Strength in Numbers: 2500 Trips to the River

Casey McFarland

It’s estimated that only 5% of the world’s bird species use mud in their nest construction, and only a small percentage of those use only mud. Cliff Swallows (Petrochelidon pyrrhonota) are a species well known to many, even non-birders, because of the ubiquitous nest colonies they build under eves and ledges on canyon walls and buildings alike. Heavily clustered, appealing to the eye, and made purely of mud, these igloo-esque Cliff Swallow structures are unique among nests, even compared to those of their close cousins. (Compare this description to the materials in the Barn Swallow (Hirundo rustica) nest pictured for this post.) These mud houses are among the nests that place birds distinctly as accomplished architects.

And how does a critter using only its beak attach and build an impressive clay igloo to a concrete wall? With a lot of trips to puddles, rivers, and creeks, and an impressive know-how of masonry and building materials.

Looking closely at the surface of the nest one can see each mouthful of mud collected and placed. Researchers in Europe counted collection trips made by House Martins (Delichon Urbica)—which build a structure similar to that of our Barn Swallow—and found that the birds made around 2500 collection forays to complete their nest!

In addition to the sheer number of trips and distance covered (an unrelated study estimated another species traveled around 14 miles just to line its nest with downy material) the strategy and skill involved is stunning. To avoid weak spots and cracks, Cliff Swallows must assure that a new layer of mud added to an already drying portion of the nest wall will adhere correctly; they vibrate their mouthful of mud to partially liquefy it as it is placed, creating a solid bonding and grouting effect. They also know when to take a break to allow for appropriate drying before moving on to the next building stage. With careful examination, one might see layers in the nest wall that reflect construction stages, as well as portions that were built with mud collected from different locales. Look closely at the photo above: coloration differences reveal that various mud sources were used during building. Here, a House Sparrow puts this old nest to use.