Brood parasite
From Wikipedia, the free encyclopedia
Brood parasites are a sub-category of kleptoparasites occurring among birds, fish or insects, that lay their eggs in the nests of other species to be raised by the host. This relieves the parent parasites from the investment of rearing young or building nests, enabling them to feed only themselves, and to lay more eggs. They usually only lay one egg per nest, although in some cases, particularly the cowbirds, several females may use the same host nest.
Contents |
[edit] Avian brood parasites
Among birds, brood parasites include the Old World cuckoos in Eurasia and Australia, cowbirds and Black-headed Ducks in the Americas, and indigobirds, whydahs, and the honeyguides in Africa. Most avian brood parasites are specialists which will only parasitize a single host species or a small group of closely related host species, but four out of the five parasitic cowbirds are generalists, which parasitize a wide variety of hosts; the Brown-headed Cowbird has 221 known hosts.
The Common Cuckoo presents an interesting case in which the species as a whole parasitizes a wide variety of hosts, but individual females specialize in a single species. Genes regulating egg coloration appear to be passed down exclusively along the maternal line, allowing females to lay mimetic eggs in the nest of the species they specialize in. Females are thought to imprint upon the host species which raised them, and subsequently only parasitize nests of that species. Male Common Cuckoos will fertilize females of all lines, maintaining sufficient gene flow between the different maternal lines to prevent speciation.
[edit] Common adaptations of avian brood parasites
Among specialist avian brood parasites, mimetic eggs are a nearly universal adaptation. There is even some evidence that the generalist Brown-headed Cowbird may have evolved an egg coloration mimicking a number of their hosts (Brian Peer, Scott Robinson, and James Herkert in The Auk 117(4):892-901).
Most avian brood parasites will remove a host egg when they lay one of their own in a nest. Depending upon the species, this can happen either in the same visit to the host nest or in a separate visit before or after the parasitism. This could function either to prevent the host species from realizing their nest has been parasitized or to reduce competition for the parasitic nestling once it hatches.
Most avian brood parasites have very short egg incubation periods and rapid nestling growth. This gives the parasitic nestling a head start on growth over its nestmates, allowing it to outcompete them. In cases where the host nestlings are significantally smaller than the parasite nestling, the hosts will often starve to death. Some brood parasites will eliminate all their nestmates shortly after hatching, either by ejecting them from the nest or killing them with sharp mandible hooks which fall off after a few days.
[edit] Nest destruction - the Mafia hypothesis
Adult brood parasites of some species have been observed destroying unparasitized nests of their common host species. It is unclear at this point exactly what drives this behavior, but several possible explanations have been offered. One idea, known as "The Mafia Hypothesis", is that the parasites will monitor nests they have parasitized and destroy the clutches of any hosts which eject the parasitic egg. An alternate explanation is that by forcing unparasitized hosts to renest, the parasite creates more opportunities to lay their eggs. The third explanation which has been offered is the parasites do this to reduce foraging competition for the host parents scattered throughout the area taking care of their nestlings.
[edit] Common counteradaptations of hosts
Given the detrimental effects avian brood parasites can have on their hosts' reproductive success, host species have come up with various defenses against this unique threat.
Given that the cost of egg removal concurrent with parasitism is unrecoverable, the best defense for hosts is avoiding parasitism in the first place. This can take several forms, including selecting nest sites which are difficult to parasitize, starting incubation early so they are sitting on the nests when parasites visit them early in the morning, and aggressive territorial defense. Birds nesting in aggregations can also benefit from group defense.
Once parasitism has occurred, the next most optimal defense is to eject the parasitic egg. This can be done by grasp ejection if the host has a large enough bill, or otherwise by puncture ejection. Ejection behavior has some costs however, especially when host species have to deal with mimetic eggs. In that case, hosts will inevitably mistake one of their own eggs for a parasite egg on occasion and eject it. In any case, hosts will sometimes damage their own eggs while trying to eject a parasite egg.
Among hosts not exhibiting parasitic egg ejection, some will abandon parasitized nests and start over again. However, at high enough parasitism frequencies, this becomes maladaptive as the new nest will most likely become reparasitized.
[edit] Insect brood parasites
There are many different types of cuckoo bees, all of which are brood-parasitic insects, laying their eggs in the nest cells of other bees, but they are normally referred to as kleptoparasites, rather than as brood parasites. A family of Cuckoo wasps also exist, many of which lay their eggs in the nests of Potter and Mud dauber wasps, and many other lineages of wasps in various families have evolved similar habits.
