Habitat-specific differences in adult survival rates and its links to parental workload and on-nest predation.

1. Adult survival rates strongly affect population growth, but few studies have quantified if and why adult survival differs between breeding habitats. We investigated potential causes of habitat-specific adult survival rates for male and female northern wheatears (Oenanthe oenanthe L.) breeding in Swedish farmland. 2. We used multistate mark-recapture models based on 1263 breeding records between 1993 and 2007 to estimate survival rates based on habitat-type (SHORT vs. TALL ground vegetation) and breeding-success state parameters. We also used breeding-season observations from 2002 to 2007 and an experimental manipulation of ground vegetation height to identify factors influencing adult mortality. 3. Females had lower annual survival than males (0.42 +/- 0.02 vs. 0.50 +/- 0.02); this difference largely resulted from low female survival in TALL habitats because of higher nest-predation risk and the large proportion of adult females being killed on the nest (>20%) during nest predation events. 4. Among successful breeders, both sexes displayed similar survival rates, but survival was lower for breeders in TALL as compared to SHORT habitats (0.43 +/- 0.03 vs. 0.51 +/- 0.02). Experimental manipulation of ground vegetation height, controlling for individual and territory quality (n = 132), suggested the cost of rearing young to be higher in TALL habitats (survival of successful breeders in TALL vs. SHORT; 0.43 +/- 0.11 vs. 0.57 +/- 0.05). 5. Detailed observations of food provisioning behaviour during chick rearing revealed a habitat-related difference in parental workload corresponding to the observed habitat differences in adult survival for successful breeders. Adults breeding in TALL habitats were forced to forage further from the nest relative to SHORT-habitat breeders (mean +/- SE; 69 +/- 10 vs. 21 +/- 2 m), which increased the estimated daily workload for adults in TALL vs. SHORT habitats by c. 20%. 6. On-nest predation and parental workload during chick rearing combine to largely explain habitat-specific adult survival rates. The results have implications for our understanding of adult sex ratios, causes of source-sink demography and habitat-specific growth rates. Furthermore, it suggests SHORT field margins and other residual habitat elements to be important for the conservation of farmland passerines breeding in cropland plains.