These compact swallows have rounded, broad-based wings, a small head, and a medium-length, squared tail. In poor light, Cliff Swallows look brownish with dark throats and white underparts. In good light, their metallic, dark-blue backs and pale, pumpkin-colored rumps are visible. They have rich, brick-red faces and a bright buff-white forehead patch like a headlamp. Some juveniles show whitish throats in summer and fall. They are about the size of a sparrow. They have a length of 5.1 inches (13 centimeters), weighing 0.7-1.2 ounces (19-34 grams), and a wingspan of 11.0-11.8 inches (28-30 centimeters).
Most Cliff Swallows in North America have whitish foreheads. However, birds from the Mexican population (which extends into the southwest U.S.) have dark chestnut foreheads.
OccurrenceCentral America, and eastern South America to reach their wintering grounds. They migrate during daytime in groups of up to several hundred, foraging as they move. Formerly restricted to canyons, foothills, and river valleys with natural cliff faces and overhangs, Cliff Swallows have spread into a wide variety of habitats by nesting on human-made structures. They now live in grasslands, towns, broken forest, and river edges, but avoid heavy forest and deserts. In the south-central and northeastern states they are rare and localized breeders. Most colony sites are close to a water source, open fields or pastures for foraging, and a source of mud for nest building. Cliff Swallows spend the winter in grasslands, farmland, marshes, and the outskirts of towns in southern South America.
Cliff Swallows eat flying insects all year round, foraging during the day in groups of 2 to more than 1,000 birds. They feed on the wing above grassy pastures, plowed fields, and other open areas, but also over floodplain forests, canyons, and towns—often taking advantage of thermal air currents that bring together dense swarms of insects. In cool or rainy weather when insects are scarcer and thermals weaker, they may also feed over bodies of water. Cliff Swallow colonies serve as foraging information centers as parents make trips back and forth to feed nestlings: unsuccessful foragers follow their successful neighbors to food sources. Their diet consists of many types of flying insects (particularly swarming species).
Each Cliff Swallow pair first chooses a colony, then takes over an existing nest or selects a space on the colony to build a new nest. Colonies may be located on natural cliff faces and overhangs or human-made structures, and each nest is built at the juncture between a vertical wall and a horizontal overhang. The female spends more time than the male scoping out colony sites before they settle on one. An unmated male may choose a site on his own and later attract a mate. Both members of the pair help build the nest, though the male may begin building before he attracts a mate. They gather mud in their bills along slow-moving bodies of water, usually near the colony but sometimes up to a few miles away. They bring mud pellets back in their bills and mold them into place with a shaking motion. The finished nest is gourd shaped and contains 900–1,200 individual mud pellets. It measures about 8 inches long, 6 inches wide and 4.5 inches high, with walls 0.2–0.7 inches thick. The entrance, which is sometimes elongated into a tube, is about 1.7 inches high and 2 inches wide. The pair then lines their nest with dried grass and continues patching it up with mud throughout the breeding season.
Cliff Swallows have one to two broods per year with one to six eggs in each clutch. The eggs are white, creamy, or pinkish, with brown speckles or blotches. The eggs have a length of 0.7-0.9 inches (1.8-2.4 centimeters) and a width of 0.5-0.6 inches (1.3-1.5 centimeters). Incubation period is 10-19 days and nesting period is 20-26 days. At hatching, the hatchlings are helpless with bare pink skin and weighing less than a tenth of an ounce each.
Cliff Swallows are the most colonial swallow in the world, regularly forming colonies of 200-1,000 nests, with a maximum of 3,700 nests in one Nebraska site. They preen, feed, drink, and bathe in groups, and they continue sticking together in large flocks during migration and on their wintering grounds. Cliff Swallows sleep in trees for most of the year, but a breeding bird will start sleeping in the nest as soon as the structure is partially finished. They fight for nest sites by grappling in half-built nests or on the bare wall. Fighting birds sometimes fall into the water and manage to row with their wings to reach the shore. Nest owners defend their completed nests by sitting in the entrances, puffing up their head and neck feathers to look larger, and lunging at intruders. Each bird has one mate with whom it raises young, but the pair does not associate away from the nest, and both members frequently mate outside the pair bond. They are preyed on by larger birds, snakes, fire ants, weasels, and pets.
While courting and nesting, Cliff Swallows sing a series of guttural grating sounds and squeaks, usually lasting up to 6 seconds. Their most common call is a soft chur. They also give a squeak when foraging and a purr-like alarm call when predators approach the colony.
Cliff Swallows are numerous and their populations were stable recently Cliff Swallow numbers probably increased dramatically beginning in the nineteenth century as they expanded into new breeding habitats. At the same time, their breeding has been impeded by the spread of invasive House Sparrows, which often take over their nests. In the northeastern United States, those conflicting influences caused Cliff Swallow numbers to drop during the 1900s. The northeastern population is currently low. Other regions that have seen declines include the Pacific Northwest, coastal California, and the Great Lakes, although these have been balanced by increases in other parts of the continent. Management officials have successfully increased local Cliff Swallow populations by trapping House Sparrows at colony sites. The species has expanded in the southeastern United States in recent decades.
- Although the Cliff Swallow can nest solitarily, it usually nests in colonies. Colonies tend to be small in the East, but further west they can number up to 3,700 nests in one spot.
- Within a Cliff Swallow colony some swallows lay eggs in another swallow's nest.
- When young Cliff Swallows leave their nests they congregate in large groups called creches. A pair of swallows can find its own young in the creche primarily by voice. Cliff Swallows have one of the most variable juvenal plumages, and the distinctive facial markings may help the parents recognize their chicks by sight too.
- The oldest recorded Cliff Swallow was a male, and at least 11 years and 10 months old when he was recaptured and rereleased for scientific purposes in California in 2004. He had been banded in Nebraska in 1993.
- Cliff Swallows are called Golondrina Risquera (in Spanish) and Hirondelle à front blanc (in French).