Ghostly white forms drifted like white handkerchiefs on the brisk night breeze, dipping, rising, flapping, turning and tumbling over an inky black ocean. My small sailing boat rocked gently on the waves, while a handful of stars blinked between the clouds above. The shoreline lay some 30km ahead, just visible as a faint smudge on the horizon, but my gaze was fixated on these spectral beings in constant motion.
That experience was more than half-a-century ago and remains one of my favourite teenage memories. I was living on the Galápagos – as I still am today – and, back then, moving between the archipelago’s islands involved hand-steering from the open cockpit of a small boat.
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I was just 17, working as a naturalist and deckhand, taking the first very small groups of visitors to see ‘my’ islands. We travelled at night so we could spend the days exploring on land.
Those white handkerchiefs over the sea were in fact swallow-tailed gulls (Creagrus furcatus), found only in the Galápagos and in tiny numbers on Malpelo Island, off Colombia, some 1,600km to the north-east. And they weren’t aimlessly blowing past the boat: they were hunting – in near total darkness.
I watched them for hours that night, barely visible in the dim glow of the navigation lights, swooping down and back up with extraordinary agility. They emitted strange, shrill crackling noises, mostly when they suddenly dipped low over the lightless sea surface. It was utterly captivating.
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What is a swallow-tailed gull?
The swallow-tailed gull is an exceptionally beautiful bird, elegant and seemingly weightless on the wing, and exquisitely coloured when observed up-close. Its plumage is adorned in striking shades of grey, white and black, and during the breeding season, its huge black eyes are encircled by brilliant red rings.
The entire inside of its gape is decorated in a lighter shade of orange, displayed during minor disputes and when making its signature, high-pitched alarm call.
It’s also unique. It’s the only gull in the world that is nocturnal, earning it the nickname of ‘owl gull’. And, unlike its cousins, which are either coastal or even range into lakes and rivers far inland, the swallow-tailed is a pelagic seabird, living in the open sea outside of the nesting season.
How do swallow-tailed gulls mate?
Nesting can happen at any time of the year, provided food is abundant – the birds are asynchronous breeders, meaning they don’t stick to a fixed reproductive season. As a result, while colonies are never empty, not all birds are present at any given time. Each pair has its own breeding cycle of 9–10 months, roughly half of which is spent raising a single chick.
The remainder is spent far from the Galápagos, mainly off the coast of Peru, in the heart of the Humboldt Current, with the birds never making landfall. This is when their dapper black hoods moult to white, and the bright red rings around their bulging round eyes fade away. When they are ready to nest, they moult again, regaining all those contrasting colours, ready for their return to the archipelago.
As nocturnal hunters, returning to their nests before dawn, swallow-tailed gulls tend to spend their days quietly. They doze through the hottest hours, with just the occasional territorial scrap interrupting the peace. Only when the sun dips below the horizon does the colony come to life, the birds’ drawn-out screeches serving as signals for small flocks to lift off and head for the far horizon.
How do they hunt?
These nightly hunting forays aren’t random searches for food, as might be the case in other scavenging gulls. The swallow-tailed’s favourite prey items are squid and deep-sea fish, elusive sea creatures that rise from the ocean depths in the dead of night in search of plankton.
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Various other comings and goings take place in the darkness, with parent birds exchanging incubating and chick-guarding duties. Night-time on land is dangerous even for a nocturnal gull, as short-eared owls, which have equally good vision, prowl the colony for vulnerable youngsters. Other potential predators include larger gulls, hawks and herons.
Swallow-tailed gull behaviour
Sunrise marks a new hubbub of activity, and this is my favourite time to observe a colony. The birds pay no attention to my presence, and I can watch them for hours, pondering the many secrets their lives still hold.
Pairs will be variously courting, building nests from tiny pebbles or fragments of coral, engaging in protracted mating rituals, or feeding their chicks regurgitated squid. Often, panic will break out when a marauding frigatebird attempts to steal the catch, with neighbouring birds joining forces to form a screaming, whirling, defensive mob.
Parents take it in turns to brood their single egg or nestling, and might delicately preen each other’s facial feathers in a gesture of tenderness. In a courting pair, the female will sometimes ‘beg’ like a chick, peeping with her head low and beak upwards. An attentive partner will respond by regurgitating an entire squid for her.
Every so often I am left utterly incredulous by the gigantic size of some of the squid that are served up to both partners and chicks, already partially digested. How these birds manage to catch such hefty creatures in total darkness, and haul them all the way back to shore in their distended stomachs, is anyone’s guess.
Why do they hunt at night?
There is still a lot we have to learn about the swallow-tailed gull but, in recent months, remarkable research has uncovered tantalising new insights, piquing my interest in these birds all over again. A study using tiny satellite-trackers, led by ornithologist and Galápagos native Sebastián Cruz, has demonstrated that this is the only known seabird that actively prefers to fish on the darkest nights.
Cruz and his team discovered that the swallow-tailed gull has a flexible sleep schedule, adjusting its activities in response to the lunar cycle. The brighter the moon, they found, the less the birds were absent from the colony. The pattern suggests that swallow-tailed gulls forage at sea when normally deep-dwelling prey are most abundant at the surface, feeding in the darkness to avoid detection.
Interestingly, Galápagos fur seals schedule their foraging activity in a similar manner: like the gulls, they rely heavily on nocturnal squid.
The study also found that swallow-tailed gulls roam far and wide on their night-time foraging trips. Tracking individual birds travelling from distinct colonies to known feeding grounds far out at sea, the researchers revealed that the gulls were exceeding distances of 100km per night.
It’s an incredible new finding – though just how this species manages to hunt and travel in pitch darkness, in a way that is completely different to other members of its family, is still not fully understood.
What do they look like?
Look carefully at a swallow-tailed gull and there are some clear adaptations that enable its night-hunting behaviour. Most obvious are the enormous eyes, equipped with bulbous, light-gathering lenses and a tapetum lucidum – a layer of reflective tissue behind the retina that bounces the light back a second time, magnifying its intensity.
There’s also the colour of its ‘pecking spot’, located at the tip of the beak and serving as a guide for chicks begging to be fed. In most gull species this is bright red or yellow, but in the swallow-tailed, it’s white. For a species that returns to the nest under cover of darkness with its catch, paler markings are much easier to see.
What makes the swallow-tailed gull unique?
Earlier research has also reported certain features that, at the time, were not necessarily seen as relating to hunting technique – for example, the bird’s buoyant flight. Compared to other gull species, the swallow-tailed’s wings, particularly its primary feathers, are longer and highly flexible, as is its widely fanning tail. These features provide exceptional manoeuvrability in mid-air, an obvious asset when snapping at squid that skip only briefly out of the water.
Occasionally, when I’ve used a torch on a boat at night, hunting gulls have flown into the beam, revealing entire undersides in a remarkably reflective white. Watching squid leap from the water and the birds in hot pursuit, I couldn’t help but wonder if those flashing white wings, even when illuminated only by stars, could be sufficiently disruptive to flush prey into the air.
Finally, there is that strange crackling call, something between a rattle and a creak. Intriguingly, it reminds me of the sound that dolphins make underwater. Could it be that the swallow-tailed gull, along with all its other unique characteristics, is also the only seabird to somehow use echolocation to detect its airborne prey?
While echolocation – in which an animal can ‘visualise’ its surroundings by interpreting a sound as it bounces off surfaces such as the sea – is a widespread navigational tool among bats, it is rare in the avian world, known only from cave-nesting oilbirds in the South American rainforest and some swiftlets, who likewise inhabit dark caverns.
Sebastián Cruz recently embarked on a new study using tiny sound recorders temporarily attached to the hunting gulls. Originally sceptical about the possibility of seabird echolocation, he was excited to discover that the swallow-tailed gull emits at least two types of click pattern – a simple volley and a complex burst.
The regular volleys seem to correspond to some form of echolocation, while the rapid-fire bursts are more typically associated with a focused ‘read’ of a moving object. The latter may be connected to those squid-chasing episodes that I myself have witnessed, though of course we can’t be sure.
Next on Sebastián’s agenda is an expedition at sea, recording the bird’s vocalisations while using only infra-red cameras to rule out the potential effect of a ship’s lights, which have been a factor during my own observations. I’m excited to see what more his team will learn about the hunting adaptations of this enigmatic bird.
There are more than 50 species of gulls worldwide, from the Arctic to the Antarctic, and from the high Andes mountains to the open seas.
Anyone who simply thinks of ‘seagulls’ (a misnomer, as they are collectively called gulls, not seagulls) as those raucous and pesky sandwich-robbers at a beach picnic is sorely mistaken. None could be further from that characterisation than the mysterious, elegant swallow-tailed gull – which is, at last, starting to give up some of its secrets.
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