Seasonal variation in nest placement by the California Gnatcatcher

Nest placement of open-nesting bird species may affect risk of nest predati on, 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 habita t specific environmental conditions that might affect the relationship betw een nest placement and reproductive success. Using data collected during 19 94 and 1995, I investigated seasonal and habitat specific patterns of nest placement in a population of California Gnatcatchers (Polioptila californic a) that breeds over a 5 month period. Nest and substrate (plant in which ne st is built) height increased and vegetative concealment of nests decreased seasonally, but these variables were not related to habitat type. Substrat e height varied with substrate species in 1994, and use of individual subst rate 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 respon ses to changing environmental conditions that may affect reproductive succe ss has yet to be determined.