Bicknell's thrush

Bicknell's thrush is just slightly smaller than the other northern migratory Catharus thrushes, with an average length of approximately and a weight ranging generally from 26 to 30g. Both sexes are identical in the field and are roughly the same size, although males average slightly larger in wing length. Adults are olive-brown to brownish on the upperparts (head, nape, back) contrasting with chestnut-tinged tail. The contrast is, however, less evident in worn plumage. The underparts are off-white with gray on the flanks; the breast is off-white with buffy wash, showing dusky spots that becomes more diffuse toward the sides and the lower breast. They have pink legs, a faint grey eye ring, and gray cheeks. Two-thirds of the lower mandible is yellow colored, while the tip of the lower mandible and upper mandible is blackish. They average slightly smaller than the very similar gray-cheeked thrush but are all but indistinguishable in outward appearance. The song is a jumbled series of flute-like tones ending on a higher note. == Taxonomy ==

A member of the family Turdidae, Bicknell's thrush belongs to the genus Catharus. This genus includes twelve species, five of which are found in North America. The closest relative of the Bicknell's thrush is its sister species, the gray-cheeked thrush. Together, the gray-cheeked and Bicknell's thrush form a cryptic species pair, and were indeed formerly considered conspecific. However, DNA analysis showed a divergence between the two species about 1 million years ago. ==Habitat and range==

Bicknell's thrush scattered breeding range extends from southeastern Quebec to Nova Scotia in Canada and the sky islands of northern New England and the Adirondacks and Catskills in New York. It is the rarest and most secretive of the breeding thrushes in North America, and it is the only bird species whose breeding range is entirely restricted to the northeastern part of the continent. It is a habitat specialist in its breeding range. It is known to favor high altitude coniferous forests affected by strong winds and heavy ice conditions. They usually breed at higher elevations, normally nesting above . However, they do not only live in this habitat, they also inhabit successional forests that have recently been affected by the forest industry. During the winter, they live in broadleaf forests at various elevations but generally preferring higher elevations. Wintering individuals have been recorded from the mountain ranges of the Cordillera Central, Sierra de Baoruco, and Cordillera Septentrional in the Dominican Republic, the Massif de la Hotte in Haiti, the Cordillera Central in Puerto Rico, the Blue Mountains in Jamaica, and the Sierra Maestra in eastern Cuba. ==Behavior==

Vocalization Vocalizing mostly at dawn and dusk, the song is mostly performed by the male, but sometimes also by females. Other calls include a growl call given in alarm situations "crr-rr-rr" and a flight call "cree-e-e". Breeding Bicknell's thrushes have an unusual mating system in which females mate with more than one male. Such a practice, known as polyandry, is not known to occur in other thrushes. As many as four males perform duties connected with one nest, including bringing food for the nestlings. It is possible that females decide to mate with more males when preys are less abundant. Pair formation is thought to occur after female arrival to breeding site in late May. and continue to do so during migration, and on the wintering grounds. They forage on both insects and wild fruits in winter. They usually forage on the forest floor, but also catch flies, and glean insects from the foliage of trees. Individuals eating fruits on wintering ground forage higher in the tree than those eating insects. Foraging technique varies but Bicknell's thrush mostly searches by pausing and peering, marked by hops and short flights. Sometimes food scratch on the ground, especially in wintering range. ==Conservation and threats==

Population monitoring Mountain Birdwatch data suggest that Bicknell's thrush populations have declined by an average of -3.88% per year (95% Bayesian credible interval: -5.27% to -2.52%) in northern New England and northeastern New York since 2010. The steepest declines (>60% decline between 2010 and 2022, 95% Bayesian credible interval: -75.31% to -33.33%, ) were observed in the Catskill Mountains in New York, USA, the southernmost portion of the Bicknell's thrush global breeding range. At that rate, it is possible that Bicknell's thrush may be extirpated entirely from the Catskills by 2050, with an estimated population loss of >95%. The Catskills, however, likely harbor less than 5% of the U.S. Bicknell's thrush population. The species has already been extirpated from Massachusetts, with the last record there being on Mount Greylock. Furthermore, recreational development, telecommunication towers and windmills increase is a major cause of habitat fragmentation and deterioration. Industrial forestry practices, although possibly harmful, may be modified to aid in conservation efforts to protect Bicknell's thrush. While more study is needed, the bird's apparent acceptance of certain commercial second-growth forest gives promise to possibilities of man-made "growing" Bicknell's thrush habitats in the future. Non-native species are potential risks throughout the thrush's range; introduced moose on Cape Breton Island consume the saplings of the tree species birds breed on, and invasive brown and black rats in the Caribbean may pose a potential threat. Mercury level increases greatly with altitude and its concentration bioaccumulate in the food web, probably explaining why it decreases as breeding season advances and birds begin to feed more on fruits. Near ski trails, maintaining vegetation on edges in a gradual slope and keeping large forested "islands" between leaves better habitat for Bicknell's thrush. Limiting vegetation management outside of the breeding season near mountain tops is another way to diminish disturbance. In areas where infrastructures are built in favorable Bicknell's thrush habitat, restoration of non-permanent modifications in the environment by reforesting is recommended. Signs or barriers along trails leading to the infrastructure to minimize disturbance is another suggested measure. As regenerating forests are known to be a good habitat for this species, assuring a continuous forest regeneration after clear-cuts benefits Bicknell's thrush. On winter range, reforestation of agricultural lands is proposed way to protect this declining thrush. ==References==