Reprinted from an Aug 20, 2005 column in the Morgantown Dominion Post
In April I was in South Texas on a bowhunt for wild hogs. Every morning as we returned from the morning hunt there were thousands of brown-headed cowbirds perched on power lines along the roads. They were everywhere.
Being an old birder, I came to know the cowbirds forty years ago. They are not hard to find. Cowbirds are common in agricultural areas, and favor the edges of fragmented forests. When you get into totally forested areas, cowbirds are not found in high numbers. The cowbird apparently got it’s name from being seen with cows. Before that, as we settled the West, cowbirds were found feeding on insects kicked up by huge herds of bison (buffalo).
OK, so why do people dislike cowbirds? The reason is simple. Cowbirds do not build nests. They are brood parasites ... they lay eggs in other birds nest. Always. The female simply finds a nest of another songbird species and lays eggs in that nest.
This brood parasitism isn’t totally random. Research shows that female cowbirds perch along wooded edges, and watch for birds going to their nests. Actually, some research suggests that the cowbirds cue in on bird vocalizations that some species give as they leave their nest. When she hears or sees the host leave her nest, the cowbird zips in and in a few seconds, deposits an egg.
It seems that cowbirds chose host nests at such a time that their eggs will hatch just before the host eggs hatch, thus giving the cowbird chicks an advantage. In addition, they seem to select hosts whose chicks are smaller than the cowbird, again giving cowbird chicks an advantage. The bigger chick gets the food when the mother comes to the nest. And later on the cowbird chick will often evict the host species chick. Nature can seem cruel to us.
The eggs of cowbirds are whitetish-tan with brown speckling. You would think that when such an egg is laid in a robins nest where her eggs are blue, the mother would spot the culprit and get rid of it. Well, the robin does just that. However, other birds with solid blue eggs, such as the wood thrush do not. Go figure. The catbird will also get rid of cowbird eggs. Cardinals simply abandon their nest when a cowbird egg shows up.
Even though I saw thousands of cowbirds in Texas, birds that will migrate north each spring, nationwide cowbird numbers are on the decline. Not a big decline mind you, but every year for the past 20 years or so, cowbird numbers have declined slightly. Even with this decline, cowbirds can negatively impact songbirds on a local level. For example, one study of the Black-capped vireo in Texas showed that 90 percent of all nests were parasitized by cowbirds. But other studies show that parasitized birds often renest, so at the end of the year, total reproduction is not affected very much.
Even so, there have been efforts to control cowbirds especially in local areas when they parasitize nests of endangered bird species. The Kirtland’s warbler is found locally in Michigan and in the early 70's there were only 200 pairs left. Habitat loss was a key, but cowbirds were also parasitizing nests. A cowbird control program helped to increase warbler numbers to 400 pairs by 1996.
The real problem here is forest fragmentation. Not clearcutting, at least not in the long run because new growth timber will replace what is lost. No, development, such as we see all around Morgantown, and most other cities in America, that removes timber and creates openings and fragmented forests, is what favors cowbirds. Other bird species will sometimes lay eggs in another species nest, but the cowbird does it all the time. Just something for you to know the next time your sitting in your tree stand or taking a hike, and you see a brown-headed cowbird.
