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