For the Birds Radio Program: Blackpoll Warbler
Laura talks about a warbler that looks entirely different in fall than in spring.
One of the most intriguing bird migration stories is that of the Blackpoll Warbler. These are little birds, but their weights fluctuate wildly, anywhere from one-third to three-quarters of an ounce. The normal day-to-day weight through most of the year is 11 grams, but before migration they lay on another full 10 grams of pure fat to burn on their long flight. It’s easy to get accurate weights of migrating Blackpolls because so many die at lighthouses and radio and TV towers—all ornithologists have to do during spring and fall is run out at the crack of dawn after the first foggy night and check out the base of the tower nearest them, and they can usually find several. Sadly, it’s their vulnerability to lights at night that makes them so easily studied during migration. They summer so far north that only a few ornithologists ever get to study their breeding habits.
Although all warblers deposit fat before migrating, few need as much as Blackpolls. Warblers fly long distances at night, but when dawn breaks they come down for the day to eat and rest. Unlike their relatives, many Blackpolls actually strike out over the Atlantic Ocean from northeastern North America flying toward Bermuda, where the trade winds assist them in moving southwesterly toward the coast of South America. The exertion required for this 86-hour journey would be the equivalent of a human running 4 minute miles for 80 hours. Most warblers couldn’t survive without supplemental food somewhere, even with the extra fat, because they aren’t as fuel efficient as Blackpolls, which burn about 25 percent less fat per hour of flight than other warblers.
Blackpoll Warblers are equally efficient in their coloring. While most warblers require yellow, orange, green, or even red to make a splash, adult male Blackpolls in spring fashion the bare minimum of black and white into an elegantly handsome plumage. This time of year, though, Blackpolls are a horse of a different color. Females look pretty much the way they always do, which means nondescript, ranging from whitish to grayish to greenish to yellowish beneath, with a variable amount of streaking, and upperparts varying from gray to olive gray to olive green. Males go from dramatic black and white to looking dull olive like females. Oddly, the warbler they most resemble in fall is the Bay-breasted Warbler, which is very closely related although it looks nothing at all like the Blackpoll in breeding plumage. One absolutely certain way of distinguishing them in fall plumage is to look at the soles of the feet. They’re always yellow in Blackpolls, but never more than yellowish gray in Bay-breasts. The only trick with this method is getting the birds to show the bottoms of their feet. Their song is very distinctive, if you can hear frequencies at 8–10 kilohertz, which is as high as any warbler goes. Of course, most adults in their forties or above can’t hear well at that range—another reason these birds may be more common than sightings might indicate.
In spring and summer, Blackpolls glean for insects in the middle and upper levels of trees, though they nest only 2–6 feet off the ground. In fall, they spend most of their time in low branches, so although identifying them may make your head spin, at least they won’t give you a sore neck.