Tue, 12 Apr 2022

— or is it?

I remember long ago when, during my first guide’s course given by Charles Darwin Station scientists nearly 50 years ago, I was delving deep into the evolutionary literature on Galápagos. Somewhere I came across two lines in the same chapter that really made me wonder, on the islands’ two endemic gulls: Lava Gulls are dark so they CAN’T be seen; Swallow-tailed Gulls are white so they CAN be seen.  But why? Why should two fairly closely related seabirds evolve opposite colouration for opposing reasons. To begin to understand the difference — which of course are still only theories — we need to look closely at each species’ lifestyle, and its respective competitors.

Lava Gull, predominantly dark plumage.

Swallow-tailed Gull, predominantly white plumage.

Lava Gulls are coastal scavengers, and in that sense they resemble a number of other gull species around the world.  With plenty of pickings to be had around sea lion colonies and the nesting areas of other seabirds (where food brought back for nest-bound chicks is often spilled), that should provide for an easy life. So why are there only 500 or so individual Lava Gulls in existence, making them the rarest gull in the world? The answer surely lies in their arch-enemy: frigatebirds. Magnificent Frigatebirds are also coastal scavengers, but with a huge advantage. With the lightest body-to-wing-surface ratio of any bird alive, frigates scavenge by hanging on the wind, surveying huge stretches of coastline from on high with almost

Lava Gulls blend in remarkably with their coastal habitat.

Taking full advantage of their cryptic coloration, Lava Gulls use lava rocks to disguise their nest.

Lava Gulls keep their bright colours circumspect.

no energy expenditure, so efficient is their kite-like body plan. And to make matters worse for the gulls, in addition to extraordinary flight and vision, the frigatebird’s negative dihedral wing shape (their centre of gravity is set above their wing-loading level) makes their flight-to-gravity engineering so inherently unstable that they can literally fly backwards, or even upside down.  For the Lava Gull, that means a frigatebird can rapidly swoop down to snatch any newfound titbits directly from its bill within seconds.

No wonder, then, that Lava Gull colouration has been selected for the best impersonation possible of the lava shoreline after which they are named!

Left: Despite their size, scavenging frigatebirds are the most agile flyers of all seabirds.

Below: A pair of Lava Gulls scavenging around nesting Blue-footed Boobies.

…then comes striding over for a closer look.

So, here comes the next question:  Why have Swallow-tailed Gulls, also restricted to the Galápagos Islands and their oceanic environs, opted evolutionarily for a nearly opposite colour scheme? Unlike the Lava Gull, the Swallow-tailed Gull is primarily white, with delicate pink-orange feet and thin, bright-orange skin around its enormous eyes? Surely, with the superlative night vision that these outsized eyes provide, seeing each other can’t be the primary purpose of their whiteness.

Instead, I’m inclined to look at the behaviour of their prey species, mostly squid and deep-sea fishes that rise to the surface of the ocean under the cover of darkness. These illusive prey are very light-sensitive, as can be seen when shining a bright torch across the sea on a dark night, inducing them to leap well clear of the surface, perhaps mistaking the light from the torch’s panning beam for that of bioluminescence created by approaching predators. It stands to reason, then, that flashing white wings catching the moonlight just above the surface might have the same effect?

Right: With very buoyant flight, Swallow-tailed Gulls are more manoeuvrable in flight than other gulls.

But here comes another twist: While engaged in chasing such fast-leaping prey, the nocturnal Swallow-tails emit an almost non-stop series of very distinctive crackling calls that are amazingly reminiscent of the bursts of echolocation clicks made by hunting dolphins. They sometimes make the same guttural sounds on land as part of their courtship, especially near dusk, and at such times a lateral expansion of their throats can be observed, as can be seen in the photo below.

Could the Swallow-tailed Gulls be pinpointing their leaping prey by adding a form of echolocation to backup their extraordinary night vision?  The only other birds known to echolocate are Oilbirds who feed on palm nuts in tropical forests at night, and roost in lightless caves during the day.  
— But an echolocating seabird????

An adult sets out to fish at dusk. That they possess extraordinary night vision is obvious.

A bilateral expansion of the voice box can be seen when emitting the very distinctive rattle-call

Even under the full moon (right), nighttime photography at sea is very difficult. As they shun bright lights, it is almost impossible to photograph a Swallow-tailed Gull snatching its prey (below).

Snoozing during daytime while protecting the single chick from both sun and predators like frigatebirds. 

From the decks of night-cruising ships, I’ve often seen Swallow-tailed Gulls converge on the sea-surface disturbance caused by the ship’s passage, repeatedly dipping down and snatching large prey from the wavetops or even in mid-air.  At such times it seems their crackles reach fever-pitch, but are they echolocating or just arguing with each other?  Time and again, I’ve tried to photograph their behaviour, but this is almost impossible in such low light levels, and the gulls shun any spotlight directed on them as they hunt.  If my hunch were to be correct, how then would one study such behaviour? Now there’s [sea]food for thought!

After an active night feeding at sea, the giant size of prey items (mostly squid) brought back to the chicks is really quite astounding.

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Photos and information appearing on this site are copyrighted. © Tui De Roy / Roving Tortoise Photos 

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All rights reserved. 

Photos and information appearing on this site are copyrighted. © Tui De Roy / Roving Tortoise Photos 

Its components cannot be shared, reproduced or reused in any way. 

Please contact me for any reuse licenses.  Thank you for your understanding.