Warbler Guy: Do any wood-warblers use cavities for nesting?

Lynnette in Elm Grove, WI asks an interesting question.

Jeanne Marie Acceturo is our guest writer who has ALL the answers for you:

One hundred fourteen wood-warbler species nest in the Americas and Bahama Islands. But only two of them use cavities for nesting: the Prothonotary Warbler (Protonotaria citrea) (PRWA, bottom photo) and Lucy's Warbler (Vermivora luciae) (LUWA, top photo).

Newly returning males of both species will soon arrive after migrating. They’ll define and defend territories before females arrive a week or two later (in April for the bulk of most breeding sites in the United States).

Males do all the wooing. Females make all the choices. Her big decisions: Which male has excellent taste in real estate? What constitutes a million dollar listing in the wood-warbler world?

Unlike PRWA and LUWA, other wood-warbler species prefer nesting sites within dense shrubs, atop a tree branch, or in a nook under a tangle of ground cover. Our two cavity nesting wood-warblers prefer otherwise, instead often utilizing dead snags or live trees where standing water provides a moat directly below the nest site.

Last year’s Downy Woodpecker hole? To a PRWA, it’s the perfect choice. Bald cypress, willow and sweet gum are the typical host trees. Male PRWAs make a platform of moss inside each suitable cavity in their territory. The females construct the nest and include mosses and liverworts.

Why? Perhaps it’s because damp nesting material increases humidity inside the cavity so that eggs are less likely to dry out.

But that still doesn’t explain why PRWAs use cavities. The mystery remains. One theory relates to the added protection obtained from living inside a limb hidden from predators such as marauding rat snakes.

In an all-too-familiar story, however, much of their preferred bottomland hardwood forest is disappearing. People have destroyed 90 percent of this habitat for logging, agriculture and other development. Fortunately, there’s good news: PRWAs sometimes use nest boxes when they’re erected in suitable locations.

Lucy's Warbler, the West Coast first cousin of the more East Coast based PRWA, also nests near water. But where’s a LUWA likely to find water within the lower Sonoran desert habitat that it favors? Not too many places. So LUWA changes its game plan: old Gila Woodpecker holes or abandoned Verdin nests sometimes become Home Sweet Home. So do crevices behind loose tree bark, natural cavities, or, less commonly, spaces among large networks of loosely tangled roots in riverbanks.

Populations of LUWAs have dropped for an obvious reason: the Southwest has been developed intensely. Unfortunately, LUWA rejects nest boxes, but where LUWA’s natural habitat of mesquite forest has been lost to logging and water diversion, introduced (and invasive) tamarisk trees pinch hit as substitute nesting sites.

Are either PRWA or LUWA listed as threatened or endangered? Not yet, but conservation of these two species’ remaining populations presents a cautionarytale — one that depends on habitat conservation as a rule and not an option.

Lucy's Warbler Photo: © Dominic Sherony; Prothonotary Warbler Photo: © Mdf

Warbler Guy, Help Me?: I’m in Florida and wondering if the photos here are western or eastern subspecies of Palm Warbler?

Thanks for the question, Mr. King. (Both photos courtesy of an excellent birder, Jim McGinity.)

Your photos are both “Western” Palm Warbler, despite your East Coast location.

That’s because during the “winter,” it’s possible to see both the subspecies that occur within the Palm Warbler species.

More details follow:

Telling both subspecies apart in the winter/non-breeding season may and can be challenging. Underline the last sentence (!)

To wit: Although you can see both subspecies during the non-breeding season/winter in Florida, the pale “Western” subspecies (Dendroica palmarum palmarum) often adds a touch more of yellow under its belly during the fall (through winter).
For this reason, some population members of “Western” may look like the “Yellow” (East Coast” subspecies, D. p. chrysolepis. The latter (the “Yellow” East Coast version) is much brighter during the breeding season than the “Western” Palm Warbler subspecies.

Got all that?

If not, here's a MAJOR tip.....

It’s an excellent field mark that helps you tell the two subspecies apart:
the yellow eye-ring that’s shown in the “Yellow” Palm Warbler (D. p. chrysolepis).
In contrast, the “Western” Palm has a WHITE eye-ring.

Even more details about the two subspecies yellow color follows:
“Western” yellow has a contrasting pattern where the yellow ends and the rump is a duller green (but more yellow in the “Yellow” East Coast subspecies (D. p. chrysolepis).

Note there are intergrades and, indeed, a potential breeding area in Canada where both subspecies may successfully mate with each other and have viable offspring.

Despite all the details I’ve already present, this account oversimplifies how to identify both subspecies from one another. Excellent drawings of both subspecies appear in “A Field Guide to the Warblers of North America” (Jon Dunn and Kimball Garrett) (plate #20, page 82). More details about how to tell them apart are described on p. 366 in this book.

The plates are “Must See” viewing for warbler fans like you, Mr. King, but I hope the account above answers your question.

If not, please ask me more questions.

That’s why I’m here as “The Warbler Guy.”

Population Explosion?: Kirtland's Warbler Responds To Wildlife/Habitat Management

See the current quiz on the right side?

Now look at the above graphic that shows the annual population changes of Kirtland's Warbler in Michigan during the last 20 years.

Hmmmm.......Do you think the current majority of folks who answered the quiz are correct? Incorrect?

Truth be told, the Michigan DNR's habitat management plan is a huge victory for the federally endangered Kirtland's.
Their populations in 12 central-northcentral Michigan counties continue to thrive. Prescribed fires have helped create optimum conditions in their Jack Pine community whereby managers have helped Kirtland's rebound from the brink of extinction in the late 1980s.

For more information, see:

http://www.michigan.gov/dnr/0,1607,7-153-10370_12145_12202-32591--,00.html#quickfacts

Of course, Wisconsin breeding populations of Kirtland's are also big news. During the past three breeding seasons, Wisconsin has hosted newborns in at least two counties. For more details about the discovery of Kirtland's Warbler in Wisconsin and an overview of their breeding abundance, see one or more of my June and July, 2009 updates/articles in the archives section of this wood-warbler blog.