Origins and Evolution of Bird Migration: A Critique of Winger et al.

The most widely held theory for the origin of bird migration states that temperate zone resident birds evolved over thousands of years the necessary adaptations to allow successful movement in winter to less seasonal regions (e.g., the tropics). A problem for this theory is the fact nearly half of species that migrate to the tropics have breeding populations there as well, indicative of tropical, rather than temperate, origin. Winger et al. (2014a,b) have proposed a mechanism explaining how temperate zone species might have invaded the tropics as migrants and remained as resident breeders ("Migration Drop-Off"). The methods used to reach these conclusions involve no ecological arguments; rather they use extant species' ranges along with phylogenetic data to construct a model for inferring the biogeographic history of migratory lineages. The fundamental error in Winger et al. lies in the assumption that a species’ range is a conservative genetic attribute evolved over millennia. Data from field studies and the crowd-source site, eBird, demonstrate that this assumption is incorrect. Ranges for humdreds of bird species have changed by hundreds of kilometers in a matter of years, evidently as a result of dispersal and changes in seasonal food availability (Rappole et al. 2011, Rappole 2013: Chapter 8). In addition, breeding, wintering, and residency ranges have appeared, disappeared, and/or fundamentally shifted for tens of thousands of years during the past 2.58 million years of the Quaternary for hundreds of species. These facts taken together with the ecological issues involved with a species entering a complex ecosystem and creating a niche for itself argue against a "migration drop-off" explanation. Dispersal provides a more likely hypothesis for the origins of migration.