Dec/Jan 2010 home page

  Bird “Mafia”
Ed Mirsky | Hoska Drive


Graphic Art Olson

Hay! Hay! Hay youse, Bell’s Vireo. We’re the cowbird gang, and we’re in the neighborhood now. Here’s the deal: I see you laid an egg in your nest and you’re planning on raising a family in my neighborhood.

It’s like this: My girls are going to lay their eggs in your nests so they don’t have to waste my time taking care of our chicks. And here’s the deal. If you toss one of my wife’s eggs out of your nest or you hurt one of my chicks, I’ll send my peoples over to violently ransack and destroy your nests. So if you’re going to live in my neighborhood, you’ll raise my chicks.

Bell’s Vireo. Photo Tom Grey

If the above sound like retaliatory “mafia” behavior to you, you’re not alone. That’s what scientists have dubbed it, and that’s what Brown-headed Cowbirds do (Hoover and Robinson. 2006).

Least Bell’s Vireos have been dealt a one-two punch. Prior to the destruction of California’s riparian lands for agriculture and urban development during the mid 1800s and early 1900s, Bell’s vireos were one of the most abundant birds in the State. They suffered precipitous declines between 1930 and 1985. By 1986, only an estimated 300 pairs remained in the central valley with few elsewhere.

Brown-headed Cowbird
Photo Tom Grey

Then, around the 1960s the Brown-headed Cowbird made its way into California, aided by the spread of agriculture and ranching in the plains states and southwestern deserts. Historically, cowbirds followed herds of migrating bison, taking advantage of the food the bison kicked up in their wake. They are still associated to an extent with large mammals such as cows and horses.

Bell’s vireos build open nests in dense riparian habitats, and low numbers are present along the San Dieguito River east of I-5. Brown-headed cowbirds are obligatory brood parasites. That means they never build their own nests, although as noted above, they are genetically programmed to depredate host nests.

A female cowbird generally chooses an open cup-nest to parasitize, and usually lays one egg per nest. She waits until the host bird has at least one egg in its nest, she removes one egg from the host nest, and then she lays her egg in the nest. This process is continued for about a month, and single female may lay up to 40 eggs per breeding season.

Cowbird chicks typically hatch earlier than their host’s eggs (10 to 12 days vs.12 to 17 days), and cowbirds chicks are bigger, stronger, and eat more than their host chicks to the detriment of the host chick and the host species.

Large numbers of cowbirds (30 or more!) are attracted to the horse stables adjacent to the river east of I-5 when the horses are fed. They represent a threat to the breeding success of the endangered Bell’s vireo, whose habitat is being choked by the non-native invasive freshwater Salt Cedar (Tamarix).


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