One of the sneakiest ways some female birds ensure the survival of their young is by laying their eggs in another birds nest. This practice is called “brood parasitism” or “nest parasitism”. Not all species engage in this act. In this article we will look at why birds lay eggs in other bird’s nests, how they get away with it, and some species that commonly display this behavior.
Here’s Why Birds Lay Eggs in Other Birds’ Nests
Brood parasites are animals that strategically get other animals to raise their young. This has been observed in birds, fish and insects. For birds, this typically means a mother will find nests of other species, and sneak in when the nest is not be attended and lay her egg. They may even destroy a nest, just so they can follow the mother bird as she builds a new nest to time the depositing of her egg just right.
Usually only one “intruder egg” is laid, but sometime more. By depositing an egg in several nests per season, she the chances that more of her offspring will survive. Also, she does not have to put any energy into building her own nest, incubating eggs, finding food for the young or protecting the nest. All that work gets passed to the bird that ends up raising the intruder egg, called the “host” bird.
How is brood parasitism successful?
Egg Mimicry: To aid in tricking the host, some brood parasites have evolved egg coloration and/or size to match that of the host eggs, in order to hide the intruder egg. For example, if the host species lays brown eggs with speckles, it would certainly help if the egg depositing bird also had brown eggs with speckles.
Thicker Eggshells: Eggshells of brood parasite eggs are often thicker than the hosts eggs. This could be to prevent accidental or intended damage by the host, or to ensure the egg does not crack if it is laid on top of other eggs already in the nest. Think of it as extra insurance that the parasite egg will hatch.
Short Incubation: Many brood parasite eggs have a relatively short incubation, and the babies grow rapidly once hatched. By hatching before the host’s eggs, they can get food first and grow large before the other young in the nest. Sometimes this means they can destroy the unhatched eggs or push out the smaller host young from the nest.
Nestling Mimicry: sometimes the parasitic young are able to mimic the babies of the host bird. They may have similar plumage, a similar looking mouth, and a similar sounding begging call. Some species may not be able to distinguish the parasitic baby, and won’t risk ejecting it from the nest.
How do birds defend themselves intruder eggs?
Just as brood parasite species have evolved to dump their eggs in other birds nests, so too have birds evolved methods to prevent this. It is often referred to as an “arms race”, with both species developing and evolving new strategies to outwit the other.
Hiding nests: Some species try to avoid imposter eggs by making their nests hard to access. They may nest deep within dense thickets and tangles, or may construct enclosed nests with small openings.
Defending nests: Sometimes the best defense is a strong offense. They will spend very little time off the nest so intruders don’t have a chance to sneak in. If a brood parasite does come around, they will viciously attack them.
Egg expulsion: While many species aren’t able to recognize an intruder egg, some are. They may take note of the size, shape and color and decide to push the egg out of the nest.