What is Brood Parasitism?Birds that lay their eggs in the nests of other (host) birds are called brood parasites. They leave the foster parents to incubate and rear their young. This behaviour is surprisingly common. The parasitic bird is freed from the duties of nest building, egg incubation and rearing young. The energy saved by this behaviour can then be used for egg production. Here, a clutch of 5 Red-gartered Coot eggs (Fulica armillata) is parasitized by one Black-headed Duck egg, (Heteronetta atricapilla). Although a number of different birds practice brood parasitism, two of the best known examples are the New World Cowbirds and the European Cuckoo.
|3 American Goldfinch
1 Brown-headed Cowbird
* from South America
|3 Red-winged Blackbird
1 Brown-headed Cowbird
|2 Ringed Warbling-Finch
1 Shiny Cowbird
Ordinarily, the cowbird will lay an egg in several hosts' nests. The hosts may or may not be the
same species. Most hosts do not react negatively toward a cowbird during nest building or egg
laying. However, many birds do react to strange objects in their nest by removing them. If the
parasite's egg does not resemble the host's egg in size and colour, the host may remove it. Robins and Catbirds will remove the cowbird's egg from the nest. Sometimes a host will desert the nest because the cowbird egg is present. Other prospective hosts will build a new nest floor and cover the parasitized clutch.
CuckoosMany species of cuckoos are brood parasites. Brood parasitism in European Cuckoos is more sophisticated than in cowbirds. Cuckoo eggs tend to mimic the size and markings of a specific host species. Females are engrained to parasitize a particular bird species. Hosts, on the other hand, have evolved the ability to detect all but the best mimics of their own eggs. A female cuckoo knows which host species she will most successfully parasitize by her own experience with that species. She probably was incubated and reared and therefore imprinted on that particular species. There are four Sedge Warbler eggs here and one Cuckoo egg.
Parasitic hatchlings often have mouth markings similar to those of the host. These hatchlings may even make the host's calls. Both the markings and the calls will reduce the likelihood that the host recognizes the parasite nestling as foreign.
Cuckoo eggs require a shorter incubation time than do the host's eggs. Typically cuckoo eggs need to be incubated for 12 1/2 days while the host's eggs need 13 or 14 days. The early hatching is advantageous to the parasite. Cuckoo hatchlings have evolved a mechanism to further increase their success over that of the host hatchlings. When the cuckoo hatches, it is altricial. It is blind and naked and gapes for food. Once the nestling is about 10 hours old, any solid object that touches the sensitive depression on the cuckoo's back initiates a pushing reflex. The young cuckoo manipulates the object (host's egg or nestling) to the edge of the nest and pushes it over. The cuckoo continues evicting its nest mates until it is the only object left in the nest. This instinct of the cuckoo nestling disappears in 3 1/2 to 4 days.