The swallow-tailed gull is mostly endemic to the Galápagos Islands with an estimated population of about 35,000. Their red eye rims encircle irises unique to all other species of gulls. They amplify light at night. This allows the swallow-tailed gull to be the only nocturnal feeding seabird in the world. Other birds on Plaza Sur include the Nazca and blue-footed boobies plus red-billed tropicbirds.