Large-billed Tern

BirdSouth America

12 Aug

Written By Alexander Julius Jensen

Phaetusa simplex

The large-billed tern uses its disproportionately long and thick beak to catch fish — either by plunge-diving into the water or skimming the surface with its beak as it flies. It has a commensal relationship with the tucuxi river dolphin, which often flushes fish towards the surface while hunting.

Travelling Terns

Clockwise from top left; an Arctic tern (*Sterna paradisaea*), a greater crested tern (*Thalasseus bergii*), a sooty tern (*Onychoprion fuscatus*), and a little tern (*Sternula albifrons*).

Terns are great travellers. You could spot Arctic terns soaring above the frozen fjords of Svalbard and royal terns alighting on tropical islands in the Caribbean; moustached Inca terns travel the west coast of South America, while common terns abound on European shores, just as whiskered terns do throughout India; the little tern frequents Japan while flocks of greater crested terns crowd the Australian coasts; even the most remote islands of the Pacific host these seafaring birds — the snowy white and sooty terns traversed these water long before the Polynesians.

There are some 40 species of terns ¹, of which the majority look pretty similar; recognisable by the black caps they wear atop their heads and their long, slender bills. There are, of course, exceptions, as well as a few oddities. If you took, say, a pretty regular tern, like the greater crested tern — with its grey-white plumage and black cap — and supersized its yellow beak, it would look something like the large-billed tern.

Big-beak Fishing

A large-billed tern with a fish in its large bill — did it catch it or steal it?

In all respects but one, the large-billed tern looks pretty unremarkable. Its body is grey and white, perhaps a little darker along its back than most terns. Its feet are fairly small, greenish-yellow in colour, and webbed. It is pretty large for a tern, with a wingspan of about 90 cm (35 in) — it doesn't, however, challenge the world's largest; the Caspian tern with its 127 cm (50 in) wingspan. But no other tern can match the size of the large-billed tern's, well, disproportionally large bill.

A black skimmer (*Rynchops niger*) — a master skim-fishing with an oversived bottom mandible.

This well-endowed bird can be found throughout South America, using the continent's great waterways to reach inland rivers and lakes. It also congregates along the coastlines — particularly during breeding season — in bays and estuaries, wetlands and coastal mangroves. Fish can be found in saltwater as well as fresh, and large-billed tern isn't picky. It can hunt as most other terns, and gulls, do — by plunge-diving into the water and snatching a fish from the shallows in its bill — but its hefty facial appendage allows for another form of fishing. The large-billed tern takes note from the black skimmer's book. The black skimmer has an asymmetrically large lower mandible. As it glides above the water, it dips this lower mandible below the surface, "skimming" the water. When it feels its bill hit a fish, the skimmer snaps it shut, seizing its prey. While the large-billed tern's bill has equally massive mandibles, it employs a similar skimming technique.²

Primarily a piscivore, the large-billed tern will occasionally take crustaceans and insects as well. It often engages in dramatic aerial chases with the latter, swerving after the fast-flying invertebrates — whether its large bill affects its aerodynamics, I don't know. But, insects aren't the only thing it pursues on the wing. While the large-billed tern may live and nest in mixed-species flocks, it is not a cordial neighbour. It has an aggressive and competitive disposition, often fighting other birds for their food, chasing them through the sky in an attempt to steal their catch. It has a less antagonistic relationship with a surprising aquatic animal: the tucuxi, a river dolphin ³. As this dolphin hunts, it flushes fish to the surface, where the tern can more easily snatch them. The tern gives the dolphin nothing in return, although it doesn't harm the dolphin either — what's known as a commensal relationship.

The tucuxi (Sotalia fluviatilis), or Guianian River dolphin, which has a commensal relationship with the large-billed tern.

Clockwise from top left; a black tern (*Chlidonias niger*), an Inca tern (*Larosterna inca*), a brown noddy (*Anous stolidus*), and a white tern (*Gygis alba*).

¹ The terns are either placed in their own family (Sternidae) or a subfamily of the Laridae family — which includes gulls and skimmers.

While most tern species look alike, there are a few outliers. The white tern has entirely white plumage and large, inky black eyes — a resident of the world's tropical islands. The black tern is its dark twin, with an all-black body and silver wings during its breeding season — the white-winged tern has a similarly striking breeding plumage. But, undeniably the most unusual tern (even more so than the big-billed) is the Inca tern. Its range roughly overlaps with the borders of the largest empire in pre-Columbian America, from which it gets its name. It has an almost uniformly grey body, rich red feet and bill, yellow patches on either side of its face, and its most conspicuous feature; a pair of satiny white whiskers, often curled like a handsome moustache.

Among the terns are 5 species of birds known as noddies (in the genus Anous). Wholly grey-white with dark bills, the noddies are named for their nodding courtship displays. The brown, black, and lesser noddies kind of look like reverse terns — with dark bodies and light caps — while the blue and grey noddies look like white terns that were dropped soot.

² The skimmers constitute a genus (Rynchops) with three species, found in the Americas (black skimmer), Africa (African skimmer), and Asia (Indian skimmer) respectively. All have large, protruding lower mandibles that they use to "skim-fish".

The range of the black skimmer and large-billed tern overlap significantly, they even share nesting sites. When it comes to skimming, perhaps the large-billed tern began emulating the skimmer's behaviour and over time evolved a beak to fully exploit the hunting technique — such a theory would require more information about the geographical distribution of the two species' ancestors and perhaps how inclined terns are to mimic other bird's behaviour, although that's a difficult query.

I say that the tern emulated the black skimmer because the skimmer likely evolved its skimming behaviour and bill first — as it seems more adapted for the hunting technique, and it certainly didn't copy it from the tern, as its relatives (who also practice skimming) are found the world over. However, it could equally likely just be a case of convergent evolution.

³ The tucuxi (Sotalia fluviatilis), also known as the Guianian River dolphin — not to be confused with its close relative, the Guiana dolphin (Sotalia guianensis) — is endemic to the Amazon and Orinoco basins. It looks somewhat like a miniature bottlenose dolphin and lives in tight pods that together hunt a variety of fish. It is considered an endangered species; threatened mostly by the fishing industry, often caught as by-catch in fishing nets, and by pollution from dams and industry. It is the only species of oceanic dolphin (family Delphinidae) known to live in fresh water and several indigenous South American cultures believe these dolphins to be protectors that carry the drowned to shore for burial.

**Where Does It Live?**

⛰️ Rivers and lakes, coastlines, bays and estuaries, wetlands and coastal mangroves.
📍 Most of South America; east of the Andes and north of the Pampas.

‘Least Concern’ as of 01 October, 2016.

IUCN

Size // Small
Wingspan // 86 - 91 cm (34 - 36 in)
Length // 38 - 42 cm (15 - 16.5 in)
Weight // 208 - 247 grams (7.3 - 8.7 oz)
Activity: Diurnal ☀️
Lifestyle: Social 👥
Lifespan: Up to 20 years
Diet: Carnivore (Piscivore)
Favorite Food: Freshwater and saltwater fish 🐟
Class: Aves
Order: Charadriiformes
Family: Laridae
Genus: Phaetusa
Species: P. simplex

A commensal relationship between two species implies that one benefits — in this case, the large-billed tern benefits because it can easier catch fish flushed to the surface — while the other isn't affected for good or ill — the tucuxi dolphin is unbothered.
The large-billed tern borrows a fishing technique from the skimmer birds. As their name implies, these three species of waterbirds skim the surface of streams to catch fish, but they are especially fit for the task — with their oversized, protruding lower mandibles.
As well as diving and skimming, the large-billed tern pursues insects on the wing with surprising aerial nimbleness. It's also a kleptoparasite; fighting other birds to steal their food. The large-billed gull's usual prey is fish between 4 and 12 centimetres (1.5 - 5 inches) long.
It hunts in both fresh and saltwater, frequenting habitats such as rivers and lakes, coastlines, bays and estuaries, wetlands and coastal mangroves.
Like many seabirds, the large-billed tern is noisy. It squeals (“squeee-ee”) and squeaks (“ink-onk") like a nasally parrot. It greets other terns with raspy “kew” and “kree” contact calls.
It nests on beaches, sandbars and floodplains — with different breeding seasons in different regions depending on water level. Both parents care for their clutch (1 - 3 eggs) and their hatchlings. They will mob any potential threat.
The large-billed tern lives throughout much of South America; east of the Andes and north of the Pampas.
There are around 40 species of tern and they are found on every continent (including Antarctica). Most look pretty similar — with grey-white plumage, black caps, and long slender bills — a few are exceptional; such as the black tern, white tern, and the white-winged tern, and especially the moustachioed Inca tern.