Black and White and Upside Down

By Bill Chaisson
Of a Feather
As I stand with my new toy, the Merlin app, holding up my phone to the sky just after dawn on successive mornings, I am sometimes surprised by the variety of calls and songs it detects. On Aug. 2, for example, it heard white-breasted nuthatch, common yellowthroat, indigo bunting, yellow-rumped warbler, black-capped chickadee, northern cardinal, red-winged blackbird, cedar waxwing, American robin, gray catbird, house wren, brown creeper, veery, hermit thrush, red-breasted nuthatch, northern parula, and black-and-white warbler.

This time of year, most of these species have ceased singing—on Aug. 2 the indigo bunting was a prominent exception, but now he too has stopped—and their presence is announced by one- or two-note calls that emerge from the undergrowth or filter down from the canopy. Sadly, several of these birds’ calls are getting to be near or beyond my range of hearing. So, they would go unnoticed were it not for my technological aid.

But I can still hear the black-and-white warbler (Mniotilta varia), one of the stranger species in the large and diverse family of Parulidae. Visually, it is a rather conservative bird. True to its common name, its plumage is composed entirely of alternating black and white stripes.

As birdsoftheworld.org notes, it is the only member of its genus, the name of which means “moss-plucking” and refers to its nuthatch-like method of foraging. In an impressive example of convergent evolution, this warbler has evolved the capability of crawling up, down and around trunks and branches nearly as adroitly as a nuthatch and more acrobatically than a creeper or a woodpecker. Its bill is large for a warbler and slightly curved, which allows it to probe in bark crevices for insects, although this does not prevent it from also foliage-gleaning in the manner associated with most members of its family.

Black-and-white warblers are common. They have a wide geographic distribution, are found in a variety of habitats, and forage everywhere from the forest floor to the top of the canopy. My local birds live along a woods road that is lined on one side by thick deciduous saplings beneath a narrow stand of mature deciduous trees in front of a grove of mature hemlocks. On the other side is an old pasture that is full of thick stands of birch saplings and a grove of mature deciduous trees. Through the summer I regularly heard their see-see-see song as I passed through there and saw them quite often, as they are not particularly shy as warblers go.

When we lived in Unity, our house was surrounded by lawns and pastures that were not far along in old-field succession, mostly full of goldenrod and milkweed. And yet a brood of half-sized fuzzy black-and-white warblers suddenly appeared one day in the lilacs at the head of the driveway, foraging nonchalantly as I stood watching about 5 feet away. I hadn’t seen any mature members of the species all spring or summer, but they had obviously bred somewhere nearby at the woodland’s edge.

And yet scientific studies of its habitat preference suggest it prefers extensive mature deciduous or mixed deciduous-coniferous woodlands over much of its broad range. Its distribution clearly shows its preference for forests. It is found from the Canadian Rockies to the Maritime provinces in the taiga (presumably in the deciduous parts) and northern hardwood assemblages, but also in a broad swath through the Appalachians and out into the woodlands of east Texas but avoiding the piney woods of the Deep South and the scrub of the Florida peninsula. It does not seem to have altered its range since the 19th century, as it is still found everywhere Audubon found it, but not beyond.

It is a fun bird to watch because it can crawl down a vertical tree trunk upside down. While it is fun to watch nuthatches do this, it is a novelty to see a warbler to it, like seeing a quarterback throw a baseball in the strike zone. The adaptations for this behavior include an unusually long hind toe and claw, a short tarsus (upper part of the leg) and a lengthened forelimb.

There is almost no difference between male and female plumage, or even between adult and juvenile plumage. The fledglings I saw in Unity were merely smaller, scruffier looking versions of their parents. The only other similar looking warbler in the Northeast is the male blackpoll, which has a solid black cap and white cheeks and much more white than black below. The blackpoll, however, is a northern species confined to spruce-fir forests, mostly from the White Mountains northward.

This time of year, the black-and-white warblers will leave New Hampshire for their wintering grounds, which extend through the Caribbean from the southern United States to northern South America. However, they are among the first warblers to return here in the spring, arriving in late April. This early arrival is possible because of their skill at extracting insect eggs and larvae from crevices. Other warblers need to wait for the leaves to emerge to pick insects off them.

Because it is a broadly occurring habitat, populations of this species are generally stable. However, it prefers continuous woodland and declines in abundance when the forest is fragmented by development. In New Hampshire, suburbanization has caused it to become less common in the southeastern part of the state. However, it has become more common north of the White Mountains because logging has allowed deciduous trees to spread into better drained lowland sites there.

M. varia nests on the ground, where it is vulnerable to predation by domestic animals. So, even a cleared acre or so in the middle of the woods for a house that is home to cats and dogs represents a potential impact on local populations of black-and-white warblers in ostensibly rural areas.

Bill Chaisson has been a birdwatcher for over 50 years. He lives and works in Wilmot.