The Owl Gull: Swallow-tailed Gull’s Nocturnal Hunting Unveiled
New GPS tracking data confirms the Swallow-tailed Gull as the only seabird that exclusively forages at night.
In the Galápagos Islands, where an array of seabirds compete for food in the daylight, one species has found its own niche in the darkness. The Swallow-tailed Gull - often dubbed the ‘Owl of the Seas’ - is the only known seabird to rely entirely on nocturnal foraging. A new study published in Marine Ornithology has provided the most definitive proof yet, using GPS tracking to map the nightly voyages of these enigmatic birds as they hunt in the black expanse of the Pacific Ocean.
A Lifetime in the Dark
Unlike most gulls, which are opportunistic scavengers that hunt in broad daylight, the Swallow-tailed Gull has evolved remarkable adaptations for night-time hunting. Its oversized eyes and specialised retinas enhance light sensitivity, allowing it to spot bioluminescent squid and myctophid fish - the main components of its diet. Unlike other seabirds, it also lacks a measurable daily melatonin rhythm, making it uniquely suited to an existence governed by the lunar cycle rather than the rise and fall of the sun.
Prior studies had strongly suggested this nocturnal lifestyle, as these gulls are conspicuously absent from their breeding colonies at night. However, until now, direct evidence of their activity at sea had been scarce. Researchers sought to fill this gap by equipping 182 Swallow-tailed Gulls across four breeding colonies with GPS loggers, recording their precise movements over a three-year period.
Mapping the Midnight Hunt
The results were unambiguous: every single foraging trip recorded took place at night. Birds departed their nests at dusk and returned before dawn, often travelling up to 100 km offshore. Analysis of movement patterns showed that they spent roughly half their time at sea in active foraging, with periods of intensive searching interspersed with fast commuting flights. Notably, none of the tagged birds foraged during the day - when they simply rested on the ocean’s surface, drifting with the currents.
Interestingly, foraging behaviour varied by location. Birds from the northern colony at Genovesa Island completed shorter trips, sometimes even making multiple foraging excursions in a single night. Meanwhile, those from the southern Española colony travelled greater distances, likely due to differences in prey availability. Despite these variations, all birds demonstrated strong site fidelity, returning to the same general feeding grounds night after night.
The Evolution of a Nocturnal Niche
Why have Swallow-tailed Gulls embraced the darkness when virtually all their relatives are diurnal? The answer likely lies in a combination of ecological pressures and evolutionary opportunity.
During the day, the Galápagos’ nutrient-poor waters host intense competition among seabirds, many of which rely on large predatory fish to drive prey to the surface. In contrast, at night, a different phenomenon occurs: diel vertical migration (DVM). Many deep-sea organisms, including small squid and fish, rise to the surface under the cover of darkness. By evolving to exploit this underutilised food source, the Swallow-tailed Gull has sidestepped the pressures of daylight competition.
Another likely factor is predator avoidance. While daylight exposes many seabirds to kleptoparasitism by frigatebirds - aggressive aerial thieves that force others to surrender their food - hunting at night allows the Swallow-tailed Gull to evade such harassment. Furthermore, by resting at sea during the day, they minimise the risk of predation themselves.
Conservation Implications
The discovery that Swallow-tailed Gulls maintain strict fidelity to specific nocturnal foraging areas carries important conservation implications. Many of their preferred hunting grounds fall within the Galápagos Marine Reserve, a protected zone that restricts industrial fishing. However, as climate change alters oceanic food webs and commercial fisheries expand in the region, their food supply may come under increasing threat.
Understanding the factors that drive the nocturnal habits of these gulls is vital in shaping marine conservation strategies. Researchers argue that future protections should not just consider breeding colonies but also the at-sea feeding zones where these birds spend the majority of their time.
A Glimpse into a Hidden World
The Swallow-tailed Gull’s nocturnal habits have long remained a mystery, hidden beneath the cover of night. Thanks to advancements in tracking technology, scientists can now confirm what has long been suspected - these birds are true creatures of darkness, adapted to a lifestyle that sets them apart from all other seabirds.
As researchers continue to uncover the secrets of nocturnal foraging in marine birds, the Swallow-tailed Gull stands as a fascinating example of how evolution carves out ecological niches in the most unexpected ways. In the moonlit waters of the Galápagos, the ‘Owl of the Seas’ reigns supreme - one of nature’s most remarkable night hunters.
19 Mar 2025
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