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DNA in Common Tern Chick Droppings Closely Matched the Fish Adults Delivered

Adult common tern feeding a fish to a juvenile
Research · Ecology

A three-year study of common tern (Sterna hirundo) chicks found that DNA in fecal samples could provide a quantitative picture of their fish diet. Researchers compared prey identified from dietary DNA with direct observations of fish delivered to chicks at a breeding colony on the Isles of Shoals in New Hampshire, USA.

The strongest match came from relative read abundance, which measures the proportion of DNA sequences assigned to each prey type. At the colony level, this measure was highly correlated with the relative biomass estimated from visual feeding observations, with a correlation of r = 0.94. In practical terms, prey types that made up a larger share of the fish biomass seen being delivered also tended to make up a larger share of the DNA reads. This colony-level match does not mean that each individual dropping reproduced the exact proportions of a chick’s recent meals.

Relative read abundance also tracked year-to-year changes in prey biomass better than frequency of occurrence, a simpler DNA measure based on whether each prey type was present or absent. Frequency of occurrence tended to inflate the estimated importance of prey species when compared with relative biomass.

The authors conclude that dietary DNA can be used not only to identify prey at high taxonomic resolution, but also as a quantitative tool for monitoring common tern diets. They highlight applications for tracking changes in forage-fish availability and food-web dynamics.

Sources and image credit

  1. Clucas, G. V., Stillman, A. N., & Craig, E. C. (2026). Proportional dietary DNA as a quantitative metric to monitor the diet composition of piscivorous terns. Ornithology, ukag040. https://doi.org/10.1093/ornithology/ukag040Primary peer-reviewed research article, first published 26 August 2026. Over three breeding seasons, dietary DNA from common tern chick feces was compared with visual feeding observations; relative read abundance correlated strongly with relative prey biomass at colony level (r = 0.94).
  2. Clucas, G. V., Stillman, A. N., Seavey, J., & Craig, E. C. (2023). From presence/absence to reliable prey proportions: A field test of fecal metabarcoding for characterizing seabird diets [Conference abstract]. Waterbird Society Annual Meeting.Supporting source documenting the study colony on the Isles of Shoals, New Hampshire, USA, and the 2017–2019 field period.

Image: Alexis Lours — adult Common Tern (Sterna hirundo) delivering a small fish to a juvenile in France, photographed 5 August 2025; Wikimedia Commons. Representative feeding image, not birds from the study colony. CC BY 4.0. · CC BY 4.0 · Source

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