A Complex in Context: Bird Surveys at the Oak Grove Unit of the San Felipe Valley Wildlife Area Complex

We thanked last December’s rains for putting a temporary dent in the drought, but this project reminded us how long it can take to see the effects permeate the ecosystem.

As part of a five-year project assisting the California Department of Fish and Wildlife (CDFW), our Museum biologists have been conducting bird surveys of the CDFW’s Oak Grove Unit of the San Felipe Valley Wildlife Area Complex in north-central San Diego County.

A big surprise was finding multiple pairs of California Gnatcatchers (Polioptila californica) breeding where this species has not been observed before. The birds were mixed in with Black-tailed Gnatcatchers (P. melanura) in desert transition habitat at over 3,000 feet in elevation. This exciting find recalls the intermixing of these two species documented in the nearby Aguanga area in Riverside County, although those occurrences were at a much higher elevation and in a different plant community. Further work will refine the extent of the gnatcatcher population in this area and which habitats they are favoring.

Despite this new discovery, many other birds had disappointingly low numbers. For instance, after an intensive survey effort, Phil Unitt and Lea Squires found only 2 Horned Larks and no Brewer’s Sparrows. This is in stark contrast to Phil’s survey for the San Diego County Bird Atlas in 1999, which turned up 40 Horned Larks and 30 Brewer’s Sparrows in the same area.

Why the difference? The winter of 1997-98 was wet, and the luxuriant plant growth in spring 1998 left abundant food for seed-eating birds the following winter. At the time of our January 2022 surveys, December’s rains had not yet had time to work their magic.

For many species, whether frugivorous, insectivorous, or granivorous, the Bird Atlas’s five years of data demonstrate a spike in numbers and distribution expansion in winter 1998-99, followed by a slump and contraction through 2001-02—the driest year in San Diego history. The data from the Atlas and collection give us the long-term perspective needed to put any single survey’s results into context.

The Oak Grove Unit (open to limited public access) encompasses an unusual mix of coastal and desert species, such as the California and Black-tailed Gnatcatchers. The only similar areas nearby, in Dameron Valley and Aguanga, are subject to piecemeal development. Thus, we are especially thankful to the CDFW for conserving a large block of this biogeographically interesting habitat, and we are ever grateful for the opportunity to revisit it.