Brood Parasitism: Definition & Examples

Instructor: Lauren Posey

Lauren has taught intermediate reading in an English Language Institute, and she has her Master's degree in Linguistics.

In this lesson, you will learn about brood parasitism, which is a method used by some birds instead of raising their own young. We'll also look at examples of parasitic birds.

What is Brood Parasitism?

For most animals (and people), having babies goes hand in hand with the responsibility of raising and feeding them. Imagine, though, if an animal had its baby and tricked another animal into raising it for them. In fact, this is exactly the strategy used by some species of birds. It is known as brood parasitism because a nest of baby birds is called a brood. Brood parasitism is when a bird lays its eggs in the nest of another bird. The host bird (as the owner of the nest is called) is then responsible for raising and feeding the parasite bird chick. This means the parasite bird can lay more eggs in a season than it might otherwise, but the host bird's chicks suffer as a result.

How It Works

Brood parasitism truly meets the definition of parasitism because the parasite benefits while the host is negatively affected. When the parasite bird lays her eggs, she gets rid of one or more of the host bird's eggs. In addition, the parasite bird eggs often hatch sooner and grow faster than the host bird, which means the parasite chick gets more of the resources. The parasite chicks will also sometimes kill the host chicks by pushing them out of the nest.

You might be wondering, why do the host birds put up with this? Why don't they kick the parasite eggs out of the nest? In some cases, they actually do! Parasite birds and host birds are constantly evolving; the host birds develop defenses against the parasite birds, and the parasite birds develop ways to overcome these defenses.

For example, when the host bird evolves to be able to recognize the parasite bird's eggs and remove them from the nest, the parasite bird evolves so that its eggs look exactly like those of the host bird. In other cases, the host bird leaves the parasite bird eggs in the nest because some species of birds simply can't see the difference between different types of eggs. They will raise whatever egg happens to be in their nest.

Some bird species cannot tell the difference between eggs, even when it seems obvious to us
Brown-headed cowbird egg

Facultative Parasitism

There are two different types of brood parasitism. The first is facultative parasitism. This is when a species of bird is only sometimes parasitic. That is, while they may lay their eggs in other nests of their own species, or in the nests of other species, these birds are able to build their own nests and do not rely on parasitism completely. One species of facultative parasite is the bank swallow, and another is the African weaver.

Bank swallows (left) and African weavers (right) are facultative parasites
Bank swallow and African weaver

Obligate Parasitism

The other type of brood parasitism is obligate parasitism. In this type, as you might expect from the name, the bird is obligated to be parasitic because they cannot raise their own young. Obligate brood parasites cannot build their own nests and rely completely on brood parasitism to raise their young. This is actually the more common type of brood parasitism, and obligate parasite species are more often studied and discussed.

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