Think of cliff swallows as social butterflies that come late to the party.

That’s because they like to hang out with others of their kind, with reports of as many as 3,500 nests in one area. And while tree, barn and violet-green swallows have started to return to Whatcom County — bringing the first promise of spring with them — cliff swallows aren’t expected back from their wintering grounds in the south end of South America until the end of this month.

Their name derives from their old haunts.

“Rock cliffs were their natural habitat until humans built large structures which are, in effect, ‘wooden cliffs’ to which the swallows adapted,” explains Paul Woodcock, president of the North Cascades Audubon Society.

Which is why their nests, made of mud and their saliva, are found under the eaves of buildings, especially barns, he adds. “Locally, many people call them ‘eave swallows.’ ”

Good grub to them is flying insects, which is why you’ll find them foraging in circles in the air. Even if you don’t know much about birds, you’ve probably heard of cliff swallows. They’re the ones that leave in swirling masses from San Juan Capistrano.

Cliff swallows look like barn swallows but the former are plumper with a pale gray nape, dark throat, light belly and a short, square tail. Both use mud to make their nests, though the shapes are different.

“The barn swallow’s nest is an open cup while the cliff swallow’s is globular and usually narrows at the top to a small round entrance,” Woodcock says.

Sources: Paul Woodcock; “The Sibley Guide to Birds,” by David Allen Sibley; Birdweb.org

Reach Kie Relyea at kie.relyea@bellinghamherald.com or 715-2234.