Brood Parasites
A Unique Breeding Strategy

All species of birds lay eggs, and most build a nest to hold their eggs. Some species, like nighthawks and killdeer, lay their eggs on a bare patch of ground. Whether they build a fancy woven purse like an oriole or just make a little scrape on the ground, just about all nesting birds incubate their eggs to keep them warm and then take care of their babies. But there are exceptions. A few birds don't build a nest, incubate eggs, or care for young at all. They leave the responsibility for raising their babies to other species.
This breeding strategy is called brood parasitism, and it's done by birds called brood parasites.

Ornithologists studying brood parasitism are still trying to figure out exactly why and when this unique behavior originated. Brood parasites include several species of cowbirds in North, Central, and South America; the black-headed duck of South America; some weaver-finches in Africa; honeyguides in Africa and Asia; and some cuckoos in Europe and Asia. Each group may have developed their parasitism for different reasons and in different ways.

It Works for Cowbirds
Fewer than 1% of all birds are brood parasites. Over most of North America, the only brood parasite is the brown-headed cowbird. It makes sense for the brown-headed cowbird to be a brood parasite when you consider this bird's history. Long ago, its range was fairly restricted to the short grass prairies of the Great Plains, where it lived on seeds and insects it found on disturbed soil. How did the soil get disturbed? Big, heavy bison hooves broke up the prairie sod. When cowbirds followed the bison, they could find food on the ground, some even in and around the bison droppings, which held partly-digested seeds and attracted insects.

This worked great most of the year. But during the breeding season, the cowbirds would have had a problem if they nested like other birds. Bison herds move about constantly. Imagine what would happen if a cowbird started nesting and suddenly the bison left! Without "help" from the bison, how could she possibly feed her babies or herself if she stayed? Being a brood parasite, she stayed with the bison, knowing her babies were safe in the nests of birds who found food in other ways.

How Female Cowbirds Find Nests
Brood parasitism requires a lot of energy. Robins lay an average of 12 eggs a summer if they manage to renest twice after their first brood is done, but cowbirds may lay as many as 40 eggs in a single season! How do female cowbirds find appropriate nests? They mainly use three nest-finding strategies:

  • Perching quietly in the tops of shrubs or trees, watching for nest-building activities
  • Walking quietly on the ground watching for nest-building activities
  • Flying noisily and landing, apparently intentionally, on leaves rather than on branches while flapping wings noisily. Sometimes this flurry of activity flushes nesting birds from their nests, allowing the cowbird to see where they are.

Once the cowbird locates the nest, she usually removes one egg. Sometimes she does this the day she lays her egg in the nest, sometimes before. She comes to the nest about 9 minutes before sunrise to lay her egg, and can be very quick. It usually takes only 20 - 40 seconds. One cowbird managed to lay her egg in a visit that lasted only 4 seconds!

How Nesting Birds React to a Cowbird Egg and Baby
Many nesting birds don't even seem to notice a cowbird egg among their own eggs. But if a nesting bird does discover a cowbird egg in her nest, she may

  • toss it out.
  • ignore it.
  • build a new nest bottom, covering up the cowbird egg and any of her own eggs.
  • abandon the nest.

Once the cowbird nestling hatches, the foster parents raise the nestling as if it were one of their own. Baby cowbirds are not aggressive toward the other nestlings, and never throw out other eggs or babies. If there is an abundance of food, they can all survive (except the one baby in the egg that the cowbird threw out to start with). But cowbird mothers tend to select nests of smaller species, such as sparrows and warblers, for hosts. The cowbird hatchling will be larger than the others, and when it begs, its mouth will be larger. This is a problem for the natural nestlings; each time most parent songbirds return to the nest, they feed whichever baby has the biggest mouth. Unless the cowbird is very full, its mouth is larger than the other babies, and it gets the lion's share of the food.

Cowbirds remain with their foster parents for about 13 days after they leave the nest, following and begging from adults who are often very much smaller than the baby cowbirds! But after about two weeks, the babies go off on their own, and somehow instinctively associate with other cowbirds.

Cowbird Problems
Most species that raise cowbirds don't have problems. If a pair of song sparrows raises one cowbird and one fewer song sparrow in an otherwise successful summer, they'll still have raised about 7 of their own babies, because song sparrows winter north of the tropics, and remain on their breeding grounds long enough to nest at least twice. So despite raising a cowbird, their productivity would be decreased by only about 13%.

Neotropical migrants usually only have time to breed once before they must get their bodies prepared for the long migration south. So if a chestnut-sided warbler raises a cowbird, her productivity is decreased about 20-25%. This isn't too bad for such a common warbler, but for endangered species such as Kirtland's warbler and black-capped vireo, such a loss can have a devastating effect on the whole population. So cowbirds are trapped in areas where some endangered neotropical migrants live.

Most birds recognize their mates based on what their own parents look and sound like. What would happen if brood parasites were like that? No one knows precisely how cowbirds recognize their own species. As a class, discuss your own ideas about this mystery. Then work in pairs or small groups to draw a comic strip, write a rap or haiku, or create and peform a skit that illustrates your ideas.