Effects of population density on survival in Merlins

Accurate estimation of survival probabilities is an important component of population demographics, and it permits a test of the life-history predicti on that densities influence population dynamics via suppression of survival rates. As part of a long-term study of urban-nesting Merlins (Falco columb arius), we estimated survival rates and tested fur the effects of density d ependence based on capture histories from 1,354 individuals (43 males and 1 10 females caught for the first time as adult breeding birds, and 597 males and 604 females caught for the first time as locally produced nestlings). Overall capture probabilities were 0.55 +/- SD of 0.039 per year for adults , 0.10 +/- 0.075 per year for juvenile males, and 0.58 +/- 0.23 per year fo r juvenile females. Mean survival rate of adults was 0.62 +/- 0.11 per year and did not differ significantly between males and females. Overall juveni le survival rates were 0.23 +/- 0.032 for males and 0.055 +/- 0.012 for fem ales. Band returns suggest that the discrepancy in survival rates between j uvenile males and females resulted from higher natal dispersal of females r ather than from lower survival. Survival of adults (but not juveniles) was negatively density dependent, suggesting that density-dependent declines in survival exerted a regulatory effect on population size.