Type: report

Article abstract: This report provides methods, results, discussion, and recommendations for a study of California Gnatcatcher (Polioptila californica) natal dispersal at a 200 acre (80-hectare) area of open space divided by an interstate highway, in the Lakeside area of San Diego County, California. The study area lies 23 miles east of the Pacific Ocean at the inland edge of a coastal plain and has a relatively warm, dry Mediterranean-type climate. Topography consists predominantly of moderate slopes on either site of Interstate-8 (I-8) with the more southerly parcel of steeper grade. Elevation ranges from about 700 to 1100 feet. The primary goal of the study was to evaluate whether California Gnatcatcher movement across I-8 occurs at the study area and thus shed light on whether this area serves as a functional corridor - at least for the California Gnatcatcher - as assumed in the MSCP Subarea Plan. Field work was conducted during dry conditions, and simple nest success (that is, the number of nests that produced fledged young) of the subject species was less than 50%. We found and monitored seven pairs of California Gnatcatchers in our study area. We found a total of 11 nests during the study period, an average of about 1.6 nests per pair of California Gnatcatchers; however only six of the pairs constructed nests. Of the 11 found nests, four were successful, that is, they produced young that fledged; five were abandoned prior to egg deposition; and two were destroyed by causes unknown. We banded young at the nest, or we captured juvenile California Gnatcatchers soon after fledging by strategically placing mist nets within natal territories at locations past which we expected family groups to move. We determined the banding strategy based on our assessment of whether approaching a nest would have a high or low likelihood of affecting the nest outcome. We banded all seven successfully fledged young from the site with no mortality (Table 1). Six of these were banded as fledglings; one was banded as a nestling. On 23 July 2002 Kylie Fischer documented the occurrence in the Pembroke parcel of a juvenile (left leg white; right leg silver USGS) that had been banded on 11 June 2002 in Territory C-01b-02 of the CalTrans parcel, that is, north of I-8. This bird had been observed on the CalTrans parcel during the previous week (15 July). On 13 September 2002 William Haas detected a banded juvenile (left leg purple; right leg silver USGS) on the Gatlin parcel. This bird