Black-throated Green, Townsend's, and Hermit Warblers in Mexico
Three closely related species of warbler: black-throated green, Townsend's, and hermit, have largely separate ranges during the breeding season in North America, but winter together in the tropics.
Scientists studied the birds on their wintering grounds in southeastern Mexico and found that they distributed themselves both by elevation and type of tree they foraged in.
The black-throated green warbler was found at low elevations, the hermit warbler in mid elevations, and the Townsend's warbler at the highest elevations. The hermit warbler ranged across the greatest elevation range and was often found in flocks with the other species. Black-throated green and Townsend's warblers were rarely found together.
The hermit warbler avoided competition with the other species by foraging primarily in pine trees, whereas Townsend's preferred oaks and the black-throated green foraged without regard to the type of tree. Black-throated greens foraged more often at the tops of trees and at the tips of branches and were more active feeders.
So, how did this distribution come to be? It is thought that the black-throated green warblers were the first to evolve of the three and occupied the most productive winter habitat. During a period of time between ice ages, they colonized the northern Rocky Mountains, evolved into the Townsend's warbler, and occupied the next most productive winter habitat, upslope from the black-throated greens. The hermit was the last to evolve, from the Townsend's, and occupied the least productive habitat, the pine trees in between the Townsend's and black-throated green.
This article summarizes the information in this scientific paper:
Greenberg, Russell S. and Gonzales, C. E. 2001. Non-breeding ecology differences between species in the Black-throated Green Warbler complex. The Condor, 103: 31-37.