In the aftermath of fire, bluebird species boundaries may blur

One of the most clear-cut reasons that species evolve to fill different ecological niches is competition. Two otherwise similar species that use the same resources experience strong selection favoring the use of less-similar resources, if they have the option. The classic example is Darwin’s finches, which evolved different beaks to better use different seeds, and have radiated (or are radiating) into an array of different species as a result.
However, nothing is simple in evolutionary biology. Competition also brings closely related species into contact, as they jostle for access to the resources over which they’re competing. A new paper in the American Naturalist shows that, for two North American songbird species, this kind of interaction may provide an opportunity for interbreeding where none would otherwise exist — competition creating, in this case, a reduction in the genetic isolation between those two birds.
The species in question are western bluebirds and mountain bluebirds. Their ranges overlap across much of western North America, and they both nest in tree cavities adjacent to open grasslands where theres’ good insect hunting. Western bluebirds are more aggressive than mountain bluebirds, and usually win out in direct competition for nest sites — but mountain bluebirds make up for this by exploring farther for nest sites. Because wildfires periodically clear patches of woodland in the birds’ shared habitat, there’s a regular supply of new open territories, which are usually colonized first by mountain bluebirds, then taken over by the more aggressive but slower-arriving western bluebirds.
In the earlier stages of succession, as trees begin to re-establish in wildfire clearings, they’re often occupied by both bluebird species. And this is a period when interactions may be more sustained — to the point of birds not-of-a-feather move in together. Renée Duckworth and Georgy Semenov, the authors of the new Am Nat paper, have surveyed 10 sites in Montana where artificial nesting boxes have been set up in open habitat, replicating the conditions in a patch of woodland recently cleared by fire. Together with an observation from a three-decade old study at one of the sites, they identified three mixed-species mated pairs, all male western bluebirds paired with female mountain bluebirds, and all early in the process of western bluebird takeover, when fewer than a third of the birds at the site were western bluebirds.

Bluebirds, like lots of bird species, form social pairs that are not necessarily identical to mated pairs — a male and a female that share a nesting site and raise chicks together may mate with individuals in other pairs, or with unpaired birds in the surrounding area. So the presence of mixed social pairings doesn’t necessarily mean that mountain and western bluebirds were hybridizing. To test for this, Duckworth and Semenov genotyped the offspring of the two mixed-species pairs they directly observed and a larger sample of birds from across all 10 sites, using 15 microsatellite loci. They ran standard analyses (STRUCTURE and NewHybrids) to identify birds whose genotypes were consistent with pure mountain or western bluebird ancestry and those who were likely hybrids.

This molecular data revealed two interesting facts. First, hybridization was indeed happening, and hybrid birds seem to persist in the population — even though all the mixed pairings were observed early in succession, birds with mixed ancestry were found at sites in the later stages of succession, where western bluebirds were in the majority. Second, and more interesting, the mixed social pairs mostly did not result in mixed matings. Three of four chicks raised by one of the mixed pairs, and four out of six chicks raised by the other, had un-mixed mountain bluebird ancestry. That indicates that the female mountain bluebird in each pair “cheated” on her western mate with males of her own species.
Duckworth and Semenov describe this as “a high level of extra-pair paternity”, and they hypothesize that it reflects a rather clever solution, by the two mountain bluebird females, to the problem of greater western bluebird competitiveness for nesting sites. To wit: even early in succession when they’re in the minority, western bluebirds are better able to claim and defend a nest site. In a mixed population, a mountain bluebird female might not find a male of her species with a nest site of his own, but if she nests with a western bluebird, she can mate with other mountain bluebirds on the side. (And, in all likelihood, her mate is stepping out with members of his own species, too.)
Still, these bluebird odd couples do produce some hybrid chicks, and the chicks thrive sufficiently well that mixed birds are not hard to find at sites in later stages of succession. Precisely because of their jockeying for nest sites, the two species come into contact in just the right way to trade genes — keeping these competitors close, in spite of their differences.
Reference
Duckworth RA and GA Semenov. 2017. Hybridization associated with cycles of ecological succession in a passerine bird. American Naturalist doi: 10.1086/693160