Bicknell's thrush

Vocalizing mostly at dawn and dusk, the song is mostly performed by the male, but sometimes also by females.[7] Like all North American thrushes, Bicknell's thrush song is flute-like. High pitched and vibrant, the song is composed of four phrases: “chook-chook, wee-o, wee-o, wee-o-ti-t-ter-ee”.[7] Unlike in the gray-cheeked thrush, the pitch of the last phrase is constant or rising.

The primary call is a downward whistle named the “Beer call” “beer”.[13] Other calls include growl call given in alarm situations “crr-rr-rr” and flight call “cree-e-e”

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.[14] 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.[15] Pair formation is thought to occur after female arrival to breeding site in late May.[8] The nest itself is typically a bulky cup made of twigs and moss, close to the trunk of a conifer at base of horizontal branches, usually 2 meters above ground.[7] The female builds the nest alone and lay three or four eggs per clutch. While the incubation period is about two weeks, nestlings, fed by both parents, grow rapidly, developing in 12 days from peanut-sized hatchlings to completely feathered adult-sized birds.

Ticks, blowflies, and lice are some of the parasites with which Bicknell's thrush must contend. The American red squirrel is the main predator of eggs and nestlings, according to breeding ecology.[14] Predators confirmed to hunt nesting adults have consisted of the sharp-shinned hawk, the long-tailed weasel, and the northern saw-whet owl. On the nesting grounds there are at least six other predators suspected or likely to depredate nests and at least three other predators that may attacks nesting adults, to say nothing of potential predators during migration or on wintering grounds.[4][7]

The thrush's diet consists mainly of insects, beetles and ants being the main preys. They also start eating wild fruits in late summer (mid-July),[16] and continue to do so during migration, and on the wintering grounds. They forage on both insects and wild fruits in winter.[17] 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.[18] 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.

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.[19] 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.[20] The species has already been extirpated from Massachusetts, with the last record there being on Mount Greylock.[2] The U.S. population of Bicknell's thrush is likely around 70,000, and the Canadian population estimated between 40,570 and 49,568,[1] with a global population of less than 120,000. The species was considered a "Special Concern" species by Committee on the Status of Endangered Wildlife in Canada in April 1999. This was later re-examined to consider C. bicknelli "Threatened" in November 2009, and was re-assessed again as such in December 2022.[2]

Its numbers are declining in some parts of its already limited range as a result of habitat degradation. Scientists believe that industrial pollution is one of the main reasons for the decline of the red spruce, an important element in Bicknell's thrush habitat in the United States. Airborne heavy metals may also damage high-elevation forests in the northeastern United States. Furthermore, based on expected substantial carbon dioxide increases by the end of the century, scientists predict a radical reduction of balsam fir forest in the eastern United States and Canada. If average global temperatures increase and forests change as much as predicted, Bicknell's thrush habitat is very likely to be altered in ways that may seriously affect the species' survival. Indeed, models predict that Bicknell's thrush will lose more than 50% of its breeding habitat over the next 30 years.[21] 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.[2] With climate change shifting temperature up elevation on mountains, Bicknell's thrush may suffer from asymmetrical and aggressive competition from the Swainson's thrush (Catharus swainsoni).[1]

Bicknell's thrush has a higher concentration of mercury in its blood than any lower-elevation Catharus thrush.[22] 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.[22] High mercury concentrations may cause reproductive impairment.[22]

There is also considerable concern about the degradation of Bicknell's thrush's wintering habitats. The Dominican Republic's native forests are under considerable pressure from naturally occurring events such as hurricanes, as well as changes from agricultural activities, particularly at low altitudes. The forested lands of Haiti have been almost eliminated, and in Cuba, most of the known suitable habitat exists only in protected parklands.[14]

Few management actions are known to be in place, even though management procedures for the species’ conservation have been established by the International Bicknell's Thrush Conservation Group (IBTCG).[23] 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.[24] 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.[25] On winter range, reforestation of agricultural lands is proposed way to protect this declining thrush.[26]