How a Tropical Seabird Survived 17 Summers in the Gulf of Maine
DNA evidence shows long-staying vagrant survived on unfamiliar but functionally similar fish to those eaten in its native range
A tropical seabird far from home
For 17 consecutive summers between 2005 and 2021, a Red-billed Tropicbird Phaethon aethereus returned to Seal Island, Maine - more than 2,400 km from its nearest Caribbean breeding grounds. Regularly seen occupying a rock crevice, displaying overhead and even attempting to copulate with lobster buoys, this bird became a fixture at the island’s seabird colony. Its persistence suggested not just accidental drift but an exploratory attempt at range expansion.
Testing the diet of a vagrant
To understand how this out-of-range seabird survived in the cold waters of the Gulf of Maine, researchers collected 25 faecal samples in 2020 and 2021. Using DNA metabarcoding, they identified six fish species in its diet. Remarkably, around 75% of the DNA reads were from Atlantic Saury Scomberesox saurus and Atlantic Mackerel Scomber scombrus.
Neither species had previously been recorded in the diet of Red-billed Tropicbirds, which usually feed on flyingfish Exocoetidae and needlefish Belonidae in tropical waters. Yet saury and mackerel are elongated, schooling, surface-dwelling fish - sharing many traits with the tropicbird’s typical prey. This suggests the Maine bird was able to select ecological equivalents of its usual diet despite the taxonomic differences.
Adapting to a new environment
The analysis showed consistent use of saury and mackerel across two years with very different sea surface temperatures, including record warmth in 2021. Cold-water Atlantic Herring Clupea harengus was present only in the cooler year, but saury and mackerel persisted in both, underlining the bird’s ability to cope with changing conditions.
Interestingly, these fish are rarely taken by Atlantic Puffins Fratercula arctica, terns Sterna spp., or other seabirds nesting nearby, suggesting the tropicbird may have been foraging further offshore, perhaps in association with marine mammals or tuna - a known strategy in its tropical range. The results highlight dietary flexibility as a potential advantage for long-distance dispersers.
Range expansion potential
Although prey availability appeared not to be a barrier, the Seal Island tropicbird never attracted a mate. Without a partner, its repeated attempts to establish a breeding territory came to nothing, and it has not been seen since 2021. Still, the study demonstrates that at least one ecological hurdle - food - could be overcome by a vagrant seabird far from home.
Such cases are important “natural experiments,” offering clues to how seabirds may shift ranges in response to climate change. The adaptability shown by the Seal Island Red-billed Tropicbird suggests that suitable prey may be less of a constraint than once thought, even in unfamiliar oceans.
September 2025
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