Comparative Loss of Reproductive Potential Among Increased Clutch Sizes in Relation to Climate Change in a High Trophic Avian Predator: A 44‐Year Study

ABSTRACT

Long‐term studies regarding the response of birds to climate change typically involve passerine songbirds that exhibit variations in timing of breeding and migration. The effects of climate warming on reproductive output are typically determined via tallies of advanced‐age young per nest and less often include clutch sizes or reproductive potential. Hence, there are fewer estimates among groups of birds, especially apex avian predators, regarding the extent of mortality of young offspring (i.e., embryos and hatchlings), the loss of which, besides often being undetected, comprises significant demographic and evolutionary components of avian reproductive failure. We report a significant increase in average clutch sizes and chronicle the dynamics of the reproductive potential among clutch sizes between two and seven eggs across 44 years (1980–2023) in a central North American population of Cooper's hawks ( Astur cooperii ), which is responding favorably and quickly to climate change. We found a recent and regular yearly presence of relatively high proportions of rarely reported six‐egg clutches, including a higher proportionate loss of young offspring in six‐egg clutches (0.22) compared to that in modal four‐egg (0.12) and five‐egg clutches (0.15), suggesting in part potentially higher metabolic costs associated with tending an increased number of eggs. We speculate on the maximum viable clutch size for our population and suggest that along with metabolic restraints to reproductive potential, incubating parents have physical difficulties covering the eggs in clutches of six or more eggs. Our findings support the hypothesis that avian clutch size might be driven by the number of eggs adult birds can afford to incubate, rather than exclusively being shaped evolutionarily by the demands of rearing the number of young that maximizes reproductive success. Furthermore, extending our long‐term research altered some climate‐driven trend conclusions.