Variation in Life-History Traits and Nest-Site Selection Affects Risk of Nest Predation in the California Gnatcatcher

Abstract

-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 lifehistory traits, as well as to variation in environmental variables and investigator activity. Received 2 May 1996, accepted 6 December 1996. NEST PREDATION iS the primary cause of reproductive failure in land birds (Ricklefs 1969). Many investigators have shown that life-history traits and nest-site selection affect the risk of nest predation (Nice 1957; Nolan 1978; Longcore and Jones 1969; Roseberry and Klimstra 1970; Best 1978; Best and Stauffer 1980; Martin 1988, 1993; Martin and Roper 1988; Hanski and Laurila 1993; Morton et al. 1993; Seitz and Zegers 1993; Hartley and Shepard 1994), indicating that optimal values for these traits exist in some species. The California Gnatcatcher (Polioptila californica californica) is an open-nesting, nonmigratory passerine that lives in coastal sage scrub in southern California. Breeding adults are sexually dichromatic, and males and females participate in all aspects of parental care. Nests of California Gnatcatchers in San Diego County increase seasonally in height and relative height (nest height relative to height of nest plant ), and nest height increases with height of the nest plant (Sockman unpubl. data). Nest concealment, however, apparently does not in' Present address: Department of Zoology, Washington State University, Pullman, Washington 99164, USA. E-mail: ksockman@wsu.edu crease seasonally despite the fact that foliage density increases seasonally. I attribute these findings to seasonal differences in nest placement (increasing nest height) that offset changes in overall vegetation density and serve to maintain an optimum level of concealment based on microclimate requirements and the need to minimize nest predation. Support for this hypothesis relies on data showing changes in risk of nest predation with respect to nest placement. Here, I provide these data in addition to data on basic life-history traits and how these traits affect reproductive success. Concern for the California Gnatcatcher, which is listed as a threatened species, has resulted in studies aimed at understanding its requirements. Yet, published accounts of their life history are scant. I examined fecundity, reproductive success, and duration of the incubation and nestling stages for seasonal variation and for correlations among these traits. Using these results and data on nest placement, I tested whether rate of nest predation: (1) differs seasonally and according to nest-plant species (hereafter substrate), nest-site characteristics, and disturbance by investigators (see Major 1990); (2) increases with increasing clutch size and brood size (because nestlings and parental