Bicknell's Thrush

U.S. Fish & Wildlife Service Bicknell’s Thrush Catharus bicknelli

The Bicknell’s thrush is a migratory passerine (or songbird) that summers in the northeastern U.S. and southeast Canada. This relative of the robin is an extreme habitat specialist, nesting primarily in stunted montane spruce-fir forests, found at or near the highest elevations of mountains. Conservationists throughout the species’ range consider the Bicknell’s thrush to face a multitude of threats as one of the rarest birds in North America.

Because of its specialized habitat requirements, Bicknell’s thrush populations are extremely vulnerable. Monitoring data from the last 10 years indicates a stable population in the U.S. and steep Ryder Credit: T.B. declines in the Canadian population A banded Bicknell’s thrush. due to habitat loss, predation and pollution. Climate change may also northeastern U.S. and southeastern for mixed paternity brood. be a threat to the Bicknell’s thrush. Canada. By early November, most Bicknell’s thrushes have migrated This highly specific bird prefers Characteristics and established winter territories in to nest in montane fir and spruce The Bicknell’s thrush is a medium- the broadleaf forests of the Greater forests, usually in recently disturbed sized bird, about 6.3 to 6.7 inches Antilles, the group of Caribbean areas characterized by dense in length, and weighs about an islands comprising Cuba, Puerto understory, low canopy, and an ounce. It is characterized by its light Rico, Jamaica, and Hispaniola abundance of snags, shrubs, moss, olive-brown upper parts, white (and (which includes Haiti and the stumps and deadfall for desirable sometimes slightly yellow-tinted) Dominican Republic). Here they can shelter and nesting areas. Nests belly and spotted breast, with some be found throughout middle and are constructed with twigs and are chestnut coloration on its tail and high elevation sites. usually dense and cup shaped, with wings. Outside of breeding, males a lining of moss. Clutch sizes consist and females are only distinguishable By the end of May, both males and of around three to four bluish green by the male’s larger size. females have returned to their eggs with light brown speckling. breeding grounds in the northeast. The Bicknell’s thrush was once Males typically return several days believed to be a subspecies of the earlier than females, and breeding Food grey-cheeked thrush, which it begins upon the females’ arrival. On its high elevation summer closely resembles. However, due to range, the Bicknell’s thrush is differences between the thrushes in The Bicknell’s thrush begins primarily a ground forager, eating behavior, song, habitat, distribution, breeding at about one year of age mostly insects, from larval moths, morphology and genetics, and is known to have a highly butterflies, and ants to bees, cicadas, ornithologists determined in 1995 unusual mating system. The and spiders. They may also feed that Bicknell’s thrush is its own mating system, termed “female- on fruit, such as bunchberries, species. defense polyandry,” is unique to blueberries and wild grapes. Small the Bicknell’s thrush and one other fruits of similar size compose a great Life Cycle passerine in North America. Females proportion of this thrush’s winter The Bicknell’s thrush is a migratory mate with more than one male per and migration diet. Nesting females bird that breeds in subalpine forests, breeding season, ensuring multiple often eat snails, which are believed which are composed of stunted males for feeding and protection of to provide much-needed calcium for fir and spruce thickets, in the the young. More than one male cares strong egg production. During the In addition, temperature increases of calcium removal from the already could lead to more hurricanes and nutrient-poor soils typical of the other severe weather events that Bicknell’s high elevation breeding would reduce forage opportunities habitat. Mercury deposition, and threaten long-term existence primarily from industrial sources is of the tropical forests that the also a concern. Data demonstrates Bicknell’s thrush depends on. that accumulation rates are 2 to 5 Biologists have not determined if times greater in some high elevation the Bicknell’s thrush will be able to sites than in others. In addition, adapt to these changes. older birds have higher mercury blood concentration levels than do Another potential consequence of younger birds, suggesting that these climate change for the Bicknell’s birds are accumulating mercury thrush might be the creation of through time. While this information a variance between arrival time is cause for concern, the effects that to the breeding grounds and the these mercury burdens will have on abundance of prey. Currently, the the Bicknell’s thrush are not fully arrival time is regulated by day understood. length, and the abundance of insects and fruits is linked to temperature. If Taking Action temperatures rise and spring occurs Recently, the U.S. Fish and Wildlife earlier, the food supply may also Service was petitioned to list peak earlier and late-arriving birds the Bicknell’s thrush as either a may not have adequate food supplies. threatened or endangered species As a result, the reproductive success under the Endangered Species Act.

Credit: Credit: Steve Faccio, Vermont Center for Ecostudies Credit: Steve Faccio, Vermont of late-arriving birds may suffer. As a result, the Service will conduct Bicknell’s thrush with chicks. a thorough assessment of the status Predation and threats to the Bicknell’s thrush. winter, the Bicknell’s thrush feeds on Predation may also be a threat. In In an effort to secure the future small fruits and insects at middle- the thrush’s breeding habitat, red of existing Bicknell’s thrush and high-elevation sites. squirrels live and feed mainly on populations, the Service, other spruce and fir cones in the montane management agencies and Causes for Concern forests. Red squirrel populations conservation organizations have are cyclical and dependent on the focused on predicting and monitoring Habitat Loss and Alteration production of large cone crop, which the effects of climate change, The Bicknell’s thrush has been can be highly variable from year to managing and protecting habitat, identified as a high conservation year. In years when red squirrels and restoring existing populations. priority within the international are abundant, predation of eggs and birding community, as it is nestlings can be high. Increased highly susceptible to several temperatures resulting from climate U.S. Fish & Wildlife Service threats. Immediate threats to the change could increase cone crops, 1 800/344 WILD species include habitat loss from and as a result, predation by red www.fws.gov forestry, energy and recreational squirrels may also increase. August 2012 developments on the breeding While red squirrel predation grounds and habitat loss from of nestlings and eggs has been subsistence farming and logging on recognized as a concern for some the wintering ground. A potentially time, predation of adults wasn’t significant threat in the future is the well documented. However, recently, potential loss of habitat that could researchers conducting studies result from climate change, which on the wintering grounds in the might mean a nearly complete loss of Dominican Republic documented this species’ high elevation breeding predation of adult Bicknell’s thrush habitat in the U.S. over the course of by introduced black and Norway the next century. rats. This predation is believed to occur while adults are night As the climate warms, the montane roosting. Researchers have not yet spruce and fir forests may disappear determined the effects of red squirrel from the Bicknell’s thrush’s current or rate predation and its impacts to breeding range within the northeast Bicknell’s thrush populations. U.S. These coniferous trees currently exist as “islands” of suitable habitat Pollution and are predicted to “migrate” Biologists have also identified northward and upwards in elevation pollution from industrial sources as as temperatures increase. This a concern for the Bicknell’s thrush vegetative migration is expected and its habitat. The decline of high to result in the nearly complete elevation red spruce has been linked elimination of Bicknell’s thrush to accumulation of acidic ions in the Credit: Credit: Steve Faccio, Vermont Center for Ecostudies Credit: Steve Faccio, Vermont habitat. atmosphere. This could be a result Bicknell’s thrush.