forages mainly among dead leaf clusters; another breeds only in jack pine forests of a certain age; yet another warbler with bark-like black-and-white streaking specializes in picking invertebrates off the trunks and limbs of trees. Whether we realize it consciously or not, these evolutionary adaptations help us to identify birds. That buzzy, insect-like call is likely to mean different sparrow species in a grassy field and a saltmarsh; the same goes for a trilled song heard in a swamp and a suburb. Without some awareness of these species' adaptations to ecological conditions, birding by ear and birding by GISS would be a lot more difficult.
Literature.org, TalkOrigins, Bartleby, or Project Gutenberg if you want to read it.
For other evolution-related reading on this blog, see my review of Pilgrim on the Great Bird Continent, an article on the evolution of wood warblers, the announcement of a new crossbill species in Idaho, an analysis of the evolution of waterfowl genitals, and a more recent post on new research into birds of the Galápagos.