Good question, Neal in the “Midwest,” per your question in the “comments” section of the 8/7/09 article I authored here.
Wood-warbler migration actually begins in the summer by July or August for many species, if you also add in their initial travel in various directions away from breeding grounds (i.e., a behavior that is called “dispersal”). That is to say, unlike the more direct, dawdle-free behavior wood-warblers exhibit as they travel north on favorable tail-winds during spring migration, their post-breeding migration south is often a stuttering, stop-and-start, protracted itinerary.
Weather patterns in August through early October are a principal factor as to when many bird species (including wood-warblers) initiate migration and, subsequently, how far they travel on each segment of their southward night-time journeys. With north and northwest winds at their backs, wood-warblers may travel as many as 100 to 150 miles during a night’s journey.
Then again, feeding layovers for one or more days may occur where wood-warblers find abundant food resources. Likewise, binge eating sessions may be required when their body fat reserves deplete. Excess rain and unfavorable wind conditions also force wood-warblers into holding patterns until conditions improve.
Given the background information mentioned above, note it’s merely a brief and general summary of autumn bird migration patterns. More exact, consider that post-breeding season migration is an obligatory behavior most wood-warblers perform following dispersal from their breeding grounds.
Greeting the season’s initial freezing low temperatures, wood-warblers rudely discover their primary food source — insects, during the breeding season — rapidly disappear from the landscape. Consequently, migration emerges as an imperative survival behavior.
How many warblers migrate? For the majority of North America’s (north of Mexico) 52 annually present warbler species, their journeys begin by mid- to late summer (at the earliest) and no later than early to late autumn. So, for example, “early birds” such Yellow Warbler may begin dispersing/migrating in July, while other populations of the same species may wait until early October to leave for southern latitudes where they’ll spend the non-breeding season.
Other late summer, early-to-leave migrants include Tennessee Warbler, American Redstart, and Louisiana Waterthrush, all of which begin dispersing from northern breeding territories in July. True, uninterrupted migration toward “winter” habitat may not occur for these species (and many other North American wood-warblers) until beyond July and as late as October (and, more rarely, November and December) for some populations.
Only a handful of warbler species either do not migrate or remain annually (or periodically) throughout the winter in higher latitudes as far north as southern Wisconsin, Ohio, New York, and some New England states. This select club of bravehearts includes at least one subspecies of the Common Yellowthroat (among its 13 subspecies that live in North America), Yellow-Rumped Warbler, Palm Warbler, Orange-Crowned Warbler, and Yellow-Breasted Chat.
Which wood-warbler is the earliest to leave its breeding grounds? A leading contender is a subspecies of Orange-Crowned Warbler populations that breeds on the West Coast. Where it breeds in the San Francisco Bay Area, for example, Orange-Crowned often leave its dry and dormant environs by June, retreating to foothill and Sierra Madre mountainous habitat as intermediary “staging ground” habitats where cooler and moister conditions dominate and, thus, host abundant insect food resources.
For more details about the typical annual dates when each North American breeding wood-warbler disperses and/or migrates, see Jon Dunn and Kimball Garrett’s “Warbler” field guide (A list of the best warbler field guides and resources appears in a 10/21/08 article on this blog, so please feel free to scroll down the page and click multiple times on "older posts" to find it.).
More simplified information relating to autumn migration times for most USA-breeding wood-warbler species appears in a “Warbler Tips ID Chart” at my Web site: www.warblerwatch.com (After arriving at this site’s home page, click on the button title “Warbler Tips ID Chart.”)
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