Morphology Insufficient to Detect Hybrid Sparrows

Identification of hybrids based on morphological characteristics can be extremely difficult. Hybrids may express a mosaic of parental traits or display extreme phenotypes compared to parental forms (a phenomenon known as transgressive segregation). In addition, extensive backcrossing can result in a continuous gradient of phenotypes across a hybrid zone.

Jennifer Walsh (University of New Hampshire) and colleagues took a closer look at the hybrid zone between Saltmarsh Sparrow (Ammodramus caudacutus) and Nelson’s Sparrow (A. nelsoni). Based on genetic markers, they were able to identify eight recent hybrids, 44 backcrosses to Nelson’s Sparrow and 98 backcrosses to Saltmarsh Sparrow. However, using the morphological variation across the hybrid zone they could only distinguish between Nelson’s and Saltmarsh groups; backcrosses were indistinguishable from the pure parental species. These results indicate that hybrid identification without genetic data will likely lead to a substantial overestimation of the proportion of genetically pure individuals in a population. Hybrid zone research should be a combination of genetic and morphological approaches.

More information on hybridization in the genus Ammodramus and the family Emberizidae can be found here.

A Nelson’s Sparrow (or is it a backcross?)

References

Walsh, J., Shriver, W. G., Olsen, B. J., O’Brien, K. M. & Kovach, A. I. (2015). Relationship of phenotypic variation and genetic admixture in the Saltmarsh-Nelson’s sparrow hybrid zone. The Auk 132, 704-716.

Giant Petrel Hybrids are Fertile

Northern and Southern Giant Petrels (Macronectes halli and M. giganteus) breed sympatrically on five island groups: South Georgia, the Prince Edward Islands, Iles Crozet, Iles Kerguelen and Macquarie Island. Hybridization between these species has been reported on some of these islands. In a recent paper, Brown et al. (2015) provide the first evidence that the hybrid offspring of these species are fertile and that back-crossing occurs. A known hybrid male, that was ringed as a chick, was obserbed breeding and successfully raised five chicks with a female Northern Giant Petrel.

More information on hybridization in the Procellariiformes can be found here.

A Southern Giant Petrel

Reference

Brown, R. M., Techow, N. M. S. M., Wood, A. G. & Phillips, R. A. (2015). Hybridization and Back-Crossing in Giant Petrels (Macronectes giganteus and M. halli) at Bird Island, South Georgia, and a Summary of Hybridization in Seabirds. Plos One 10.