Variation in life-history traits and nest-site selection affects risk of nest predation in the California Gnatcatcher

I monitored nests and reproduction of California Gnatcatchers (Polioptila californica californica) at Miramar Naval Air Station, California, in 1994 and 1995 to determine correlates of nest placement, life-history traits, and nest predation. One in four nests survived to fledging, with the majority of losses caused by predation. Predation rate over the entire nesting cycle did not change seasonally and was lower in nests with full clutch sizes than in those with submodal clutch sizes. When the nest sample was divided into thirds according to nest height and then according to rate of disturbance by investigators, the upper and lower thirds suffered greater rates of predation than the middle third, and nests disturbed most often by investigators suffered greater predation rates than those disturbed less frequently. Finally, the probability of incubation lasting the full period (at least 14 days) dropped seasonally from about 0.9 for nests initiated in mid-March to less than 0.2 for nest initiated in early July, despite there being no apparent relationship between incubation length and mean daily high or low temperatures over the laying and incubation stages. These results indicate that reproductive success is related to variation in nest-site selection and life-history traits, as well as to variation in environmental variables and investigator activity.