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 3 April and 7 July 2006. All drainages containing riparian habitat suitable for vireos were surveyed three to seven times. A small number of other sites containing more marginal habitat were periodically checked throughout the season for vireos. Seven hundred and eighteen territorial male vireos were detected on 19 out of the 23 drainages surveyed. Ninety-one percent of all vireo territories occurred on the eight most populated drainages, with the Santa Margarita River containing 56% of all territories on Base. Two thirds of all male vireos were confirmed as paired. The majority of vireo territories (75%) occurred in habitat characterized as Willow Riparian. An additional 9% of birds occupied willow habitat co-dominated by cottonwoods (Populus fremontii) or sycamores (Platanus racemosa). Nine percent of territories were found in Riparian Scrub, dominated by Baccharis salicifolia and/or Salix exigua. Habitat characterized by upland vegetation was used by 6% of vireos. Four territories (1%) were placed in habitat dominated by white alder (Alnus rhombifolia), while less than 1% of vireo territories were placed in habitat composed entirely of non-native vegetation. Invasive exotic plants were found to be dominant or co-dominant in approximately 6% of all vireo territories. The primary exotic plants found in vireo territories were poison hemlock (Conium maculatum), black mustard (Brassica nigra), giant reed (Arundo donax), and tamarisk (Tamarix ramosissima). Nesting activity was monitored in 47 territories within two A. donax Removal and two Reference monitoring areas. Nest success of pairs breeding in Removal sites was higher than pairs at Reference sites. More pairs at Removal sites (85%) were successful in fledging young from at least one nest than pairs at Reference sites (73%). However, pairs at Reference sites fledged more young per successful nest than pairs at Removal sites, resulting in no statistical difference in the average number of young fledged per pair between treatments. Successful and unsuccessful nests within Reference and Removal sites did not differ statistically in average nest height, height of the host plant, or the distance the nest was placed from the edge of the host plant. Sixty-two to 64% of nests were placed in S. lasiolepis or S. exigua. An additional 10% of all mo