Avian hybrid zones have been studied extensively for decades. You would think that ornithologists looked at these zones from all possible angles. Nonetheless, Renée A. Duckworth and Georgy A. Semenov provide a fresh perspective on a particular hybrid zone between two Sialia species.
Ecological succession refers to the situation where the species composition of a community changes over time. Mostly, succession occurs after an environmental disturbance, such as a forest fire. The resulting habitat is first colonized by the most dispersive species. Later on, other species – often better competitors – invade and start to replace the early arrivers.
This is what happened in the northwestern United States. The open meadows of Montana are not the ideal habitat for bluebirds (genus Sialia), but the placement of artificial nest boxes provided the ideal conditions for these cavity breeders. So, the introduction of nestboxes can be considered the ‘environmental disturbance’. The first species to take advantage of these new nesting places was the Mountain Bluebird (S. currucoides). The more aggressive Western Bluebirds (S. mexicana) were delayed in their arrival, but once they showed up, their numbers increased rapidly.
Using data from 10 populations from 2001 to 2014, Duckworth and Semenov investigated the occurrence of matings between these two Bluebird species during different stages of the ecological succession. In addition, to check whether these heterospecific pairs actually produced hybrids (you never know if there are extra-pair copulations!), they also looked into the genetics of these birds using microsatellites.
The results show that heterospecific matings only occurred during the early stages of succession. This is in accordance with Hubb’s principle, which states that hybridization is more likely when one species is rare. In this case, the first wave of invading Western Bluebirds were outnumbered by Mountain Bluebirds. Unable to find another Western Bluebird to mate with, they ‘settled’ for a Mountain Bluebird. Later in the succession, the number of Western Bluebirds increased and the chances of hybridization diminished.
Although the frequency of hybridization seems to decline during the ecological succession, the genetic effects of interbreeding are still measurable in later generations. This could have important evolutionary consequences for these species.
Duckworth, R. A. & Semenov, G. A. (2017). Hybridization Associated with Cycles of Ecological Succession in a Passerine Bird. The American Naturalist 190.
Thanks to Renée A. Duckworth for sending me a copy of this paper. The information has been added to the Turdidae page.