Thrushes are passerine birds that occur worldwide. Hybridization has been documented in several genera, namely Sialia, Myadestes, Catharus, and Turdus.
The Swainson’s Thrush (C. swainsoni) is separated into genetically distinct coastal and continental populations that diverged during the Pleistocene. The coastal populations migrate along the Pacific Coast to Central America, while the continental birds prefer an eastern route to Panama and South America (Ruegg, Hijmans & Moritz, 2006b; Ruegg & Smith, 2002). Both populations meet in a hybrid zone that coincides with a migratory divide (Ruegg, 2008). Hybrids show a variability of migratory strategies. Some exhibit intermediate migration routes, while other use one parental route in autumn and the other parental route in spring (Delmore & Irwin, 2014). Selection against hybrids seems moderately strong. The transmission, distribution and phylogenetic relationships among avian blood parasites in this contact zone have also been studied, but provided no insights into possible selection against hybrids (Svensson et al., 2007).
What mechanisms are responsible for the divergence of these populations? Ruegg et al. (2006a) found a correlation between ecological and acoustic distance after correcting for genetic distance, which suggests ecological selection is driving song divergence. Also, short-distance ecotypes arrive earlier on breeding grounds, which might contribute to premating isolation (Ruegg, Anderson & Slabbekoorn, 2012). A genome-wide study of divergence revealed distinct genomic islands on 15 of the 23 chromosomes and accelerated divergence of the Z-chromosome. Migration-linked genes were highly divergent, but located outside of the genomic islands (Delmore et al., 2015; Ruegg et al., 2014).
A similar situation has been described for the Hermit Thrush (C. guttatus), where birds from populations that span a migratory divide display a great variety of migration routes. Genetic analyses also revealed a nonbreeding location for birds from the hybrid zone (Alvarado, Fuller & Smith, 2014).
In the north-western United States, Mountain Bluebirds (S. currucoides) and Western Bluebirds (S. mexicana) are hybridizing in an ecological succession setting. The open meadows of Montana are not the ideal habitat for bluebird, but the placement of artificial nest boxes created the ideal conditions for these cavity breeders. The first species to take advantage of these new nesting places is the Mountain Bluebird. The more aggressive Western Bluebirds are delayed in their arrival, but once they show up, their numbers increase rapidly. 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 are outnumbered by Mountain Bluebirds. Unable to find another Western Bluebird to mate with, they ‘settle’ for a Mountain Bluebird. Later on in the succession, the number of Western Bluebirds increases and the chances of hybridization diminish. Although the frequency of hybridization seems to decline during the ecological succession, the genetic effects of interbreeding are still measurable in later generations (Duckworth & Semenov, 2017).
Wilson et al. (2009) discuss the taxonomic status of Karoo Thrush (T. smithi) and Olive Thrush (T. olivaceus) and point to overlapping distributions where hybridization might occur.
Alvarado, A. H., Fuller, T. L. & Smith, T. B. (2014). Integrative tracking methods elucidate the evolutionary dynamics of a migratory divide. Ecology and Evolution 4, 3456-3469.
Delmore, K. E., Hubner, S., Kane, N. C., Schuster, R., Andrew, R. L., Camara, F., Guigo, R. & Irwin, D. E. (2015). Genomic analysis of a migratory divide reveals candidate genes for migration and implicates selective sweeps in generating islands of differentiation. Molecular Ecology 24, 1873-1888.
Delmore, K. E. & Irwin, D. E. (2014). Hybrid songbirds employ intermediate routes in a migratory divide. Ecology Letters 17, 1211-1218.
Duckworth, R. A. & Semenov, G. A. (2017). Hybridization Associated with Cycles of Ecological Succession in a Passerine Bird. The American Naturalist 190.
Ruegg, K. (2008). Genetic, morphological, and ecological characterization of a hybrid zone that spans a migratory divide. Evolution 62, 452-466.
Ruegg, K., Anderson, E. C., Boone, J., Pouls, J. & Smith, T. B. (2014). A role for migration-linked genes and genomic islands in divergence of a songbird. Molecular Ecology 23, 4757-4769.
Ruegg, K., Anderson, E. C. & Slabbekoorn, H. (2012). Differences in timing of migration and response to sexual signalling drive asymmetric hybridization across a migratory divide. Journal of Evolutionary Biology 25, 1741-1750.
Ruegg, K., Slabbekoorn, H., Clegg, S. & Smith, T. B. (2006a). Divergence in mating signals correlates with ecological variation in the migratory songbird, Swainson’s thrush (Catharus ustulatus). Molecular Ecology 15, 3147-3156.
Ruegg, K. C., Hijmans, R. J. & Moritz, C. (2006b). Climate change and the origin of migratory pathways in the Swainson’s thrush, Catharus ustulatus. Journal of Biogeography 33, 1172-1182.
Ruegg, K. C. & Smith, T. B. (2002). Not as the crow flies: a historical explanation for circuitous migration in Swainson’s thrush (Catharus ustulatus). Proceedings of the Royal Society B-Biological Sciences 269, 1375-1381.
Svensson, L. M. E., Ruegg, K. C., Sekercioglu, C. H. & Sehgal, R. N. M. (2007). Widespread and structured distributions of blood parasite haplotypes across a migratory divide of the Swainson’s thrush (Catharus ustulatus). Journal of Parasitology 93, 1488-1495.
Wilson, J. W., Symes, C. T., Brown, M., Bonnevie, B. T., de Swardt, D. H. & Hanmer, D. (2009). A re-evaluation of morphological differences in the Karoo Thrush Turdus smithi – Olive Thrush Turdus olivaceus species complex. Ostrich 80, 171-175.