Blackpoll warbler

Migrating adult in a fruiting flowering dogwood tree

The blackpoll has a deliberate feeding style with occasional flitting, hovering, and hawking around branches. They are primarily insectivorous. The species appears to be quite a generalist, preying on a great diversity of adult and larval insects and spiders. Documented insect prey for the species includes lice, locusts, cankerworms, mosquitoes, webworms, ants, termites, gnats, aphids, and sawflies. It has been suggested that this species may be a spruce budworm specialist, but there is no obvious connection between the population trends of the two species.[6] The blackpoll will opt for berries during migration and in winter. They often forage high in trees, and sometimes catch insects while in flight.

Their breeding habitats are coniferous woodlands, especially those in which spruce trees grow. The bird's breeding ranges extend to the taiga. Blackpoll warblers commonly nest in a relatively low site of a conifer. They lay 3–5 eggs in a cup-shaped nest, rarely up to 9. The eggs are incubated for around 12 days and the young leave the nest when they are only 10 days old before they can fly well. Their parents feed them for a total of around two weeks. Mated females usually begin second nests right away and leave post-fledging parental duties to their mates. The high incidence of double brooding, coupled with and partly a function of low nest predation and parasitism rates, results in high annual productivity for this species.[7]

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.[8] 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 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.[3]^: 66 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.[9] Part of the fall migratory route of the blackpoll warbler is over the Atlantic Ocean from the northeastern United States to the Greater and Lesser Antilles or northern South America. Island stopovers at Bermuda and other places are evidence of migratory pathways.[10] 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.[3]^: 72

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 the 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).[11]