In this article we look at the swallows evolution and explore how they successfully adapted to human habitats. The planet we live on has changed dramatically. As humankind expands, we shape the landscape to meet our own needs. As our environments balloon, the natural world recedes, and the animals that inhabited those spaces are forced to adapt to survive.
Some animals achieve this by living among us. A select few have become so successful at exploiting human environments that their populations have boomed. One of these animals is the Swallow.
The Swallow is part of the Hirundinidae family of birds and contains over 90 species worldwide. Of these, 45 evolved as mud nesters.
Barn Swallow
The Barn Swallow (Hirundo rustica) actively seeks out human architecture as one of their preferred breeding locations. Many modern buildings have ideal ledges, overhangs, beams, and gaps that can form the basis for their nesting sites.
Common House Martin
Another member of the swallow family that does the same is the Common House Martin (Delichon urbica). The Common House Martin construct nests using mud, mixed with saliva, often interwoven with grasses, and lined with feathers and moss. So successful are they that for many of us, the spectacle of large quantities of Swallows during the breeding season has become a common sight.
Swallows Nesting Evolution
But what drove evolution to cause these birds to build such unique nests?
At a basic level, many birds seek out nest sites, either by laying their eggs directly into natural hollows or by building nest cups or platforms. This works well for some, particularly larger birds able to defend their young from predators, but smaller birds rely on concealment. All ground-nesting birds are, however, susceptible to various predators.
For this reason, many birds took to building nests off the ground in trees and rocky ledges, including natural or carved-out hollows. But even this strategy is prone to aerial predators and several tree-climbing mammals and reptiles.
Swallow development followed the evolutionary path of carving out hollows of up to 1m in length into steep sandy cliffs, riverbanks, or gravel pits. This makes the nest site inaccessible for many predators.
Sand Martin
This strategy is used to great effect by the bird here, the Sand Martin (Riparia riparia).
However, the technique is only possible when the soil is soft enough for them to dig into. So the limiting factor that prevents them from expanding their breeding ranges is the number of available nest sites.
Some members of the Swallow family eventually diversified into mud nesting, enabling them to expand their range by colonizing areas that were previously unsuitable to support breeding. Mud nesting swallows construct their nests by collecting mud. Beak-full by beak-full throughout several weeks and shape them into a nest, almost like hundreds of little bricks in a house.
Cliff Swallow
Cliff Swallows (Petrochelidon pyrrhonota) build a completely enclosed nest site with a single access door. This keeps their young safe from predators and the environment. Mud nesting does offer benefits to Swallows other than being able to expand their breeding ranges. Swallows have evolved to be social birds. Some nest sites contain thousands of little mud houses, each with its own perimeter.
There are tradeoffs to nesting in these fragile-looking nests. The climate needed to be dry enough for the nest to set and harden. However, large parts of the African continent provided ideal conditions for this, and through the centuries, so did Eurasia and North America.
Barn Swallows Evolution
This also broadly coincided with human expansion, and as a consequence of this, the Barn Swallow branched off as a separate species, taking advantage of man-made structures to build their own houses. The Barn Swallow is now considered the most prolific and widespread swallow species worldwide.
REFERENCES
Schield, D.R. et al. (2024) ‘Phylogeny and historical biogeography of the swallow family (Hirundinidae) inferred from comparisons of thousands of UCE loci’, Molecular phylogenetics and evolution, 197, pp. 108111-. [Accessed 13/05/2025]
