Bell's Vireo at San Dieguito River Park

In the 1980's the California population of Bell's Vireo (the Least Bell's Vireo) was down to only about 300 pairs. Two causes of the decline of this once abundant species are really one cause: Man.

Clearing of riparian woodlands in California was a direct attack on the habitat of this small bird. Clearing of forests in general allowed the Brown-headed Cowbird to expand from its historic habitat of Great Plains grasslands to spread across North America. The cowbirds parasitize the nests of smaller songbirds, laying their eggs in the smaller bird's nest to be raised by their foster parents.

In just 15 years of protection as an endangered bird in 1984, the population rebounded by a factor of six (Unitt 2004). And now another successful 15 years have elapsed. How are they protected? Remaining habitat is protected and enhanced. Cowbirds in their habitats are trapped and destroyed. Not an ideal or permanent solution, but necessary now.

Least Bell's Vireo. Near Escondido, California. April 3, 2016. Greg Gillson

Bell's Vireos need a dense canopy with a dense understory in damp stream bottoms. So the San Dieguito River bottom is perfect for them.

The Least Bell's Vireo is a drab gray above and whiter below, different from the Eastern Bell's Vireo, which are more greenish above and yellower below. They have rather faint "spectacles," a white eye ring with attached pale lores to the bill. The wing bars are likewise faint, often only one bar is obvious.

Males give two husky song phrases, one following the other immediately. The first ends with a rising inflection, the second is the same but ends with falling inflection. Thus it sounds as if it is asking, and then answering, a question. Here's a wonderful YouTube video of a singing bird by Don DesJardin.