Abstract
Seabird parents use a conservative breeding strategy that favours long-term survival over intensive parental investment, particularly under harsh conditions. Here, we examine whether variation in several physiological indicators reflects the balance between parental investment and survival in common murres (Uria aalge) under a wide range of foraging conditions. Blood samples were taken from adults during mid-chick rearing from 2007 to 2014 and analysed for corticosterone (CORT, stress hormone), beta-hydroxybutyrate (BUTY, lipid metabolism reflecting ongoing mass loss), and haematocrit (reflecting blood oxygen capacity). These measures, plus body mass, were related to three levels of food availability (good, intermediate, and poor years) for capelin, the main forage fish for murres in this colony. Adult body mass and chick-feeding rates were higher in good years than in poor years and heavier murres were more likely to fledge a chick than lighter birds. Contrary to prediction, BUTY levels were higher in good years than in intermediate and poor years. Murres lose body mass just after their chicks hatch and these results for BUTY suggest that mass loss may be delayed in good years. CORT levels were higher in intermediate years than in good or poor years. Higher CORT levels in intermediate years may reflect the necessity of increasing foraging effort, whereas extra effort is not needed in good years and it is unlikely to increase foraging success in poor years. Haematocrit levels were higher in poor years than in good years, a difference that may reflect either their poorer condition or increased diving requirements when food is less available. Our long-term data set provided insight into how decisions about resource allocation under different foraging conditions are relating to physiological indicators, a relationship that is relevant to understanding how seabirds may respond to changes in marine ecosystems as ocean temperatures continue to rise.
Highlights
Parents must balance the costs of care for offspring with possible negative consequences for their future survival and reproduction
Mass did not vary with capture date, suggesting that mass had already stabilized by the time murres were captured in mid-chick rearing
This observation fits with findings that breeding murres lose body mass right after their chicks hatch, with mass stabilizing later in chick rearing
Summary
Parents must balance the costs of care for offspring with possible negative consequences for their future survival and reproduction. These trade-offs are key to the prudent parent model (Drent and Daan, 1980; Bókony et al, 2009), as life history parameters determine how much can be invested in current offspring without compromising future reproductive success. Several factors make reproductive costs for murres (Uria spp.) high, even among long-lived seabirds. Due to their diving-adapted wings, murres have elevated flight costs associated with extremely high wing loading (Ainley et al, 2002; Elliott et al, 2013). Under poor foraging conditions, common murres U. aalge decrease their chick provisioning rates (Harris and Wanless, 1988; Burger and Piatt, 1990; Harding et al, 2007; Wilhelm et al, 2008), which is consistent with predictions for low brood value species (Bókony et al, 2009; Breuner, 2010)
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