How Do Swallows Build Their Nests? Mud Cups Explained

Step by step: building a mud cup

Nest building starts soon after the barn swallow returns from its winter quarters. The male picks a site and shows it to a female with song and circling flights. Once paired, the two birds build together, although the female usually does more of the work.

The basic material is mud. The swallows land at the edge of a puddle, pond or wet farmyard, scoop up a small pellet of mud in the bill and carry it back to the nest site. Each pellet is pressed against the wall or beam and smoothed into place with quick sideways movements of the head. Grass stems, straw and horse or cattle hair are mixed in, which works like the straw in old mud bricks and stops the nest from cracking as it dries.

The pair lays down the mud in layers, letting each layer dry a little before adding the next. Building a brand-new nest takes about one to two weeks, and many hundreds of trips. Finally the cup is lined with fine grass and feathers, which keep the eggs warm and soft.

What a barn swallow nest looks like

A finished barn swallow nest is a neat, open half-cup, made of brown mud with visible pellets and wisps of grass. It is usually fixed to a vertical surface just below a ceiling, or placed on top of a beam, a ledge, a light fitting or a nail. The open top lets the parents fly straight in and out.

Other swallows build quite different homes:

The swift, which looks similar in flight, does not use mud at all. It builds a flat nest of feathers and plant fluff caught in the air, glued together with saliva, inside holes in buildings.

Why mud matters

Mud nesting has advantages: the material is free and abundant on farms, and a dry mud cup is strong and long lasting. But it depends on the weather. In a dry spring, puddles and wet soil can be hard to find, and nest building may be delayed. In very wet weather, freshly laid mud may not dry, and heavy new nests sometimes fall.

Mud also has to be the right texture. If it is too sandy it crumbles, and if it is too runny it slumps before it sets, so swallows sometimes fly some distance from the nest to find a suitable puddle or bank.

Why swallows nest near people

Before people built barns, barn swallows nested in caves, rock overhangs and cliffs. Today, almost all of them nest in human structures, and in North America natural nest sites are now rarely used.

Buildings offer several benefits. Roofs protect the nest from rain and from birds of prey such as hawks, and the open doors and windows of barns let swallows fly in and out freely. Livestock attract large numbers of flies, giving parents a rich supply of food close to the nest. As farming changes, though, closed modern sheds and fewer animals mean fewer nesting places and fewer insects for swallows.

Reused nests and broods per season

Because building a nest takes so much effort, old nests are highly valued. Barn swallows readily repair a nest from the previous year with a fresh layer of mud and new lining, and many nests are used for 10–15 years; one was reported to have been in use for 48 years. Adults often return to the same farm, and sometimes the same nest, each spring.

Most pairs raise two broods per summer in the same nest, and occasionally three in warmer regions. The female lays four or five eggs on average, incubation lasts 14–19 days, and the chicks leave the nest after 18–23 days. Young swallows from the first brood sometimes help their parents feed the second brood. In autumn, the families leave for their winter quarters, described in where do swallows go in winter?

Swallow nests and the law

In much of Europe and North America, swallow nests are protected by law while they are in use. Old folk beliefs also protected them: farmers once thought that destroying a swallow’s nest would bring bad luck, such as cows giving bloody milk. Today, conservationists encourage farmers and homeowners to leave barn doors or windows open in summer and to leave nests in place, because a pair returning to an existing nest can start breeding sooner.