At best, it’s a bit confusing and startling. Some people even use emotion-laden words such as “heart-breaking” or “sad.” No telling how you might feel the first time you see a small bird such as a Song Sparrow working like crazy to feed a begging cowbird youngster more than twice its size. But one thing for sure—you’ll wonder what the heck is up with that.
Brown-headed Cowbirds used to follow herds of bison in shortgrass prairie, feeding on stirred-up insects. As the only brood parasite likely in southern California—and as North America’s best known, and arguably most reviled, obligate brood parasite—the Brown-headed Cowbird lays her eggs in the nests of many species, relying on her “hosts” (some humans say “victims”) to incubate her eggs and care for her young. Brood parasitism in cowbirds complemented the cowbirds’ following the wandering bison herds very well. (Often people claim that this brood parasitism evolved with following the bison. However, this species moved up from South America, where it was a brood parasite long before it ever saw a bison.) When the herds moved on, so did the cowbirds’ food source and, therefore, the cowbirds. So laying eggs in other species’ nests allowed them to follow the herds and not to be tied to the energy-intensive duties of and raising their own offspring. (Nature’s deadbeat parents, some say.)
Although most average smaller clutches, one female cowbird can lay more than 40 eggs in a single breeding season; almost daily she chooses another nest in which to deposit a single egg. Nationwide, Brown-headed Cowbirds have parasitized more than 200 species. San Diego County Bird Atlas observers recorded 16 “foster” species—most frequently, Common Yellowthroat, Song Sparrow, Hooded Oriole, Hutton’s Vireo, and Bell’s Vireo. Some species with a long history of exposure to cowbird parasitism (e.g., Bullock’s Orioles) recognize the interloper’s egg and remove it or abandon the nest altogether and start anew. Others don’t recognize the intruder’s egg or physically cannot remove the large egg, so they raise the intruder’s offspring. Cowbirds impact the reproductive success of the host species by removing one of the host eggs before leaving their egg as well as producing eggs that hatch earlier than most hosts’ eggs (after about 11 days, compared to about 14 days). The larger, aggressive young cowbirds also out-compete their nest mates for food and space in the nest. Even the scientific name (Molothrus ater) reflects this nestling. Molothrus arises from Greek molobros—a greedy beggar, parasite, or intruder. Ater (Latin, “black, dark-colored”) refers to the male’s dark body color.
At a distance, a male Brown-headed Cowbird does appear black. But up close and in a strong light, his plumage also shows green iridescences topped off with a dull brown hood. Females are nondescript, cloaked in a dull brownish gray. These cowbirds are noticeably smaller than all other members of the icterid family (e.g., blackbirds, grackles) that they often hang with. The male sings a song that is quiet but distinctive, if you tune your ears just right—a bubbly gurgle followed by a short musical whistle
The near extermination of bison in the late 1800s restricted this “buffalo-bird” to low-elevation grasslands of the West as cattle replaced bison. (The common name, “cowbird,” describes the present-day habit of feeding around cattle; “Brown-headed,” to the dull brown hood of the male.) These cowbirds have now dispersed widely across most of North America as settlement by Europeans changed the grasslands, prairies, and forests into the agricultural and suburban landscapes of today. Brown-headed Cowbirds have been rather recent arrivals in San Diego County, first appearing here only about a century ago.
Brown-headed Cowbirds are birds of a flock. In warmer seasons, they forage for insects among the “flocks” of hoofed mammals. In winter, they mix with blackbirds and other icterids in search of grain. In such mixed avian flocks, Brown-headed Cowbirds easily stand out as the smallest; the females, the palest. Although some of this species winter here, San Diego County Bird Atlas records suggest that most of the cowbird breeding population arrives in early April.
Although cowbirds have made a success of the practice, brood parasitism in birds remains rare (~1% of all bird species). It’s an intriguing strategy. A good deal of species-specific information is transferred to baby songbirds by their parents—for instance, what characteristics to look for in a mate and what songs to respond to. So how can a baby cowbird learn to be a cowbird if it’s raised by, say, a Song Sparrow? In nearly every test where the offspring of two songbird species were switched early in the nestling phase, the swapped juveniles readily adopted the behavior and mate choices of their foster species (referred to as sexual imprinting). Yet when raised in isolation, Brown-headed Cowbirds respond to a cowbird’s song; nestlings also increase their begging in response to cowbird calls, indicating the hard-wired and early onset of species recognition. Timing appears to be critical—captive juvenile brood parasites will sexually imprint on their host’s phenotype if the association extends beyond the typical short timeframe that cowbirds remain in the nest (about 11 days from hatching to fledging).
So again arises the question of how cowbirds learn to identify with other members of their species. A recent study offers some thoughts. Adult birds don’t feed fledglings during the night, which decreases their reliance on foster parents at that time. Therefore, departing at sunset may represent the perfect chance to explore. Fledgling cowbirds leaving their birth areas prior to sunset—perhaps with a genetic predisposition for certain habitats—could encounter fellow cowbirds and potentially accompany adults to a communal night-time roost, helping youngsters not only find suitable foraging locations but also learn the song and behaviors of future breeding partners. Come morning, the kids can head back to their natal habitat and mooch off their foster parents some more.
When cowbirds followed nomadic bison, local bird “victim” populations faced only intermittent, sporadic parasitism. Now, though, with stationary cattle herds and, consequently, stationary cowbirds, potential host species face constant, unremitting pressure and frequent breeding failures. High densities of cowbirds and severely impacted avian communities have brought heartfelt appeals for programs to trap and remove cowbirds from host breeding areas. Unfortunately, such trapping programs are expensive, labor-intensive, and rarely permanently effective. They work best for endangered host species with limited breeding areas. For example, we came across a trapping project along the lower San Luis Rey River, attempting to increase the breeding efforts of the endangered Least Bell’s Vireos and Southwestern Willow Flycatchers that could use that habitat.
One faction states that extensive cowbird parasitism is simply a result of cowbirds’ being successful cowbirds. Any intervention uses resources that most agencies these days don’t have, so just let Nature take its course. The host species will adapt or they’ll disappear. Evolution and succession in progress. Alternatively, since we humans have destroyed bison herds and created new landscape conditions that enable cowbirds to thrive at the expense of more marginal species, perhaps we have an obligation to protect the species now endangered by cowbirds. Arguments can be made for both sides—where do you stand?
Photo Credit- Steve Brad