A Rare Find in the Least Tern Colony

Julia Hoopes

It was a foggy morning on the coast when monitors spotted something strange. Sitting on the sand among the California least terns was a bright white bird! This bird was nothing like any of the other birds commonly seen in the area. It was not mottled, marbled, speckled or shaded like other shorebirds. It was stark white with the faintest shade of gray around its black beak. It was the same size and shape of a least tern fledgling. Could this strange white bird be a California least tern?

Typical least tern fledglings have a mix of grays, whites, blacks and even some rusty gold in their feathers. This strange white bird had none of these markings. Monitors watched the mysterious bird, hoping to gain clues from its behavior.

 All around, fledgling terns were testing their flying skills. Many of the adult terns were flying, hovering and plunging into the water to catch tasty little fish. Although least tern fledglings can fly, they still rely on their parents to supply them with food. Adult least terns pull fish from the water, return to the shore and call to notify their young that they have returned with food. With anticipation for a meal, the fledglings will return the call.

The monitors noticed an adult calling and circling above with a fish in its mouth and, sure enough, the  white bird responded! Down flew the adult to deliver the fish to the bright white bird. That confirmed the monitors’ suspicions – it was indeed a least tern fledgling. But what could explain this fledgling’s lack of color?

Birds can suffer from a genetic condition called albinism causing their body to be unable to produce melanin - the substance that gives pigment to eyes, skin and feathers.  This wasn't the case with our white tern, however. The mysterious bird did have some pigment. Its beak was black, and there was gray around its face. This bright white bird was actually suffering from a similar genetic mutation called leucism. Leucism is like albinism however it only affects the melanin in a bird’s feathers. The mystery was solved! Right there in the least tern colony was a leucistic fledgling.

Although beautiful, this fledgling faces many challenges because of its rare condition. Least tern fledglings are not strong flyers and they rely on their coloration to help them blend into their environment and hide from predators. A bright white bird is very easy to spot, making it a target for avian predators like northern harriers and peregrine falcons. Another problem is that melanin is an important structural feature of feathers. Without melanin, this bird’s feathers may be less effective insulators and more susceptible to wear and tear.

All of the least terns have a big journey ahead of them as they begin their migration southward but perhaps next year a mysterious white bird will be spotted in the least tern colony and we will know the leucistic tern survived.

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