Today in the Project Lab, I prepared a study skin of a Song Sparrow (Melospiza melodia). Although not as prominent as their other sparrow contemporaries, I often hear Song Sparrows speaking to each other in Golden Gate Park through series of “barks,” saying “here I am” and “where are you?” These small birds can be identified by their streaked breast with central dark spot and heavy malar stripe. As I walk to work, I start to wonder what interesting things can be learned from this bird. What in our collections can help us understand more about Song Sparrows?
The International Union for Conservation of Nature (IUCN) currently classifies the Song Sparrow as a species of “least concern.” Species in this category are seen as stable populations that presently do not fit into the threatened or extinct category. Although they are a species of “least concern,” it doesn’t mean they don’t face fierce competition from other birds. Song Sparrows can be victim of brood parasitism, which is when one bird species lays their eggs in other bird species’ nests. So how does this happen, and why?
Bird nests come in a wide spectrum of shapes, sizes and materials. Some birds build completely enclosed nests and other birds lay their eggs directly onto the ground. Song Sparrows are open cup nesters, meaning their nests look like a cup or a bowl. Eggs are laid at the bottom of the “cup” and the parent provides warmth and cover by sitting inside the nest. When these nests are left unattended, brood parasites can swoop in and place their own eggs into the exposed nest.
Insert photo of nests.
Brown-headed Cowbirds (Molothrus ater) will lay their eggs in nests of Song Sparrows when the moment is right. Both lay eggs that look very similar, which may be an adaptation that allows brood parasites to deceive the host. For Song Sparrows, sometimes the egg is noticed right away and the parent will kick the Cowbird egg out of the nest, but other times the parent will unwittingly raise the Cowbird chick, occasionally to the detriment of its own young. The Cowbird adult is effectively relieved of parental duties and can invest energy into foraging and breeding, rather than nest building and raising young.
Brood parasitism is seen in cuckoos, some ducks, and a few different passerines. There are many theories as to how this strategy has arisen over time. The relationship between the brood parasite and its host can be complicated, constantly evolving with one trying to outwit the other. Song Sparrows and Brown-headed Cowbirds are only one example of this fascinating phenomenon. Eggs and nests here in the Academy’s collection can be used to study brood parasitism and how it evolves over time. How much of a role does egg mimicry play? How much of a role does nest shape play? These are questions that can utilize the amazing egg and nest collection right here in our very own museum.
Curatorial Assistant and Specimen Preparator
Ornithology & Mammalogy Department