Brood Parasitism

What is Brood Parasitism?

Red-gartered Coot eggs 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

Brown-headed Cowbirds (Molothrus ater) are common brood parasites in North America. A cowbird female will find a prospective host's nest and make frequent checks of it while the host is absent. After her host has laid two or more eggs, but before incubation actually begins, the female cowbird will sneak into the nest and deposit an egg. The egg will be laid just before sunrise, while the host is gone. The cowbird may remove one or all of the host's eggs. This serves to reduce competition for her egg, but may also force renesting by the host so that the Cowbird can better time her egg drop. She pierces the egg with her sharp bill, carries it away and eats it.

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.


Cuckoo eggs Many 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.