New Zealand falcon prey selection may not be driven by preference based on prey nutritional content

Little is known about how diurnal raptors, as apex predators, select their prey. It has been hypothesised that they are opportunistic, taking prey according to availability, and that they select prey based on prey size. The threatened New Zealand falcon or kārearea (Falco novaeseelandiae) is New Zealand’s only remaining endemic bird of prey. A previous study on prey caught by kārearea during the breeding season suggested that introduced avian prey were taken more often than expected, and endemic avian prey taken less often than expected, based on their abundance. There is a growing interest in the role that nutrients play in prey selection by predators. We used the nutritional geometry framework in a field study to determine the role that nutritional composition plays in prey choice. We built on an existing dataset to assess whether prey selection by kārearea can be explained based on prey body-mass, abundance, or nutritional characteristics. We determined the protein-to-lipid ratio and ash content of individuals across 16 species of prey and potential prey, including both endemic and introduced species, and modelled these against known prey consumption based on our earlier work. We found limited evidence for selective predation based on nutrient balancing. Instead, the relative abundance of each species in the surrounding habitat and the endemicity of each species were the most important predictors, with species body-mass playing only a minor role in prey choice. To investigate the apparent selection for introduced over endemic prey, future research could compare the behavioural adaptations of endemic birds against their natural predator with behaviours of introduced birds.