Type: report

Article abstract: Surveys for the endangered least Bell's vireo (Vireo bellii pusillus) were conducted at Marine Corps Base Camp Pendleton, California, between 2 April and 11 July 2007. Drainages containing riparian habitat suitable for vireos were surveyed three to seven times. Because of difficulties in maintaining radio contact with Range Control the upper Santa Margarita River was surveyed once for vireos. Seven hundred and seven territorial male vireos were detected on 19 out of the 23 drainages/sites surveyed. Ninety-one percent of all vireo territories occurred on the seven most populated drainages, with the Santa Margarita River containing 58% of all territories on Base. Eighty percent of male vireos were confirmed as paired. In 2007, the overall size of the vireo population was similar to the 2006 population, differing by only 11 territories, and was within the range of approximately 700-1000 territories observed on Marine Corps Base Camp Pendleton for the past 13 years. The number of territories on 87% (20/23) of drainages surveyed differed by fewer than four territories compared to 2006, while 65% (15/23) of drainages differed by one or fewer territories. Six drainages increased in vireo numbers, 10 decreased, and 7 showed no change. Overall, the vireo population on Base decreased by 2% in 2007 compared to 2006. The majority of vireo territories occurred in habitat characterized as Willow Riparian, with 74% of males in the study area found in this habitat. An additional 6% of birds occupied willow habitat co-dominated by cottonwoods or sycamores. Thirteen percent of territories were found in Riparian Scrub, dominated by Baccharis salicifolia and/or Salix exigua. Six percent of the vireos used drier habitats including areas dominated by a mix of sycamores and oaks (1% of total) or upland vegetation (5%). Fewer than 1% of vireo territories were placed in habitat dominated by solely non-native vegetation. Nesting activity was monitored in 48 territories within the A. donax Removal and Reference monitoring areas. A total of 108 nests were monitored during the breeding season; however, 10 of these were not completed and were excluded from calculations of nest success and productivity. Nest success of pairs breeding in Reference and Removal sites did not differ significantly. Forty-seven percent (25/53) of Reference nests and 49% (22/45) of Removal nests successfully fledged young. Predation was believed to be the primary source of nest failu