Brood parasitism

Brood parasitism is a subclass of parasitism and phenomenon and behavioural pattern of animals that rely on others to raise their young. The strategy appears among birds, insects and fish. The brood parasite manipulates a host, either of the same or of another species, to raise its young as if it were its own, usually using egg mimicry, with eggs that resemble the host's. The strategy involves a form of aggressive mimicry called Kirbyan mimicry.

The evolutionary strategy relieves the parasitic parents from the investment of rearing young. This benefit comes at the cost of provoking an evolutionary arms race between parasite and host as they coevolve: many hosts have developed strong defenses against brood parasitism, such as recognizing and ejecting parasitic eggs, or abandoning parasitized nests and starting over. It is less obvious why most hosts do care for parasite nestlings, given that for example cuckoo chicks differ markedly from host chicks in size and appearance. One explanation, the mafia hypothesis, proposes that parasitic adults retaliate by destroying host nests where rejection has occurred; there is experimental evidence to support this. Intraspecific brood parasitism also occurs, as in many duck species. Here there is no visible difference between host and parasite eggs, which may be why the parasite eggs are so readily accepted. In eider ducks, the first and second eggs in a nest are especially subject to predation, perhaps explaining why they are often laid in another eider nest.

Brood parasitism is an evolutionary strategy that relieves the parasitic parents from the investment of rearing young or building nests for the young by getting the host to raise their young for them. This enables the parasitic parents to spend more time on other activities such as foraging and producing further offspring.

Among specialist avian brood parasites, mimetic eggs are a nearly universal adaptation. The generalist brown-headed cowbird may have evolved an egg coloration mimicking a number of their hosts. Size may also be important for the incubation and survival of parasitic species; it may be beneficial for parasitic eggs to be similar in size to the eggs of the host species.

The eggshells of brood parasites are often thicker than those of the hosts. For example, two studies of cuckoos parasiting great reed warblers reported thickness ratios of 1.02 : 0.87 and 1.04 : 0.81. The function of this thick eggshell is debated. One hypothesis, the puncture resistance hypothesis, states that the thicker eggshells serve to prevent hosts from breaking the eggshell, thus killing the embryo inside. This is supported by a study in which marsh warblers damaged their own eggs more often when attempting to break cuckoo eggs, but incurred less damage when trying to puncture great reed warbler eggs put in the nest by researchers. Another hypothesis is the laying damage hypothesis, which postulates that the eggshells are adapted to damage the eggs of the host when the former is being laid, and prevent the parasite's eggs from being damaged when the host lays its eggs. In support of this hypothesis, eggs of the shiny cowbird parasitizing the house wren and the chalk-browed mockingbird and the brown-headed cowbird parasitizing the house wren and the red-winged blackbird damaged the host's eggs when dropped, and sustained little damage when host eggs were dropped on them.

Most avian brood parasites have very short egg incubation periods and rapid nestling growth. In many brood parasites, such as cuckoos and honeyguides, this short egg incubation period is due to internal incubation periods up to 24 hours longer in cuckoos than hosts. Some non-parasitic cuckoos also have longer internal incubation periods, suggesting that this longer internal incubation period was not an adaptation following brood parasitism, but predisposed birds to become brood parasites. This is likely facilitated by a heavier yolk in the egg providing more nutrients. Being larger than the hosts on hatching is a further adaptation to being a brood parasite.

Bird parasites mitigate the risk of egg loss by distributing eggs amongst a number of different hosts. As such behaviours damage the host, they often result in an evolutionary arms race between parasite and host as they coevolve. Some host species have strong rejection defenses, forcing the parasitic species to evolve excellent mimicry. In other species, hosts do not defend against parasites, and the parasitic mimicry is poor.

Intraspecific brood parasitism among coots significantly increases the reproductive fitness of the parasite, but only about half of the eggs laid parasitically in other coot nests survive. This implies that coots have somewhat effective anti-parasitism strategies. Similarly, the parasitic offspring of bearded reedlings, compared to offspring in non-parasitic nests, tend to develop much more slowly and often do not reach full maturity.