Fluctuating Populations of House Wrens and Bewick's Wrens in Foothills of the Western Sierra Nevada of California

Abstract

In foothills of the western Sierra Nevada, 31 km east of Madera, California, we studied whether Bewick's Wrens (Thryomanes bewickii) tend to be excluded from an area occupied by House Wrens (Troglodytes aedon) as reported in several studies in the eastern United States. Neither point counts from 1985 to 1997 nor spot mapping from 1989 to 1993 suggests any interaction between these species. The study period included the most prolonged drought and the most severe period of winter weather in the 64-year climatological record from the study site. House Wren abundance was significantly related to the 4-year running average of annual precipitation and to the lowest temperature recorded in the winter months preceding survey counts. We speculate that House Wrens cannot maintain their numbers by local recruitment during droughts in the oak-pine woodlands that dominate our study area and that recovery of their population following droughts involves recruitment from other areas, probably riparian zones and other mesic habitats elsewhere in the Sierra foothills and Central Valley of California. There was no influence of precipitation on abundance of Bewick's Wrens and, although they apparently sustained heavy mortality during a period of extreme winter weather, their numbers otherwise were not related to the lowest temperature during the winter preceding counts.