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 publication:
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.
We studied the ecological distribution and foraging behavior of three species in the Black-throated Green Warbler (Dendroica virens) species group in the Eastern Highlands of Chiapas, Mexico. The three species, Black-throated Green, Hermit (D. occidentalis) and Townsends (D. townsendi) Warblers are largely allopatric on the breeding grounds and variably sympatric during the winter. We surveyed the composition of 166 mixed-species flocks and recorded habitat variables associated with the flock location along an elevational transect. We found that Black-throated Green and Townsends Warblers segregated by altitude,whereas Hermit Warbler overlapped extensively with both other species. Even though Townsends and Hermit Warblers co-occurred commonly in the same flocks, the latter showed strong selective use of pine trees and the former showed weaker selectivity of oak trees. Black-throated Green Warbler showed no tree-type selection. The combined differences in elevational distribution and tree-type use resulted in very low ecological overlap for the three species. The Black-throated Green Warbler was the most distinct in its foraging behavior, occurring more often in the outer and upper branches of trees, and using hanging, aerial, and hovering maneuvers more than its congeners. In addition, it showed a higher movement frequency and a lower rate of between-branch flights. The Black-throated Green Warbler also is the most morphologically distinct as a result of its overall smaller body size, which is consistent for the trend towards larger body size in more conifer-specialized species in this genus. We propose that as new breeding populations were founded, and speciation occurred, individuals of the more derived species occupied decreasingly productive winter habitats.
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