San Diego Management & Monitoring Program


Distribution and Breeding Activities of the Least Bell's Vireo and Southwestern Willow Flycatcher at the San Luis Rey River, San Diego County, California

Type: report

Article abstract: Surveys and monitoring for least Bell's vireos (Vireo bellii pusillus) and southwestern willow flycatchers (Empidonax traillii extimus) were conducted on the San Luis Rey River, San Diego County CA, between 1 April and 21 August 2006. Vireo surveys were conducted from Interstate 15 west approximately 6.5 km to Mission Road. Southwestern willow flycatchers were surveyed in the same area, as well as downstream between Sante Fe Road and a point approximately 1 km upstream on the San Luis Rey River (Guajome Regional Park). Fifty-three territorial male least Bell's vireos were observed within the study area, 50 of which (94 percent) were confirmed as paired. Nine transient male vireos were also detected. Within the section of river consistently monitored since 2003, vireo numbers declined from 46 territories in 2005 to 31 in 2006. For the three years prior to 2006 the number of resident territorial males had remained relatively constant, varying from 40 to 46 territorial males. Nesting activity at 99 nests within 43 vireo territories was monitored. Thirty-six percent of nests were successful, fledging at least one vireo young, while 64 percent failed. Sixty percent of vireo nests whose contents were observed were parasitized by brown-headed cowbirds (Molothrus ater). Nest predation and cowbird parasitism accounted for 66 and 21 percent of failures, respectively. However, biologists "rescued" parasitized nests by removing cowbird eggs shortly after they were laid, allowing some to fledge young. Without "rescuing" it is likely that nest success would have been only 15 percent. In total, 86 vireo young fledged from 35 nests, and pairs fledged on average 2.2 young by the end of the breeding season. Thirteen least Bell's vireos banded prior to the 2006 breeding season were resighted within the study area. All had been banded as nestlings on the San Luis Rey River. Eight of the thirteen possessed a unique combination of color bands or were recaptured during the 2006 season and therefore could be identified to individual. Two of the eight were banded as nestlings outside of the study area and dispersed 14 and 4.9 km into the study area. All other uniquely color banded vireos fledged from and dispersed within the study area. The extent of their dispersal ranged from 0.8 to 4.4 km. Five other adult vireos that had been banded as nestlings with a single federal band were target netted, but attempts to recapture them were unsuccessful. Two additional adult vireos w

Number of pages: 30

Authors: Rourke, James W.; Kus, Barbara;

Year: 2007

Prepared for: District 11, Department of Transportation, State of California;

Prepared by: U.S. Geological Survey, San Diego Field Station;

Keywords: Bell's vireo; breeding; endangered species; least Bell's Vireo; southwestern willow flycatcher; Vireo bellii pusillus; willow flycatcher;

Species: Least Bell's vireo; Southwestern willow flycatcher