| Summary: | We examined the effects of mate and site fidelity on Western Sandpiper (Calidris mauri) nesting success in a breeding population on the Yukon-Kuskokwim River Delta, Alaska (1998–2005). We estimated return rates (percentage of banded birds that returned to the site in the subsequent season) and mate fidelity among 533 individually marked birds (296 females and 237 males) and monitored 430 nests with sufficient data for nest-survival analyses. Return rates were lower for females (40%) than for males (65%). Annual divorce rates (both members of a pair returned and each mated with a new individual) ranged between 10 and 29%, with 4–26% of pairs reuniting annually. Reuniting pairs initiated clutches earlier than newly formed pairs, and clutches that were initiated early in the season had higher nest-success rates than those initiated later in the season. However, when initiation date was controlled, mate fidelity did not explain significant variation in daily nest survival rate. When we controlled for seasonal variation, nests tended by individuals with prior breeding-site experience had higher daily survival rates than those tended by birds breeding at the site for the first time. This effect was greater for males than for females. We also observed annual and seasonal variation in nest survival, as well as variation associated with nest age. Future study of the proximate causes of temporal variation would add considerably to our understanding of temporal effects on individual behavior, fitness, and population dynamics.Efectos de la Fidelidad de Pareja y de Sitio sobre la Supervivencia de Nidos en Calidris mauri
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