Cliff swallows and the extended phenotype








Some birds prefer to build nests in trees or simply nest in tree cavities. One species, in particular, prefers to build a nest out of mud on the side of an outhouse. Searching along the banks of a river for mud may sound like a dirty job. However, if you are a cliff swallow (Petrochelidon pyrrhonota), it’s simply a way of life. Cliff swallows find mud and gather it in their bills to use as the primary nest building material. The swallows bring the mud pellets back to the side of a cliff, cave, or even a man-made structure (in Yellowstone, they like outhouses) to construct a conical, gourd-shaped nest. What’s the point of collecting mud and other debris to construct an odd-looking nest? Why do cliff swallows prefer to build these types of nests in colonies? To begin to answer some of these questions, it is important to look at the reason(s) for having nests.

There is a huge amount of variation in nest design among animal taxa. Some mammals dig burrows, fish construct craters, and termites build mounds. Within the bird family, the there is still quite a variation. Even though the design is different, all nests basically serve as a place where animals lay eggs and/or raise their young. Why would cliff swallows go through all the trouble of building a gourd-shaped nest attached to the side of a building in close proximity to other nests (in a colony)? What is the role of natural and sexual selection in nest building? Do environmental factors influence construction?

Nests can certainly impact the fitness of individual birds. The greater the fitness of an individual, the greater the chances of surviving and reproducing. This is seen in several ways. First of all, the location of nests as well as the design of nests both contribute to predator avoidance. Also, building nests in a colony can increase social interactions which, in turn, can increase the chances of survival. These strategies show that natural selection exerts strong selective pressures on the design and placement of nests, but natural selection does not explain it all.

In addition to being influenced by natural selection, nest design may also be the result of extended phenotypic signals through sexual selection pressures (Mainwaring, Hartley, Lambrechts, & Deeming, 2014). In one of his landmark books, Richard Dawkins defined the extended phenotype as “an animal artefact, like any other phenotypic product whose variation is influenced by a gene” (1982). Some species of birds have individuals who signal their quality through physical signals like bright plumage. Other individual birds signal quality through behavioral signals like dancing. Is it possible that nest building be just like bright plumage or fancy dancing? Bowerbirds build structures that seem to be extended phenotypic signals whose sole purpose is to attract a mate. If nest-building behavior and nest design is an extended phenotype in cliff swallows that plays a role in sexual selection, then there must be some costs associated with the builder (Andersson 1982). Ectoparasitism may provide these costs.

Cliff swallows are highly social birds. They feed, preen, gather mud, and participate in most activities socially. As we have mentioned, they also nest together in colonies (Brown 1985). It is this sociality that may explain why the swallows are parasitized by hematophagous swallow bugs (Oeciacus vicarious) and fleas (Ceratophyllus celcus). Swallow bugs are found almost exclusively on cliff swallows and can remain permanently in nests, except for a period of dispersal in the spring in which they hitch a ride at the base of the swallows’ feathers (C. R. Brown & Brown, 1992). Research has shown that swallow bugs actually lower nestling body mass by 3.4 grams and survivorship by 50% in large colonies, but not in small colonies (C. R. Brown & Brown, 1992). Thus, parasitism is a serious cost of nesting in colonies for cliff swallows. The presence of parasites probably contributes to the selection of nests or the need to build new nests.

One particular study showed that colony sites vary in their suitability (Charles R Brown, Brown, Brown, & Brown, 2000). The researchers showed that the colony sites that had the highest reproductive success relative to others tended to be re-used the following year, even by new immigrants. This may show that these birds select nesting sites that have the least concentration of parasites, and thus may help increase the chances of offspring survival. Even though females probably do most of the nest site selecting, single males have been observed picking or even building a new nest to attract a mate (Charles R Brown et al., 2000).

There is very strong evidence that nest-building behaviors and nest design in cliff swallows act as extended phenotypic signals. So, the next time you see these birds zipping in and out of their nests hanging from an outhouse eave, there may be a purpose behind their business.


Andersson, M. (1982). Sexual selection, natural selection and quality advertisement. Biol. J. Linn. Soc., 17:375-393.

Brown, C. R., Brown, C. R., Brown, M. B., & Brown, M. B. (2000). Breeding habitat selection in cliff swallows: the effect of conspecific reproductive success on colony choice. Journal of Animal Ecology, 133–142.

Brown, C. R., & Brown, M. B. (1992). Ectoparasitism as a cause of natal dispersal in cliff swallows. Ecology, 73(5), 1718–1723.

Dawkins, R. (1982). The Extended Phenotype. Oxford:Oxford University Press.

Mainwaring, M. C., Hartley, I. R., Lambrechts, M. M., & Deeming, D. C. (2014). The design and function of birds’ nests. Ecology and Evolution, 4(20), 3909–28.


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