Blackpoll Warbler

The blackpoll warbler is a fairly small bird which attains the weight of a ball point pen. However, it is one of the larger of the diverse genus Setophaga (formerly Dendroica). In the species, body length can vary from 12.5 to 15 cm (4.9 to 5.9 in) and wingspan can range from 20 to 25 cm (7.9 to 9.8 in). Body mass can vary from 9.7 to 21 g (0.34 to 0.74 oz), with an average bird anywhere between 12 and 15 g (0.42 and 0.53 oz). Among standard measurements, the wing chord is 6.6 to 8 cm (2.6 to 3.1 in), the tail is 4.5 to 5.4 cm (1.8 to 2.1 in), the bill is 0.8 to 1.2 cm (0.31 to 0.47 in) and the tarsus is 1.8 to 2 cm (0.71 to 0.79 in). The summer male blackpoll warblers have dark-streaked brown backs, white faces and black crowns. Their underparts are white with black streaks, and they display two white wing bars. The adult females essentially resemble washed-out versions of the summer males, and in particular, the females lack the strong head patterns, and their crowns and faces are shades of gray. Another outstanding physical characteristic of the species are the bright orange, pink legs. Non-breeding birds of this species have greenish heads, dark-streaked greenish upperparts and yellowish breasts, with the yellow extending to the belly in young birds. Their wing bars are always present.

The blackpoll warbler's transoceanic flight has been the subject of over twenty-five scientific studies. Sources of data include radar observations, bird banding and weights taken, dead birds recovered from field sites and fatal obstacles. It is unknown if they feed on insects while in flight. Blackpoll warblers have the longest migration of any species of New World warbler. This is likely the reason that they are one of the later warblers to appear in spring migration, after one or more short overwater flights and a relatively prolonged movement overland after through North America anytime from early May to mid-June. The peak of their migration is in late May, when most warblers are on their breeding grounds. In the fall the birds migrate from their breeding grounds across the northern latitudes. They converge on the Northeastern United States south to Virginia starting in mid-August. Most blackpolls fly directly from northeastern North America over the Atlantic Ocean to their winter range. Data from nocturnal accidents, banding stations and sightings have shown that blackpolls are rare autumn migrants south of Cape Hatteras, North Carolina, whereas north of Cape Hatteras they are common. Part of the fall migratory route of the blackpoll warbler is over the Atlantic Ocean from the northeastern United States to Puerto Rico, the Lesser Antilles, or northern South America. Island stopovers at Bermuda and other places are evidence of migratory pathways. To accomplish this flight, the blackpoll warbler nearly doubles its body mass in staging areas and takes advantage of a shift in prevailing wind direction to direct it to its destination. When they fly southward over the Atlantic they burn, 0.08 g of fat every hour. This route averages 3,000 km (1,900 mi) over water, requiring a potentially nonstop flight of around 72 to 88 hours. They travel at a speed of about 27 mph (43 km/h). Blackpolls can weigh more than 20 g (0.71 oz) when they leave the United States and lose 4 or more grams by the time they reach South America. Some of the blackpolls land in Bermuda before going on. Some birds, often with lower body weights, do not make it. Using a tiny light level geolocator biologists have proven that the black poll flies an average of 2540 km (2270 to 2770 km) non-stop over an average of 62 h, up to 3 days, corresponding to about 41 km/h. In 2013, 37 blackpolls from Vermont and Nova Scotia carried a miniaturized geolocator weighing 0.5 g with harness on their back. The device recorded light-levels, from which longitudes and latitudes could be estimated, and in 2014 the scientists recovered five of the original 37. Four of the five birds departed from western Nova Scotia between September 25 and October 21, and traveled at speeds between 10.7 and 13.4 meters per second. The study revealed that the spring migration overland and the autumn routes overwater were "dramatically different". When the flight distance per body mass was compared to other birds, only the ruby-throated hummingbird might travel more kilometres per gram (estimated around 210–280 km/g vs. 233 km/g for blackpolls).