Warbler Guy, do you know when the warblers begin their autumn migration?

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.”)

Can you please share the answers to your latest quizzes?

Yes, Jeremy S. (Tucson, AZ). Here’s the answers:

1. Can you name the two hybrid forms that sometimes result when Golden-Winged and Blue-Winged Warbler mate?

Answer:

Lawrence’s and Brewster’s

2. Can you name two wood-warbler species that are breeding endemics to one USA state?

Tropical Parula and Golden-Cheeked

3. On the West Coast, which wood-warbler is one of the earliest dispersing species after nesting?

Answer:

Orange-Crowned Warbler, given many disperse and/or migrate from the Bay Area by June.

4. Among the species listed in the 5/29/09 article, which one is considered extinct?

Answer:

Bachman’s Warbler

5. Common Yellowthroat was often previously named differently in many field guides as XXXXXX Yellowthroat?

Answer:

Maryland Yellowthroat

6. Beyond Michigan's breeding population, where else do researchers believe the Kirtland's Warbler regularly to periodically breeds?

Answer:

Wisconsin (as only one documented nest has been found in Ontario during the 1945 breeding season), while Kirtland’s has bred from 2007 through 2009 in Wisconsin.

7. Which common wood-warbler's breeding range is split into an eastern and western subspecies breeding population?

Answer:

Nashville Warbler, as its two USA subspecies are split into separated (allopatric) populations while nine Yellow Warbler species occur in the USA among the 43 total subspecies within North, Central and South America.

Warbler Guy, do you know why New World warblers are usually more colorful than Old World warblers?

(Townsend's Warbler, above)

Good question, Antoine (in Richmond, VA).

The answer is not obvious nor proven in scientific research.

My explanation relates to how birds evolved and new species became established throughout North America (i.e., a portion of the New World) over eons of time.

As a first step, think about the geographic spot or “epicenter” where our North America wood-warblers originated. That area is the Appalachian Mountain region that served as breeding areas to which tropical species of wood-warblers (such as the colorful Yellow and Black-Throated Green Warbler, to name just two of many species) began migrating and breeding within the distant past.

In turn, new, evolving wood-warbler species spread throughout North America as they colonized habitat when the glaciers melted within northern latitudes 12,000-18,000 years ago. Generation after generation of breeding isolation of these new, pioneering populations from one another allowed distinct appearances to flourish as new species.

So, for instance, “sister,” look-alike, colorful species such as the Black-Throated Green, Hermit, Townsend’s, Golden-Cheeked, and Black-Throated Gray Warbler are considered close first-cousin relatives – though all primarily breed in distinct geographical areas throughout North America that do not overlap.

In fact, experts consider these five warblers a “super species,” with the Black-Throated Green the initial precursor from which the other four look-alike species evolved and spread west and north throughout North America from their Appalachian origins.

As for the generally drab, uncolorful appearance of Old World warbler species (Sylviidae family) that primarily breed in Eurasia, their evolution occurred in a similar fashion to the aforementioned progression of our New World North American wood-warblers (the majority of which occur in the Parulidae family). That is to say, one or more drab, uncolorful original warbler species in the far past were the root species of the evolutionary tree from which other multiple species evolved and spread throughout Europe and Asia.

In short, it’s mere geographical luck that the USA’s location is within the latitudinal migration pathway that colorful neotropical/equatorial New World species chose as they evolved their migrational routes from the south in search of northern breeding territories.

Best Buys: Warbler Books

Which are two of the best wood-warbler books?

Here's two covers of publications I own and use often:

- Warblers, Jon Dunn & Kimball Garrett

- Warblers of the Americas: An Identification Guide, by David Quinn, David Beadle, Jon Curson