Over the past several days, I have observed an amusing yet disturbing relationship on our Littleton, Colorado, farm. A tiny bushtit "parent" has been tirelessly racing among the shrubs and trees, followed by its demanding and much larger "offspring."
The bushtit, usually observed with its true relatives in active, twittering flocks, has been the victim of brood parasitism by a brown-headed cowbird. The latter species, common throughout southern Canada and the contiguous 48 States, lays its eggs in the nests of other songbirds, relieving the cowbird parents of incubation and feeding duties and allowing them to produce multiple clutches of eggs throughout the breeding season. This behavior gives them a significant competitive advantage and is obviously detrimental to the foster parent songbirds. Indeed, the aggressive cowbird nestlings, often larger than their nest mates, end up receiving most of the food delivered by the host parents and may actually kill the genetic offspring (actively or passively) in the process. The populations of some native songbirds have been significantly threatened by cowbird parasitism; while some species recognize the cowbird eggs and either discard them or abandon the nest, female cowbirds are capable of laying up to forty eggs per season.
As a naturalist, I accept the fact that the doting bushtit and the aggressive cowbird fledging are merely responding to their natural instincts. Indeed, evolution has been driven by competition and natural selection; nature neither displays favoritism nor rescues the victims of her complex web of life. Still, I must admit feeling sorry for the bushtit and know that, if she survives her ordeal, she will be relieved when the cowbird departs to join its true relatives.
