Gull the exceptions being Sabine's gull and swallow-tailed gulls, which have forked tails, and Ross's gull, which has a wedge-shaped tail. Gulls have moderately long legs, especially when compared to the similar terns, with fully webbed feet. The bill is generally heavy and slightly hooked, with the larger species having stouter bills than the smaller species. The bill colour is often yellow with a red spot for the larger white-headed species and red, dark red or black in the smaller species. The gulls are generalist feeders. Indeed, they are the least specialised of all the seabirds, and their morphology allows for
Swallow-tailed gull photoreceptor cells. A study of melatonin levels of swallow-tailed gulls found them to have no measurable daily melatonin rhythm, whereas a day-feeding gull that was chosen for comparison had the expected higher night time melatonin level for day active birds. High melatonin levels generally make birds sleepy. It is still unknown whether the melatonin levels are a cause or an effect of the swallow-tailed gulls' nocturnal activity. The swallow-tailed gull is a near-endemic breeding bird of the Galápagos Islands, although a few pairs nest on Malpelo Island off the coast of Colombia. When not breeding, it is totally pelagic, flying