Type: report

Article abstract: Surveys for the endangered least Bell's vireo (Vireo bellii pusillus) were conducted along the San Diego River between 31 March and 23 June 2008. Riparian habitat suitable for vireos from Interstate 5 to the El Capitan Reservoir was surveyed three times. Fifty-nine territorial male vireos were detected, 43 of which were confirmed as paired. Nine transient vireos were also detected. Most (98%) vireo territories occurred in four of six sections surveyed: Santee (34%), Park (31%), Lakeside (19%), and Gorge (15%). The Valley and El Capitan survey sections contained one or no territories. The number of territorial least Bell's vireos detected in 2008 was similar to the number detected in 1997 and approximately double the number detected in 1987. The distribution of vireos in 2008 shifted relative to that in previous years: more vireos were detected in the Park and Lakeside survey sections and fewer vireos were detected in the Valley, Gorge, Santee, and El Capitan survey sections than in 1997. The majority of vireo territories occurred in habitat characterized as mixed willow (Salix spp.) riparian, with 68% of males in the study area found in this habitat. The remaining 32% of males occupied willow habitat co-dominated by cottonwoods (Populus fremontii). Only one territory was dominated by exotic vegetation (salt-cedar; Tamarix ramosissima). A total of 74 least Bell's vireos were banded during the 2008 season. These included 21 adult vireos and 53 nestlings that were banded for the first time in 2008. The 21 adult vireos were banded with unique color combinations. The nestlings were banded with a single light blue numbered federal band (49 on the left leg and four on the right leg). Three other vireos that had been banded prior to 2008 were resighted along the San Diego River in 2008. One was banded as an adult in 2006 near its 2008 territory. Two others were banded as nestlings in 2007, one on the Santa Margarita River on Marine Corps Base Camp Pendleton and one on the Sweetwater River. These two birds moved 39.0 + 32.5 km from their natal sites to their 2008 breeding territories. Nesting activity was monitored in 30 territories within two Treatment sites (one giant reed (Arundo donax) removal site and one cowbird control site) and two Reference sites. Treatments had not occurred by the 2008 breeding season, so these results represent a pretreatment baseline. A total of 68 nests were monitored during the breeding season; howeve