Summit Outside: Outsourcing raising your young
November 17, 2012
Don’t want to build your own nest, spend time sitting on eggs, feeding young, yet nature calls you to reproduce, why not lay your egg in someone else’s nest and let them raise your young. This type of behavior is called brood or nest parasitism.
One definition of a parasite is: A person who habitually relies on or exploits others and gives nothing in return. In this case it’s a feathered creature. The term “brood parasitism” and “nest parasitism” is used interchangeably.
Interspecific brood parasitism has received a great deal of attention from ornithologists. This behavior of laying eggs in another bird’s nest can occur in a number of bird species and is not uncommon in ducks and water birds like the coot.
If a bird of one species parasitizes another by laying an egg in its nest, the act is relatively easily detected by a human observer. The host and parasite eggs and the host and parasite young usually differ in size and coloration. Many species of birds however, don’t even recognize an alien egg and will raise the young of others.
Many of us might remember the children’s story: “The Ugly Duckling” by Hans Christian Anderson. Turns out mother goose hatched a swan!
Cuckoos are notorious for not wanting to be domestic: sex happens but who wants to build and sit on a nest! They will frequently lay an egg in another species nest. When the hatchling emerges, he may push his other nest mates out of the nest and the hapless, clueless, adoptive parents are left to struggle to feed a youngster that might even be larger than themselves. The only parallel I can imagine is studies done where humans have raised a chimpanzee as they would a human infant.
Another example is the brown-headed cowbirds. In fact, the genus name of cowbirds, Molothrus, means “intruder” in Latin. It tends to use the nests of migrant species as host such as warblers, tanagers, vireos and thrushes. The populations of many of these birds have been declining and parasitism by cowbirds is one of the factors.
Female cowbirds remove host eggs from the nest, nestling cowbirds then compete with the host nestling being much larger and more aggressive. In many cases these host parents cannot differentials between their own and the cowbird.
Why does this phenomenon of brood parasitism occur frequently in ducks? One suggestion is that many ducks, especially cavity nesters, suffer from a shortage of suitable nest sites and this could motivate several females to lay in the same nest. Ducks also seem to be better able than smaller birds are to find nests of other females of their own species. Also, duck young are relatively mature and able to move around after hatching, which can lessen the host’s burden when caring for the young of others. Birds that are hatched with eyes closed, with little or no down, and incapable of departing from the nest (called altricial), are less likely to survive and starvation is commonly observed in such “adopted” offspring.
Another reason ducks tend to parasitize nests is that a female duck often returns to the place of her birth. Sisters or mothers and daughters would therefore tend to nest in the same area, and in many cases, parasitize each other.
Another reason duck nests are parasitized is that many duck species do not defend the immediate vicinity of their nests during the laying period, allowing parasitic females to sneak around and lay eggs in another’s nest.
The redhead duck appears to be a most notorious parasitic duck. In one study in reservoirs in Alberta, Canada, redheads parasitized 19 percent of 685 duck nests, laying an average of 2.68 eggs per parasitized nest. Mallard nests were most frequently parasitized.
So what are the consequences to female duck that is parasitized by another of the same species? She may have her own egg output reduced. Hatching success of her own eggs and survival of her hatchlings may be reduced, and the larger brood may attract more predators.
What can the host female do in the face of parasitic attack? She can desert the clutch and start over, thus not spending her efforts on a mix of her own and “adopted” offspring. She then wastes the resources tied up in her own eggs.
She can identify the parasitic eggs and discard them.
If she senses from the presence of more eggs in the nest than she laid, she can adjust the number of eggs she lays subsequently so as to maximize survival of her own offspring. All this can occur if a female can recognize eggs that are not hers. Oddly enough there are many bird species that cannot recognize “foreign” eggs.
The American coot, unlike other parasitized species, has the ability to recognize and reject parasitic chicks from their brood. Parents aggressively reject parasite chicks by pecking them vigorously, drowning them, preventing them from entering the nest.
They learn to recognize their own chicks by imprinting on cues from the first chick that hatches. Usually the first to hatch would be their own egg. The first-hatched chick is a reference to discriminate between later-hatched chicks. Chicks that do not match the imprinted cues are then rejected. Coots are one of only three bird species in which this behavior has evolved.
Brood parasitism is one of the most interesting and strange phenomenon in nature and demonstrates the amazing relationships the myriad species of birds can have with one another.
Joanne Stolen is retired from teaching microbiology at Rutgers University.