Brown-headed Cowbirds and Brood Parasitism
Did you know that some bird species do not construct a nest and rear their young?
It’s true — Some birds are known to lay their eggs in another bird’s nest. They then go off while the “host” parent, or parents, get stuck with all the parental care of these young, often at the expense of their own offspring. And brown-headed cowbirds are notorious for this practice, called brood parasitism.
Most North Dakotans are probably familiar with brown-headed cowbirds. The males look like blackbirds with a chocolate brown head, and the females are gray. Cowbirds inhabit a variety of habitats, but generally avoid forests. They are common throughout the entire state, and are perhaps most often observed around livestock, as they feed on insects stirred-up by the livestock.
Brown-headed cowbirds are obligate brood parasites. They do not construct a nest. Instead, they lay their eggs in the nests of other species. As a result, they do not have to expend all that energy or the risks associated with incubating, and then rearing the young. They can actually increase their reproductive output by doing so.
As you might expect, this is a detriment to the host birds. Cowbirds may remove host eggs from the nest, their young often hatch earlier than some of the host eggs, are often larger, grow faster, and are more aggressive in getting fed. All this ends up reducing the reproductive output of the host birds. Some host birds, however, can identify the eggs or young of parasites and may push them out of the nest. But others may simply abandon their nest.
And there is an interesting twist here: Cowbirds and forest fragmentation appear to be causing warbler populations and some other forest nesting birds to decline over much of North America.
Woodlands in North Dakota and elsewhere are increasingly being fragmented due to changes in land use, and cowbirds travel the edges of forests. Something like 90% of cowbird parasitized nests are within a few hundred feet of the forest edge. Nests further into the forest experience little parasitism by cowbirds.
As you might expect, cowbird populations are now increasing while warblers and other forest nesting bird populations are decreasing. So, those warbler nests within a few hundred yards of the edge of small stand of trees or woodlands may not be producing enough young to maintain their populations.