Brown-headed Cowbirds

We had an interesting encounter this spring in our own backyard here at the North Cascades Basecamp with something we talk and teach about on a regular basis...  nest parasitism.  We found a ground nest of juncos with an intruder amongst them; a brow-headed cowbird chick (chick on the top-left in the photo).  Notice the new feathers already coming in on the cowbird chick, however the juncos hardly have their down.  It is also significantly larger than the other babies.

Mama and Papa junco diligently were caring for their own trio of juncos, but this babe (the cowbird, chick #4) kept growing and eating, and eventually the juncos were looking a bit lethargic due to their unwanted sibling's ability to grow faster, eat more, and even squish them out of the nest.  Here is more information about the species from the Smithsonian Migratory Bird Center.

What is a Cowbird?

The brown-headed cowbird (Molothrus ater) is a brood parasite that lays its eggs in the nests of other birds. These "foster parents", called hosts, usually raise cowbird young at the expense of their own eggs or young.
Cowbirds earned their common name from the habit of following herds of buffalo (and cattle) in search of the insect prey that were flushed up by the large grazing mammals.

Portrait of a Parasite

Cowbirds have been called songbird chickens because they can lay more eggs than any other wild bird. A single female is capable of laying nearly one egg per day at the peak of the breeding season, and produces a total of 30-40 eggs over the 2-3 month breeding period (May-July). Because female cowbirds usually lay only one egg in a host nest, this translates into 30-40 nests parasitized (usually of at least several different species) per female in one season.

Typically, female cowbirds find potential host nests by perching during early morning hours in a location with high visibility such as a dead tree in a forest opening, observing behavior of host females that are building nests, and following the unsuspecting birds directly to their nests. Once the nest location is known, the cowbird returns during the egg-laying period of the host female and deposits her own egg in the nest. By parasitizing during the host egg-laying period, the female cowbird improves her chances that the host female will be away from the nest often (allowing the cowbird access without being detected), and that the cowbird egg will be accepted by the host and receive full incubation.

In approximately 60-70% of nests parasitized, the cowbird removes one of the host eggs before laying her own egg. This is possibly to ensure that the host does not "notice" a different number of eggs in the nest, or to ensure that the cowbird egg receives full incubation (an extra egg laid in the nest could make it difficult for the female to incubate all eggs evenly, resulting in some eggs—possibly the cowbird's—failing to hatch).  Cowbird eggs require an incubation period of only 11-12 days, whereas most host species require 12-14 days, and some as many as 17 days of incubation. Consequently, cowbird young hatch earlier than the host young, affording the parasite a distinct advantage in competing for food with its younger foster siblings.

Cowbirds tend to parasitize birds smaller in size than themselves. For example, by the time they are ready to leave (fledge) the nest, most warbler or vireo (the most common cowbird hosts) nestlings weigh less than half as much as cowbird nestlings. In fact, a fledgling cowbird is larger than the adult warblers and vireos! The monstrous cowbird nestling not only can cause host young to starve by monopolizing the food supply brought by parent birds but also literally may crowd host young right out of the nest.

All of these attributes of the parasite can add up to severe effects on the host family. Impacts of parasitism are different for different host species, but usually result in loss of at least one, and often all, of the 3-5 host young. Although each cowbird female only lays one egg in a nest, a high abundance of cowbirds in an area often will lead to many cowbirds parasitizing the same limited supply of host nests. The presence of more than one cowbird in a nest almost inevitably leads to death of all host young. In addition, raising even one cowbird can potentially reduce the survival of host parents because of the increased physical effort needed to care for the ravenous intruder.