Proportional dietary DNA as a quantitative metric to monitor the diet composition of piscivorous terns

Abstract Climate change and human impacts are causing shifts in species' distributions and abundance, often disrupting predator–prey relationships. Monitoring animal diets can elucidate these relationships, potentially providing a mechanistic understanding of population declines. Dietary DNA can be used to gather data on species' diets with high-taxonomic resolution, but whether it can provide quantitative diet information has so far limited its applications. Here we compare multiple methods of estimating diet composition using both visual observations and dietary DNA in Sterna hirundo (Common Tern) chicks. Over 3 breeding seasons, we observed S. hirundo chick feeding events and collected fecal samples opportunistically from chicks in the same colony. We compared the frequency of occurrence (FOO) and relative read abundance (RRA) of prey in the fecal samples to the relative biomass recorded through visual observations and found a high correlation (r = 0.94) between RRA and relative biomass at the colony level. RRA outperformed FOO in capturing interannual changes in relative prey biomass, while FOO estimates were inflated compared to relative biomass, making it a less useful metric for estimating the proportions of prey species in the diets of S. hirundo. Using simulations, we also estimated the number of fecal samples needed to minimize the error between RRA and relative biomass in studies of this type. The high taxonomic resolution provided by dietary DNA and the high correlation between RRA and relative biomass suggests that dietary DNA can be used as a quantitative metric to monitor tern diets. This has important applications for studying the impact of changing forage fish availability on seabird populations, and for harnessing seabirds as sentinels of ecosystem health. Because dietary DNA data can be collected highly efficiently, it could provide key information about food web dynamics and forage fish availability as we move towards ecosystem-based fisheries management.