Age-specific offspring mortality economically tracks food abundance in a piscivorous seabird

Earlier offspring mortality prior to independence saves resources for kin, which should be more beneficial when food is short. Using 24 years of data on age-specific common tern (Sterna hirundo) chick mortality, best described by the Gompertz function, and estimates of energy consumption per age of mortality, we investigated how energy wasted on non-fledged chicks depends on brood size, hatching order and annual abundance of herring (Clupea harengus), the main food source. We found mortality directly after hatching (Gompertz baseline mortality) to be high and to increase with decreasing herring abundance. Mortality declined with age, at a rate relatively insensitive to herring abundance. The sensitivity of baseline mortality to herring abundance reduced energy wasted on non-fledged chicks when herring was short. Among chicks that did not fledge, last-hatched chicks were less costly than earlier hatched chicks, due to their earlier mortality. However, per hatchling produced, the least en...