brood parasitism in birds
Nest of a flyctacher showing the pale eggs of the host species and the green dotted egg of a parasitic cowbird of the genus Molothrus. Photo: Galawebdesign/Wikipedia.

Brood Parasitism in Birds

Definition: Brood Parasitism in birds is defined as the interaction in which a bird species (parasitic)  places its eggs in the nest of a host species with the intention of freeing itself from parental obligations. The parasitic species only has to find a nest to place its eggs and will not see its offspring again.

Parasitism between species is more common in micro-organisms. In general, the parasite is lodged in the host for the purpose of obtaining food, protection, and reproductive opportunities.

In the case of brood parasitism in birds, the parasitic species does not obtain food, nor does it depend on the host species for other functions, but only to take charge of incubating the eggs and feeding the chicks of the parasitic bird.

Types of Parasitism in Birds

It can be said that there is a type of passive and active brood parasitism. Passive brood parasitism occurs when a male of a formed pair mates with a female that is not his mate. In this way, this male passes his genes onto the next generation without having to invest time and effort in the incubation and care of his offspring.

Active brood parasitism is when a female and a male mate, but avoid all or part of the investment of time and effort in raising their progeny by placing their eggs in nests of species of the same or another species.

nest parasitism in birds-cowbird
Rufous-necked Sparrow (Zonotrichia capensis) feeding a chick of Shiny Cowbird (Molothrus bonariensis).

Possible Reasons for Parasitism in Birds

Ornithologists propose a series of reasons why birds parasitize the nests of other birds:

  • One of the most prevalent explanations is that brood parasites birds increase the chances of raising a greater number of offspring per year with less or no effort. This explanation is particularly valid in bird species with nidicolous chicks, that is, chicks that depend on the parents for food and protection to survive during the first weeks of life.
  • Another possibility is that parasitic birds are pairs that could not get a quality territory or were displaced from their territory and that the only possibility of breeding their chicks is to lay their eggs in nests located in quality territories and thus increase the chances of success in his progeny.
  • Another idea suggests that brood parasitism is a stable evolutionary strategy at the population level, with benefits similar to those of raising their own offspring.
  • It is also considered that brood parasitism in birds is a way to diminish competition among nestlings in a nest. That is to say, a pair that would raise four chicks in a nest, place two eggs in the nest of another bird and only have to raise two chicks.

Do Brood Parasitic Species have Preferences for Certain Types of Nests?

Brood parasite especies place their eggs in nests of birds of the same species, as well as, in nests of other species of birds.

  • Intra-specific parasitism: Occurs when the parasitic species places its eggs in nests of birds of the same species. It is the more prevalent type of brood parasitism and occurs more often in species with nidifugous chicks, that is, chicks that leave the nest shortly after hatching.
  • Inter-specific parasitism: Occurs when a brood parasite species use nests of other species to lay their egg. In some cases, the parasitic species have a preference for nests of certain species and tend to lay their eggs in such nests more often than in other species.

cowbird-nest parasitism in birds
Thrush Parasito Gigante (Molothrus oryzivorus) inspecting a nest the Oropendola (Psaracolius sp.) With the intention of placing an egg. Photo: Carlos Bran.

Types of Brood Parasitism

There are two types of brood parasitism in birds:

  • Facultative Parasitism: Occurs when the parasitic species is able to build a nest and raise their own chicks and additionally parasitizes the nests of other species.
  • Obligate Parasitism: Occurs when the parasitic species never builds a nest or raises its own chicks. All eggs are laid in nests of other birds.

How do Parasitized or Host Birds Respond?

Parasitized birds or host species show various methods of defense against brood parasites. The first defense is actively attacking and ward off parasitic species if they are close to their nest.

When the parasitic species gets to lay an egg, the host species is sometimes able to recognize the different egg and remove it from the nest, leave the nest, or bury all the eggs with nest material to lay a new clutch on top of the parasitized eggs. Other species of birds do not notice the presence of different eggs and proceed to incubate them as usual.

striped cuckoo-nest parasitism in birds
The Striped Cuckoo (Tapera naevia) is a mandatory parasitic species that never builds a nest. 

Some Brood Parasite Species

Brood parasitism in birds in the new world (the Americas), is not as developed as it is in the old world. Brood parasitism is largely limited to species of the genus Molothrus (cowbirds) of the family Icteridae, and to the striated cuckoo (Tapera Naevia) of the family Cuculidae. Some waterfowl are known to occasionally lay eggs in other nests.

The brood-parasitic cowbirds are generalists and include more than 250 species in which nest cowbirds lay their eggs or serve as host. It is not clear whether each female specializes in parasitizing a bird species only or if all females parasitize any bird species nesting in the area. What is observed is that only a few species are parasitized more frequently.

The striped cuckoo is also a generalist obligate brood parasite with a long list of host species. The cuckoo lays polychrome eggs, that is, of various colors. The cuckoo has specialized in placing colored eggs similar to the coloring of the eggs of their favorite hosts in order to prevent the eggs from being recognized and rejected by the host species.

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  • Yezerinac, Stephen M .; Weatherhead, Patrick J. (1997). Extra-Pair Mating, Male Plumage Coloration and Sexual Selection in Yellow Warblers (Dendroica petechia). Proc. R. Soc. London B.264 (1381): 527-532.
  • Massoni, Viviana (2002) Parasitism of rearing the brown thrush on the yellow wing rod. Doctoral thesis. Faculty of Exact and Natural Sciences. Buenos Aires’ University.
  • Revoreda et al. (2003) Impact of breeding parasitism of the Black-winged Tordo (Molothrus bonariensis) on the reproductive success of its hosts. Faculty of Exact and Natural Sciences, University of Buenos Aires.
  • Gálvez, David (2005), Defenses and Counter-defenses in the Parasitism of Bird Breeding: Egg Recognition and Petition. Doctoral thesis. Science Faculty. University of Granada
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