Brown-headed Cowbird (Molothrus ater)
   
 
Narrative and Science Lens
 
Audubon knew this bird as the "Cow-pen Bird," so-named for its habit of attending cows to consume associated insects and worms. He was aware that it does not nest, but instead lays each egg in the nest of another, to be raised by foster parents. He also knew the cowbird female doesn't force her way into a nest, but surreptitiously lays her egg. Sneaking an egg into the right nest is critical. The potential foster parents may eject the egg, bury it under a layer of nesting material, abandon the nest, or parent poorly.

Cowbirds prefer some hosts over others. Among species commonly seen on campus, they frequently or commonly victimize/parasitize Common Yellowthroats, Red-winged Blackbirds (See Art: Wheye, below), California Towhees, Spotted Towhees, and Song Sparrows. Cowbirds may also target Bewick’s Wrens, Dark-eyed Juncos, Bullock’s Orioles, and House Finches, although not commonly, and only rarely target Mourning Doves, White-breasted Nuthatches, Western Bluebirds, and American Robins. (See Sustainability, below)
 
  Campus Locations
Fairly common breeder in a variety of habitats virtually throughout campus--everywhere where there are hosts (everywhere) or food (usually seeds or anthropogenic food waste)--less common during the nonbreeding season.
  Campus sustainability
  Stealing Parental Care: In a 1985 report Friedmann and Kiff found more than 220 host species victimized (received a cowbird egg) and 144 species parasitized (reared cowbird young). Targets usually ranged in size from .35 ounces (kinglets) to 5.3 ounces (meadowlarks).

In central and eastern North America, where host species have coexisted with cowbirds for centuries, many hosts have evolved strategies to avoid, or at least reduce the effects of, brood parasitism.  Brown-headed Cowbirds, however, did not occur in the San Francisco Bay area historically. From a toe-hold in the SE corner of the state around 1900, the species expanded throughout most of the state by the mid-1900s, first occurring in the South Bay in 1922. As a result, California hosts have not had as much time to evolve parasitism defenses, and some host species have suffered considerable losses. On campus, a study found the California Towhee the most frequent cowbird host.
     What you can do
       1. Try to spot foster parents tending cowbird young, and share the sighting.
       2. Try to asses if these birds out-compete other species benefiting from discarded food and crumbs on patios (See, Commensal Feeding, below.)
  Science
       Essays from The Birder’s Handbook:
            Brood Parasitism; European Starlings; Range Expansion; Commensal Feeding
       References: Darley, 1982; Friedmann and Kiff, 1985; Teather and Robertson, 1986.

       Videos:
 
  Art
       Photos:
            Rohan Kamath: Brown-headed Cowbird (male and female)
       Drawings and Paintings:
            Darryl Wheye: Red-winged Blackbird and Brown-headed Cowbird, after Audubon--Too Late

 
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