Cowbirds and buffalo might not be as closely involved as we think.
Two weeks ago the birding column in the Home and Garden section of the StarTribune focused on cowbirds. I repeated the commonly held belief that cowbirds adopted their brood-parasite habit via their relationship with buffalo. Cowbirds fed on the insects and seeds disturbed by buffalo herds. Since those herds were always on the move, cowbirds could not nest in the usual fashion. While the birds were incubating and feeding hatchlings, the buffalo were moving on. So, it has been believed, cowbirds evolved into brood parasites: birds that lay their eggs in the nests of other birds, letting the host hatch and raise those chicks.
Roger Everhart, a birder in Apple Valley, wrote to tell me of new information that casts significant doubt on that idea. HIs information comes from an email post made to a national birding chat line by Alvaro Jaramillo, a senior biologist at the San Francisco Bay Bird Observatory. Jaramillo wrote of study of cowbird DNA by Dr. Scott Lanyon, professor and head of the Department of Ecology, Evolution, and Behavior at the University of Minnesota.
The upshot is the suggestion that Brown-headed Cowbirds, while they did follow buffalo herds, were parasitic before they ever found buffalo. This suggestion is based on age of the cowbird lineage, as determined by the DNA study. Jaramillo wrote, "Parasitism in the cowbirds may have evolved due to coloniality, and the safe predator-secure nests of the hosts, among other factors." (Coloniality: birds nesting in colonies.)
Jaramillo continued, "Basically, it (the evolution of brood parasite behavior) is still a mystery. However, the fact that brood parasitism has evolved in several unrelated groups such as cowbirds, two cuckoo lineages, many other birds, and even aphids it is clear that this is a good way to make a living."