Placing offspring in foster care is not limited to human society. Certain bird species, like cuckoos, have evolved to put their chicks up for adoption. The only difference being that the adoptive parents have no consent or knowledge about the agreement. This phenomenon is known as brood parasitism and several of these ‘free-loaders’ can be viewed on MalaMala Game Reserve. We may percieve it as a cruel practice, but it is fascinating.
The female cuckoo will go and seek out a potential host for her offspring and wait patiently nearby until the host leaves its nest. Host bird defense has largely been to try to keep the female cuckoo from laying her eggs in the first place, but male cuckoos often act as a decoy, luring the hosts away from their nest. The female cuckoo then slips into the nest, hastily lays an egg and flies off, never to see her offspring again. When the host returns to its nest it cannot distinguish the parasite’s egg from its own and automatically accepts the addition to its clutch. It incubates the egg then hatches and raises the chick. Different kinds of cuckoos behave differently when they hatch. Some famously push all the other eggs out of the nest, leaving themselves as the sole survivor and beneficiary. Others leave the other chicks alone and share in food the mother brings, acting as an adopted sibling, of sorts. As the cuckoo chick grows up into a demanding foster fledgling, the hosts become involuntary and defenseless slaves, responding to all the parasite’s needs. The adoptive parents are oblivious to reality despite the fact that the young parasite can be up to 3 times larger than its foster parents and look completely different. Once fully fledged, the chick will still be fed by the exhausted foster parents and, once it becomes an adult, will search for victims of its own.
Brood parasites in Southern Africa include; Cuckoos, Honeyguides and Honeybirds, Whydahs and Indigobirds as well as the Cuckoo Finch.
Cuckoos: There are 10 cuckoo species that breed in Southern Africa. Most cuckoos parasitize a number of different host species although some only parasitize one specific host. A single egg is usually laid per nest and the color of the egg usually, but not always, match those of the host. Incubation periods are usually shorter that those of the host, giving the parasite chick an advantage over the host chicks. The host usually raises the parasite chick alone, as its own eggs have been destroyed, either by the female brood parasite or by the chick itself.
Honeyguides and Honeybirds: Four honeyguides and two honeybird species occur in Southern Africa. All species, except the Green-backed Honeybird, have more than one host. Host species include hole-nesting birds such as barbets, woodpeckers and kingfishers as well as cisticolas, warblers and white-eyes. A single egg is usually laid and it does not match the color or patterns of the host. This is not necessarily because they target hole-nesting birds, where the differences would not be clearly visible in the dark anyway. Newly hatched chicks have sharp, lethal bill hooks that they use to kill the host chicks.
Whydahs and indigobirds (viduids): Eight species occur in Southern Africa; four Whydahs and four Indigobirds. Most species are host-specific although some have more than one host. Whydahs parasitize specific wax-bill and manikin species, while indigobirds parasitize specific firefinches. The eggs match those of the host and one egg is usually laid per nest. Sometimes more than one female will target the same nest and this can lead to multiple eggs being laid in the same nest by different females. These brood parasites do not remove or damage the hosts eggs when laying their own. The parasite and the host chicks may be raised together, although the host chicks chances of survival are reduced. The foster parents are fooled by the fact that the begging call, as well as the distinctive color and patterns on the parasite chicks mouth parts, mimic those of the host chicks.
Cuckoo Finch: Originally thought to be part of the weaver family, the Cuckoo Finch is now known to be more closely related to whydahs and indigobirds. It has at least 13 known host species which consists mainly of cisticolas and prinias. The eggs also match the eggs of the host, just like those of the viduids. The only difference is that the Cuckoo Finch removes the host’s eggs from their nest, and lays one to two eggs per nest.
Nobody knows exactly how brood parasitism evolved, but it is clear that it evolved independently in the three parasitizing families- cuckoos, honeyguides as well as honeybirds / viduids. We can therefore assume that this is a survival strategy of considerable advantage to those birds. They don’t have to waste any time or energy on nest building, incubation or the raising of chicks. They can rather invest that energy in laying more eggs.