Ariana (lampetia) wrote in wtf_nature,

Brood Parasitism

Birds are well known for their parental care, patiently incubating their eggs and then bringing food to their young until they are old enough to look after themselves. However, certain birds, known as "brood parasites," lay their eggs in the nests of other birds and do not provide any parental care for their own offspring. Care that the "hosts" provide to the young parasites is care denied to their own young. This often has a detrimental effect on the reproductive success of the hosts and may affect their population numbers as well.

There are two types of brood parasitism, non-obligate and obligate. Non-obligate brood parasites lay eggs in the nest of conspecifics (i.e. same species) and in their own nests. Examples include several colonial nesting species such as Bank Swallows or African Weavers. Obligate brood parasites lay eggs in nests of other species and have completely lost the ability to construct nests and incubate eggs. Examples include Brown-headed Cowbirds and European Cuckoos. About 1% of all known bird species are obligate brood parasites. Other obligate brood parasites include: all African Honeyguides, about half of the species of cuckoos, the Black-headed Duck in South America, Shiny Cowbirds, Screaming Cowbirds, Bronzed Cowbirds, and Giant Cowbirds.


Cuckoos are worse than Cowbirds

Some species of birds thrive not by carefully rearing their own young, but by pawning that task off on adults of other species. The European Cuckoo, whose distinctive call is immortalized in the sound of the "cuckoo clock," is the bird in which this habit has been most thoroughly studied. Female European Cuckoos lay their eggs only in the nests of other species of birds. A cuckoo egg usually closely mimics the eggs of the host (one of whose eggs is often removed by the cuckoo). The host may recognize the intruding egg and abandon the nest, or it may incubate and hatch the cuckoo egg. Shortly after hatching, the young European Cuckoo, using a scoop-like depression on its back, instinctively shoves over the edge of the nest any solid object that it contacts. With the disappearance of their eggs and rightful young, the foster parents are free to devote all of their care to the young cuckoo. Frequently this is an awesome task, since the cuckoo chick often grows much larger than the host adults long before it can care for itself. One of the tragicomic scenes in nature is a pair of small foster parents working like Sisyphus to keep up with the voracious appetite of an outsized young cuckoo.

Interestingly, different females within a population of European Cuckoos often parasitize different host species. Some cuckoos may specialize in parasitizing the nests of Garden Warblers; others of the same population may lay in the nests of Reed Warblers, and yet others may lay in nests of White Wagtails. The eggs of each female very closely mimic those of the host selected (even though one host may have large, densely spotted eggs, and another may have smaller, unmarked pale blue eggs), and the mimetic patterns are genetically determined. The different genetic kinds of females (called "gentes") apparently mate at random with males. How these gentes are maintained within the cuckoo populations is not fully understood

Cuckoo egg in the nest of a Reed Warbler

Cuckoo chick ejecting the competition from the nest

Reed Warbler feeding Cuckoo chick

I love this picture

I found this interesting:

Body temperatures of 11 bird species, including cuckoos, were measured in an artificial meteorological room. Ratios of change in body temperature to that in air temperature were thereby obtained for each species. Cuckoos demonstrate a remarkably high value, indicating a particularly low ability to regulate body temperature. Viewed in this light, the cuckoo's parasitic behavior is very likely an adaptation to overcome a physiological disadvantage. This in turn might be expected to reinforce delay in evolution of temperature homeostasis.


Reed Warblers, however, do have some defense against the Cuckoo invasion

Previous studies have shown that reed warblers, Acrocephalus scirpaceus, are more likely to reject a cuckoo, Cuculus canorus, egg if they have seen a cuckoo at their nest. This suggests that they would benefit from watching out for cuckoos. We tested whether presentations of a cuckoo mount near the nest (to simulate nest inspection) led to increased nest attendance by the warblers. Cuckoo presentations at completed nests before laying, when males guarded their females closely, led to desertion at 40% of nests before any eggs were laid (there were no desertions after presentations of a jay,Garrulus glandarius , a nest predator). In the remaining cases, there was no effect of the cuckoo on nest attendance before laying began, but a marked increase in male nest attendance (compared with jay and no-presentation controls) on the days the first and second eggs were laid. Cuckoo presentations at the one-egg stage led to the same increase in male nest attendance as did the prelaying presentations. Increased male nest attendance at the one–two-egg stage was not at the expense of mate guarding, because this declined anyway when laying began, and it did not lead to increased paternity loss compared with controls. Overall, 15% of broods had one or two extrapair young (6% of all young extrapair). We conclude that male reed warblers do increase nest guarding in response to cuckoos, but only after their females have begun egg laying, when there are less likely to be costs in lost paternity. Females did not increase nest guarding, perhaps because they need to spend more time foraging during the egg-laying period. Our results suggest that cuckoos should be secretive not only when they lay but also when they monitor host nests beforehand.


Didn't look as hard for this one. Hopefully not a repost.
Tags: bird, birds
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