Seasonal Variation in Nest Placement by the California Gnatcatcher

—Nest placement of open-nesting bird species may affect risk of nest predation, nest microclimate, and reproductive success. In populations that breed in multiple habitat types and over long seasons, nest placement should vary seasonally and by habitat to compensate for seasonally changing and habitat specific environmental conditions that might affect the relationship between nest placement and reproductive success. Using data collected during 1994 and 1995, I investigated seasonal and habitat specific patterns of nest placement in a population of California Gnatcatchers (Polioptila californica) that breeds over a 5 month period. Nest and substrate (plant in which nest is built) height increased and vegetative concealment of nests decreased seasonally, but these variables were not related to habitat type. Substrate height varied with substrate species in 1994, and use of individual substrate species varied seasonally. Reproductive phenology differed between the two major habitat types used by gnatcatchers in this study. Whether these seasonal and habitat specific changes in nest placement are adaptive responses to changing environmental conditions that may affect reproductive success has yet to be determined. Received 15 Feb. 2000, accepted 11 June 2000. In open nesting birds, variation in nest placement may affect predation risk (Best and Stauffer 1980, Wilcove 1985, Martin 1988, 1993, Morton et al. 1993; Sockman 1997; but see Filliater et al. 1994) and nest microclimate (Walsberg and King 1978, Walsberg 1981; but see Walsberg 1985). In populations that breed in multiple habitats and over long seasons, nest placement should vary seasonally and by habitat to compensate for seasonally changing and habitat specific environmental conditions that may affect the relationship between nest placement and reproductive success. The California Gnatcatcher (Polioptila californica) is an open nesting, non-migratory passerine that lives in the coastal sage scrub ecosystem of southern California. Its nesting season may last at least 5 months (Sockman 1997), a period over which ambient temperature and precipitation vary considerably (Fig. 1). Its range extends from the Pacific Coast to more than 100 km inland (Atwood 1980) and subsumes several habitat types. Here, I present data on its nest height, height and species of the nest substrate (plant in which the nest is built), and vegetative concealment of nests and how they change with habitat type and phenology of clutch initiation. 1 Dept. of Biology, San Diego State Univ., San Diego, CA 92182–4614. 2 Present Address: School of Biological Sciences, Center for Reproductive Biology, Washington State Univ., Pullman, WA 99164–4236; E-mail: ksockman@ wsu.edu METHODS This study was conducted at Naval Air Station Miramar in San Diego County, California. I have depicted the study site and distribution of breeding pairs previously (Sockman 1997, fig. 1). The site covers approximately 9,600 ha of hilly terrain with several extensive canyons and ridges and extends approximately 20 km from east to west. Elevation ranges from 80– 330 m. Dominant habitat types occupied by gnatcatchers are chaparral and coastal sage scrub, which together cover approximately 60% of the site (J. F. O’Leary,