kick it from the nest. It has even been noticed that host species are threatened into caring for the parasite hatchling. In some cases, the host has ejected the parasite chick from the nest, and in retaliation the brood parasite adult has destroyed the hosts’ nest. The aggressive behavior of the brood parasite, of which most remain close to the host nest to keep an eye on their eggs, then forces the often-smaller host to care for the parasite hatchling.

Scientists are still working on projects and studies regarding this very interest form of survival, the impact it has on the host population and trying confirm their intimidation theories across the board. But, when looking at brood parasites in general, it seems that the host species always suffers, and the only party that gains from this arrangement are the brood parasites.

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