SITE-SPECIFIC SURVIVAL OF BLACK-HEADED GROSBEAKS AND SPOTTED TOWHEES AT FOUR SITES WITHIN THE SACRAMENTO VALLEY, CALIFORNIA

Abstract

We estimated apparent annual survival and recapture probabilities for adult Black-headed Grosbeaks (Pheucticus melanocephalus) and Spotted Towhees (Pipilo maculatus) at four sites along the Sacramento River, California. To calculate our estimates, we used capture-recapture mist-net data collected over two time periods at four study sites: from 1993 to 1995 at Flynn, Ohm, and Sul Norte, and from 1995 to 2000 at Ohm and Phelan Island. Our primary objective was to determine whether there were site-specific differences in adult survival and recapture probabilities for each species. Such differences are rarely investigated, yet, if present, suggest site-specific differences in habitat quality, with important implications for source/sink dynamics. We found site-specific variation in Black-headed Grosbeak survival within both the 1993–1995 dataset (Flynn = 0.797 ± 0.496, Ohm = 0.158 ± 0.191, Sul Norte = 0.773 ± 0.131) and the 1995–2000 dataset (Ohm = 0.088 ± 0.090, Phelan Island = 0.664 ± 0.111). For Spotted Towhees (1993–1995 data), the most supported model assumed constant survival across sites (0.602 ± 0.240), but there was some support for site variation in survival, as well (Flynn = 0.653 ± 0.365, Ohm = 0.214 ± 0.253, Sul Norte = 0.632 ± 0.258). These results clearly suggest site variation for Black-headed Grosbeaks, and weak evidence of site variation for Spotted Towhees. For both species, the general pattern was low survival at Ohm, suggesting low-quality habitat there and/or reduced site fidelity. The magnitude of site-to-site variation in survival observed in the Black-headed Grosbeak, and suggested for Spotted Towhee, has strong implications for determining source versus sink population status. To determine source versus sink status, we conclude that investigators must not only take into account site variation in reproductive success, but also consider site-specific estimation of adult survival.