“Age-specific offspring mortality economically tracks food abundance in a piscivorous seabird”
Oscar Vedder, He Zhang, Andreas Dänhardt, and Sandra Bouwhuis (Apr 2019)
Earlier chick mortality with low food availability reduces the energy wasted on non-fledged chicks in poor years
Why does mortality peak at the start of life? One idea is that this early mortality reduces the amount of resources wasted on unsuccessful offspring, and is the result of a strategy adopted by the parents that is beneficial when unpredictable foraging conditions turn out bad. By analysing a 24-year dataset of age-specific chick mortality of common terns (fish-eating seabirds), an international team of researchers has found that with reduced food availability, fledgling success decreased in an economical fashion. When herring (the terns’ main food source) were rare, chick mortality increased, but because chicks died earlier, this did not lead to a proportional increase in energy wasted on non-fledged chicks. Disadvantaged, last-hatching, chicks were particularly cheap when they died, but, per hatchling, the chicks without siblings required the least waste of energy. The researchers suggest that parents may facilitate early mortality of excess offspring by promoting competitive asymmetries among offspring from the start, but that competition between siblings may interfere with the parents’ best interests despite such asymmetries. These results thereby support evolutionary theory on age-specific mortality, parental effects, sibling competition, and parent-offspring-conflict. The researchers conclude, “Despite it being an extremely sad sight to see so many small chicks die when there is little food, this may be nature’s way of ensuring that the parents survive and are able to reproduce in future years when food may be more abundant.”
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 energy was wasted on chicks without siblings, due to their baseline mortality being most sensitive to herring abundance. We suggest that earlier mortality of offspring when food is short facilitates economic adjustment of post-hatching parental investment to food abundance, but that such economic brood reduction may be constrained by sibling competition.