Brood Parasitism in Birds

nest parasitism in birds
The nest of a flycatcher shows 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 a host species’ nest to free itself from parental obligations. The parasitic species only has to find a nest to lay its eggs and not see its offspring again.

Parasitism between species is more common in micro-organisms. In general, the parasite is lodged in the host to obtain food, protection, and reproductive opportunities.

In 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 incubate the eggs and feed the parasitic bird’s chicks.

Types of Parasitism in Birds

It can be said that there is passive and active brood parasitism. Passive brood parasitism occurs when a male of a formed pair mates with a female other than its own mate. In this way, this male passes his genes onto the next generation without investing 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 parasite birds increase the chances of raising more offspring per year with less or no effort. This explanation is particularly valid in bird species with nidicolous chicks: chicks that depend on their 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 good territory or were displaced from their territory. The only chance of breeding their chicks is to lay their eggs in nests located in quality territories.
  • Another idea suggests that brood parasitism is a stable evolutionary strategy at the population level, with benefits similar to raising their offspring.
  • It is also considered that brood parasitism in birds helps 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 species lay their eggs in the nests of their favorite host species. However, they are not limited to their favorite host but can lay their eggs in multiple species’ nests. 

  • 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, 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 prefer certain host species and tend to lay their eggs in such nests more often than in other species.
cowbird-nest parasitism in birds
Giant Cowbird (Molothrus oryzivorus) inspecting an oropendola (Psaracolius sp.) to lay 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 can 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 the 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 warding off parasitic species if they are close to their nest.

When the parasitic species lay an egg, the host species can sometimes recognize the different egg and remove it from the nest. Other host species leave the nest or bury all the eggs with nest material to lay a new clutch on top of the parasitized nest. Other 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 of the new world (the Americas) is not as developed as in the old world. Brood parasitism is largely limited to species of the genus Molothrus (cowbirds) of the family Icteridae and the striated cuckoo (Tapera Naevia) Cuculidae. Some waterfowl are known to lay eggs in other nests occasionally.

The brood-parasitic cowbirds are generalists and include more than 250 species in which nest cowbirds lay their eggs or serve as hosts. It is unclear 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. This cuckoo lays polychrome eggs or eggs of multiple colors. The cuckoo has specialized in placing colored eggs similar to the coloring of their favorite hosts’ eggs to prevent the eggs from being recognized and rejected by the host species.

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