A House Finch’s Successful Use of a Barn Swallow Nest

The ability to adapt to and thrive in human-dominated landscapes has allowed the House Finch (Haemorhous mexicanus) to become one of the most widely distributed songbirds in North America and has allowed the Barn Swallow (Hirundo rustica) to become the widest-ranging swallow in the world (Alsop 2006). As a result of adapting to nest on artificial structures and the House Finch’s introduction to the east coast of the United States (Aldrich and Weske 1978), these species’ overlap increased greatly. Both species inhabit disturbed areas (Badyaev et al. 2012) and construct cup-shaped nests, but House Finches use twigs, grasses, leaves, and a lining of finer materials, while Barn Swallows use dried mud and grass with a lining of feathers or hair (Alsop 2006). As the chicks grow, a characteristic ring of feces forms around the edge of House Finch nests (Evenden 1957), but Barn Swallow chicks defecate over the edge rather than on the edge of a nest (Spencer 2005). Additionally, unlike House Finches, Barn Swallows reuse the same nests many times, and the nest structures themselves can persist for years (Møller 1994). Here I present an observation of a House Finch occupying an abandoned Barn Swallow nest, an occurrence documented rarely. I first observed a House Finch sitting inside a Barn Swallow nest on 18 July 2018 (Figure 1A). The nest was atop a light fixture under the roof edge of a building with a wooden exterior in Elkhorn Slough National Estuarine Reserve, California. The finch laid three eggs (Figure 1B), and all three hatched, the last on 26 July 2018, when I observed eggshell fragments around the third hatchling. Though there were Barn Swallows nearby while the House Finches used the nest, I noted no swallows harassing the House Finches. The finch treated the clay nest as platform, building a complete nest within it with outer layer, thick walls, and inside lining. In a search online, I found only three similar observations of House Finches nesting in Barn Swallow nests—in Pennsylvania (www.hiltonpond.org/ThisWeek000415.html), South Carolina (www.marys-view.blogspot.com/2007/04/hit-and-miss.html), and