Brood Parasitism

Brood parasitism is a parenting strategy in which birds do not brood their own eggs or tend to their own offspring once those eggs hatch. The eggs are deposited into the nests of other females of a different species.

brood parasitism in cuckoos
The cuckoo, a generalist brood parasite; photo by Per Harald Olsen CC BY-SA 3.0

Brood Parasitism Success Strategies

Unfortunately there isn’t really a “bright side” for host moms. Parasite chicks often employ strategies that render them more successful than the host mom’s own chicks. They may push host chicks out of the nest. They may beg louder and more frequently in order to gain a greater level of parental provisioning. Parasite chicks may repeatedly stab host chicks with specialized bill hooks evolved specifically for that purpose. Some parasite moms will even poke holes in the eggs of host moms, so that the host mom’s own chicks die an early death while the parasite eggs hatch and gain all the attention.

Ideally, the parasite eggs will hatch a few days earlier than those of the host mom. This way, parasitic chicks will already be larger and more competitive than the host chicks.

If the egg laying of parasite and host moms is out of sync, and a parasite mom has missed her opportunity to parasitize a preferred nest, she may engage in nest-farming. Nest farming moms will destroy entire egg clutches or broods of host moms to reset the process of egg laying, so she can now have a greater chance of successfully parasitizing the nest.

brood parasite shiny cowbird being fed by rufous-collared sparrow
Brood parasite Shiny Cowbird being fed by Rufous-Collared Sparrow; photo by Dario Sanches cc-by-sa-2.0

Types of Brood Parasites

Generalists leave their eggs in the nests of a wide variety of other birds. Common cuckoos are generalist parasites. Although they may have a higher rate of rejection, they can be reproductively successful in a range of environments and under a variety of conditions. Biologists predict that generalistic brood parasitic species may thrive despite human-caused changes in climate or landscape. Because of their flexibility, they can lay eggs just about anywhere, at any time.

Specialists deposit their eggs into the nests of only a very specific partner species. Specialist eggs may be virtually identical to those of host eggs. As a result, host moms must evolve intricate mechanisms to be able to identify their own offspring or their own eggs. The chances of specialized-parasite breeding success in any one nest is very high. Unlike the generalists, however, they must live where their host species lives.

How Do Brood Parasite Birds Choose a Host Mom?

Both generalist and specialist parasite moms aim to do the best by their own offspring by finding the perfect host. Think of a busy mother’s endless interviews for a top-tier nanny for her child. Parasite moms take into consideration the age of the host mom, the vocal repertoire of the host mom, and the microhabitat of the host nest.

Great spotted cuckoo moms parasitize the nests of magpie moms in many temperate areas of Europe. The volume of the magpie nests is related to the potential quality of the nest for a brood parasite in a few ways. Nest volume is an indicator of territory quality; larger nests are built in higher quality territory. Magpies in high quality territories breed earlier than those in low quality ones. The great spotted cuckoo dad will stage an attack on the magpie host parents, and while they fend off the attack, mom will swoop in and quickly lay her eggs.

Naturally, parasitic moms preferentially aim to deposit their eggs into the high-volume nests of host moms in high-quality territories. After all, if you’re going to give up your offspring to the care of a mother of a different species, there should be some measures in place to ensure that the host mother will be of high quality.

brood parasitism eggs
Eastern Phoebe nest with one Brown headed Cowbird egg; photo by Galawebdesign

How Do Brood Parasites Avoid Being Rejected?

Egg appearance is an important characteristic that may make the difference between acceptance and rejection of a specific host egg. Parasite eggs with the highest levels of color overlap with host eggs are less likely to be rejected. Because of this, many specialist species have evolved complex physiological mechanisms to fine tune the color of their eggs to more closely match those of the host.

The common cuckoo is a generalist parasite which has evolved a remarkable ability to create eggs of a wide variety of colors, textures, and patterns depending on the host being parasitized at any given time.

Parasite moms will aim to lay their eggs around the same time as the host. This is done to capitalize on the opportunity to quietly add a few extra eggs to the collection without the host mom noticing.

If the parasitic Horsfield’s bronze cuckoo lays their eggs before the host fairy-wren, the eggs will be discovered and discarded. If the cuckoo lays her eggs after the main laying period of the wren, the wren will desert the nest and none of the eggs will develop.

What Are Some Defense Strategies of Host Moms To Prevent Brood Parasitism?

Species that are commonly hosts for brood parasitism have evolved a number of frontline defenses to protect their nests from parasite species:

  • Nest location away from perching points of parasites
  • Camouflage of the nest and the host species
  • Nest proximity to predators of parasite species
  • Nest structure with small cavities
  • Dense aggregations of host species to defend the nests
  • Monogamous relationships, where mom or dad is almost always guarding the nest

If parasites get past these defenses, host moms need to be able to identify the parasitic eggs. For some, instead of preventing the laying of parasite eggs, they attempt to minimize the level of damage that occurs to their own eggs. Others can identify their own chicks over the parasite chicks and simply avoid feeding the parasites.

The entire existence of brood parasitism across the bird world represents an astonishing maternal strategy that is most certainly not what one would expect in a traditional context of motherhood. Still, brood parasitism represents a sound and successful evolutionary strategy.


This post is an excerpt from my book “Wild Moms: Motherhood in the Animal Kingdom.”

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