The weird things you come across on the Internet

What can you make of this? Look at that amazing photo. So, why doesn't mama bird let the interloper starve? Read on:

A Common Cuckoo being raised by a Reed Warbler.

Brood parasites are organisms that use the strategy of brood parasitism, a kind of kleptoparasitism found among birds, fish or insects,

In many monogamous bird species, there are extra-pair matings resulting in males outside the pair bond siring offspring and used by males to escape from the parental investment in raising their offspring.^[4] This form of cuckoldry is taken a step further when females lay their eggs in the nests of other individuals. Intraspecific brood parasitism is seen in a number of duck species with females laying their eggs in the nests of others for example in the Goldeneye, Bucephala clangula.^[5]

Interspecific 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. Seven independent origins of obligate interspecific brood parasitism in birds have been proposed. While there is still some controversy over when and how many origins of interspecific brood parasitism have occurred, recent phylogenetic analyses suggest two origins in Passeriformes (once in New World cowbirds: Icteridae, and once in African Finches: Viduidae); three origins in Old World and New World cuckoos (once in Cuculinae, Phaenicophaeinae, and in Neomorphinae-Crotophaginae); a single origin in Old World honeyguides (Indicatoridae); and in a single species of waterfowl, the black-headed duck (Heteronetta atricapilla).^[6][7][8]

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. They usually only lay one egg per nest, although in some cases, particularly the cowbirds, several females may use the same host nest.

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 among the different maternal lines.^[9]

The mechanisms of host selection by female cuckoos are somewhat unclear, though several hypotheses have been suggested in attempt to explain the choice. These include genetic inheritance of host preference, host imprinting on young birds, returning to place of birth and subsequently choosing a host randomly ("natal philopatry"), choice based on preferred nest site (nest-site hypothesis), and choice based on preferred habitat (habitat-selection hypothesis). Of these hypotheses the nest-site selection and habitat selection have been most supported by experimental analysis.^[9][10]

[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.^[11]

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 both prevents the host species from realizing their nest has been parasitized and reduces competition for the parasitic nestling once it hatches.

A Shiny Cowbird chick being fed by a Rufous-collared Sparrow

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 significantly smaller than the parasite nestling, the host nestlings 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] "Mafia hypothesis"

It has often been a question as to why the majority of the hosts of brood parasites care for the nestlings of their parasites. Not only do these brood parasites usually differ significantly in size and appearance, but it is highly probable that they reduce the reproductive success of their hosts. So what possible benefits are gained from providing this parental care? Through studies in an attempt to answer this question evolved the "Mafia hypothesis". This hypothesis revolves around host manipulations induced by behaviors of the brood parasite. Upon the detection and rejection of a brood parasite's egg, the host's nest is depredated upon, its nest destroyed and nestlings injured or killed. This threatening response is indirectly enhancing selective pressures favoring aggressive parasite behavior that may result in positive feedback between Mafia-like parasite and compliant host behaviors.^[12]

There are two avian species that have been speculated to portray this mafia-like behavior, the brown-headed cowbird of North America, Molothrus ater, and the Great Spotted Cuckoo of Europe, Clamator glandarius. The Great Spotted Cuckoo lays the majority of its eggs in the nests of the European Magpie, Pica pica. It has been observed that the Great Spotted Cuckoo repeatedly visits the nests that it has parasitised, a precondition for the Mafia hypothesis.^[12] An experiment was run by Soler et al. from April to July, 1990–1992 in the high-altitude plateau Hoya de Guadix, Spain. They observed the effects of the removal of cuckoo eggs on the reproductive success of the magpie, and measured the magpie's reaction; the egg was considered accepted if it remained in the nest, ejected if gone in between visits, or abandoned if the eggs were present but cold. If any nest contents were gone between consecutive visits, the nests were considered to have been depredated. The magpie's reproductive success was measured by number of nestlings that survived to their last visit, which was just before the nestling had been predicted to fledge from the nest. The results from these experiments show that after the removal of the parasitic eggs from the Great Spotted Cuckoo, these nests are predated at much higher rates than those where the eggs were not removed. Through the use of plasticine eggs that model those of the magpie, it was confirmed that the nest destruction was caused by the Great Spotted Cuckoo. This destruction benefits the cuckoo, for the possibility of re-nesting by the Magpie allows another chance for the cuckoo egg to be accepted.