Photo Credit: Maslowski, Steve, Public domain, via Wikimedia Commons
“I hear you, but I can’t see you” are often the words going through my head when I am surveying riparian habitats amongst least Bell’s vireo (Vireo bellii pusillus; LBV). Vireos are small, grey-green, migratory songbirds that are quite chatty and territorial but are often tucked away and out of sight. Though there are thirty-three types of vireo species worldwide, 11 of those species call San Diego County home. The least Bell’s vireo is a subspecies of the Bell’s vireo. The definition of a “species” and “subspecies” is a hot debate in the world of taxonomy as it relates to the Endangered Species Act (ESA). A new subspecies may evolve when individuals of a single species of bird (or any animal) becomes separated in some way (i.e. geographically or migration timing) that prevents breeding. Eventually, over time, two entirely separate species may form; these species will not be able to produce fertile offspring in the event that they are able to meet and mate in the future. In the case of the Bell’s vireo, the LBV and the Arizona Bell’s vireo (Vireo bellii arizonae) are both subspecies because they are separated geographically and do not interbreed. The LBV breeds entirely in California and Northern Baja California, Mexico, whereas the Arizona Bell’s vireo breeds in few areas along the lower Colorado River, but occurs primarily throughout Arizona, Utah, Nevada, and Sonora, Mexico.
The LBV is listed as an endangered subspecies under the ESA. Habitat fragmentation and parasitic, brown-headed cowbirds (Molothrus ater) are among the main reasons for the rapid decline of this subspecies. Although they are a listed subspecies, there have been signs of recovery among populations in San Diego county. These migratory birds fly north from Mexico to San Diego in March and begin to set up breeding territories. Once May comes around, breeding territories have been established, nests have been built, and some eggs have been laid. Their eggs are white with speckles and are incubated for 14 days. Adult pairs can have two broods in one nesting season between May and July.
Balk Biological assists with the long-term monitoring of these species within the prime habitat found at the Tijuana River Valley. These birds are often found in thick riparian scrub or low-lying marsh lands, and their signature call allows biologist to easily identify the tucked away birds with our ears, when the birds are often out of site. The pattern of their call brings to mind the sound of buffing a “squeaky clean” window. The quick back and forth of a towel on a window sounds very similar to the quick ups and downs of a least Bell’s vireo call. If the birds are feeling extra feisty, they cue up their raspy, low whisper chatter to let other birds know the extent of their nest territory. This year we have identified five LBV territories within a subset of the Tijuana River Valley. This is a promising sign for both the species and the recovering habitat of the mitigation site in which they reside.
Shelley Lawrence, Biologist