An Enigmatic Warbler

“Wee, wee, wee, wee, bzzz” sings my favorite yard bird.  For two months most years — mid-May to mid-July — the male cerulean warbler sings his monotonous song from dawn until dusk. The first year this happened, back in 2002, I worried that he hadn’t found a mate.  Why else would he sing on and on like some demented suitor?

Then I did a little research and learned that on their breeding grounds territorial male cerulean warblers are “vigorous and persistent singers, usually singing from the highest available forage,” according to Jon Dunn and Kimball Garrett in Warblers.

Other researchers reported that ceruleans often have favorite song perches in trees that leaf out late such as bitternut hickories, black walnuts and black locusts.  They hypothesize that the leafless trees allow the birds to broadcast their songs with little “acoustic hindrance” until late in the breeding season. And, in the case of our yard trees — black locusts and black walnuts — when they do leaf out their foliage is relatively thin.

Despite their glorious blue heads and backs, their white wing bars and bluish-black chest bands that stand out against their white throats, breasts, and bellies, male cerulean warblers can be difficult to spot high in the treetops.  And I can’t ever remember seeing the greenish-blue females with pale yellow underparts, which is why I thought the singing cerulean was an unrequited lover.

“A bird more difficult to observe I have rarely ever met with,” wrote a frustrated observer in 1919.  “Had it not been for the almost incessant singing, being heard almost constantly from daybreak until nearly dark, the task of identification would have seemed hopeless.”

Yet, in my case, seeing was believing, because for years I didn’t think we had breeding cerulean warblers.  In the 1970s and 80s cerulean breeding habitat was thought to be exclusively lowland open forests near streams or in old growth bottomland forests. Then one June day I saw a cerulean warbler in a tree beside the Far Field Road, and I wondered if they could be breeding on our dry mountaintop.

As if in answer to my question, during the first Pennsylvania Breeding Bird Atlas Project, my son Mark confirmed breeding ceruleans on our dry mountaintop in 1986 despite the Project’s Handbook, which described cerulean habitat as “mature moist or riverside forests.”

Even since, I have found singing ceruleans somewhere on our mountain every spring and summer.  So far, the earliest return date I have recorded for them is April 30 and the latest singing date July 21.  They have sung above Greenbrier and Ten Springs trails in this century, a decade after a previous owner poorly logged that portion of the property.  But he did leave bitternut hickories, a few large oaks, and several tulip trees, which may have lured ceruleans.

Other ceruleans have sung at the upper edges of both the First and Far Fields, along the powerline right-of-way, in the Far Field thicket, beside the deer exclosure, in tall trees behind our old garden site, in the black cherry forest near the spruce grove, and along Laurel Ridge Trail — all dry ridgetop sites and all in edge habitat. I have not found them along our stream in our 100-year-old deciduous forest.

Luckily, Paul B. Hamel published an updated account of cerulean warblers, based on more recent research, in The Birds of North America in 2000.  In it, he wrote that ceruleans also breed in upland deciduous second-growth as well as mature forests at elevations up to 3,000 feet. But his 19-page article had many life history gaps, and he admitted that the cerulean warbler “has been little studied.”

Because they nest high in large trees such as oaks, they are difficult to observe.  What researchers do know is that despite expanding their breeding range into the Northeast from the Mississippi Alluvial Valley and Cumberland Plateau of eastern Tennessee, eastern Kentucky, and southern West Virginia, they are one of the fastest declining songbirds (70% in 40 years) in North America.

In the same year that Hamel’s account was published, ornithologists Kenneth V. Rosenberg, Sara E. Barker, and Ronald W. Rohrbaugh of the Cornell Laboratory of Ornithology submitted An Atlas of Cerulean Warbler Populations: Final Report to USFWS: 1997-2000 Breeding Seasons [PDF]. Two hundred field persons, both volunteers and professionals, collected information on breeding ceruleans and the habitats and dominant tree species they preferred.  They canoed navigable waterways, drove along rural roads, hiked portions of the Appalachian Trail, and drove and hiked through forests and isolated woodlots from Illinois to Missouri, New Jersey to New England, eastern Tennessee to Ontario, Canada.

They located 7,669 cerulean warblers at 1,923 sites in 28 states and Ontario.  Not surprisingly, almost all the ceruleans were singing males.  They also searched 355 likely sites where they didn’t find any ceruleans.  Some states had many more volunteers than others. Unfortunately, Kentucky, which is thought to be a major breeding site in its eastern section, had very few volunteers.  But Tennessee, another important breeding area, reported the most ceruleans (1210), followed by West Virginia (1124), New York (1068), Illinois (1000), and Pennsylvania (548).

I was particularly interested in the Pennsylvania findings.  After all, Philadelphia-based bird artist, Alexander Wilson, first named and then painted the male cerulean warbler (Dendroica cerulea) after it was discovered in eastern Pennsylvania, and Titian Peale, another Philadelphia artist, painted the first female cerulean, which had been taken along the banks of the Schuylkill River in 1825.

It turns out that eastern Pennsylvania is still a hotbed of ceruleans in the Delaware River Valley on both the Pennsylvania and New Jersey riverbanks with 90 birds. This was followed by the Jennings Environmental Center and Moraine State Park in western Pennsylvania (69), the Juniata River and vicinity (42) and Peter’s Mountain and State Game Lands #211 (29), both in central Pennsylvania.  The habitats included dry slopes, riparian and lake margins.

Despite the 90 in the Delaware River Valley and the 71 in central Pennsylvania, ceruleans have historically been most numerous in southwestern Pennsylvania. Intensive surveys in that area and adjacent West Virginia found 1,400 ceruleans in what the report called “the heart of the species’ range.”  Almost as many ceruleans were found on dry slopes or ridgetop sites as in riparian or other bottomland habitats.

I was especially intrigued by the Juniata River and vicinity number in Huntington and Blair counties, since our Little Juniata River that we cross at the bottom of our mountain whenever we go out is a tributary of the Juniata River and our home is in northern Blair County. I also noted that the favorite nesting trees in dry upland sites were white and red oaks, black cherry, and maples, all of which we have as 100-year-old or older trees on our property.

In addition, ceruleans seem to prefer a tall, but broken tree canopy and large wooded tracts of at least 50 to 75 acres, but 1,300 acres is considered optimal.  Still, while those ceruleans in the Southeast use large forest tracts, those in the Northeast often breed in much smaller forests.  And in eastern Ontario maple forests of 25 acres are adequate.  So cerulean warblers may be more adaptable than previously thought.

On the other hand, their numbers keep falling at the rate of 4% a year.  Habitat loss, both on their breeding and wintering grounds, seems to be the major reason for their steady decline. Here in North America on their breeding grounds in eastern Kentucky and southwestern Virginia, mountaintop removal to mine for coal is permanently destroying mountaintop forests where ceruleans breed.

Forestry practices, as they relate to cerulean warbler habitat, have also been studied in southern Indiana by Sarah M. Register and Kamal Islam, and they concluded that “cerulean warbler habitat needs appear to be supported by 20-30 year cutting cycles combined with uneven-age management and timber stand improvement practices.”  Furthermore, clear-cutting results “in immediate habitat loss for cerulean warblers and other interior forest dwelling birds that may take years to regenerate.”

On their wintering grounds in the subtropical forests of the Andean valleys in Columbia, Venezuela, Ecuador, and Peru, shade coffee plantations that provide excellent habitat for ceruleans are being replaced by sun coffee farms which are more profitable.  Deforestation in those countries is also due to cacao and tea plantations as well as to the illegal coca trade.

In summary, Hamel says that “Land-use changes brought about by increasing populations in the breeding, migratory, and winter ranges of this species appear to be the underlying cause of the population decline of the bird…”

Here in Pennsylvania the current move to put industrial wind farms on our dry, forested mountaintops will lead to fragmentation of many of our best remaining wild areas, especially in the ridge-and-valley province.  This will not only affect cerulean warblers but many other forest-interior nesting birds.  What a pity it would be to lose what researchers in eastern Ontario call the “enigmatic Cerulean Warbler.”

Those same researchers managed to find and observe 201 cerulean nests in a second growth, 80 to 90-year-old deciduous forest of mostly sugar maple, bitternut hickory, and ash trees from 1996 to 2001.  They discovered that cerulean nest-building, by the females, takes four to seven days, egg-laying seven days, and incubation 10 to 12 days.  While the females do all the incubating and brooding, the males and females feed the three to four nestlings.  Their major predator was the blue jay.  Other studies mention cowbird parasitism as a threat, but in Ontario, despite high cowbird numbers, the researchers never observed ceruleans feeding cowbird nestlings or fledglings.  The couple of nests that had cowbird eggs were abandoned by ceruleans.

Cerulean nests are usually constructed of bark fiber, fine grass stems, weed stalks, and fine hairs.  They decorate the outside of their nests with gray or white material, such as gray shreds of bark or spider webs.  Cerulean females anchor their nests on horizontal deciduous tree limbs 30 feet or higher from the ground beneath clumps of leaves.  Researchers both in Ontario and the Mississippi Alluvial Valley have banded ceruleans and have had banded birds return to the same breeding area at least two years in a row.

That convinces me that the particularly vocal cerulean warbler who sang in our yard in 2002 and 2003 was the same bird.