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. Eighty-two territorial male vireos were detected, forty-five of which were confirmed as paired. One transient vireo was also detected. Most (96%) vireo territories occurred in four of six sections surveyed: Santee (37%), Park (28%), Lakeside (17%), and Gorge (15%). The Valley survey section contained two vireo territories and no vireos were detected in the El Capitan survey section. The number of territorial Least Bell's Vireos detected in 2009 increased 30% from 2008. Vireo numbers increased in four of the six survey sections, with the largest increase in the Santee area (50%). The majority of vireo territories occurred in habitat characterized as mixed willow (Salix spp.) riparian, with 72% of territories in the study area found in this habitat. Twenty-four percent of territories occurred in willow habitat co-dominated by cottonwoods (Populus fremontii), and one territory each occurred in willow habitat co-dominated by sycamores (Platanus racemosa), riparian scrub, and non-native vegetation. A total of 84 banded Least Bell's Vireos were observed during the 2009 season. These included 40 adult vireos (26 banded prior to 2009 and 14 banded in 2009) and 44 nestlings that were banded for the first time in 2009. Of the 40 adult vireos, 39 were banded with unique color combinations and 1 was observed with a single metal light blue service band, indicating it had been banded as a nestling in 2008. The nestlings were banded with a single light blue numbered federal band on the right leg. Seventy-two percent (80% of males and 50% of females) of adult vireos banded prior to 2009 returned to the San Diego River in 2009. All returning adults occupied the same territory that they had occupied in 2008. One additional adult vireo that was not detected in 2008 was captured and banded in 2009. This vireo fledged from a nest on the San Luis Rey River in 2007, 55 km from his 2009 breeding site. Twenty-six percent (40% of males and 13% of females) of vireos banded as nestlings in 2008 were resighted in 2009, one of which was captured and banded on Marine Corps Base Camp Pendleton (61.3 km from his natal territory). The average distance first-year vireos dispersed from the San Diego River to all sites was 10.2 ± 22