NATURAL HISTORY

BICKNELL’S THRUSH} Catharus bicknelli
FAMILY: Turdidae

DESCRIPTION: The Bicknell’s thrush is a medium-sized thrush, typically 16-17 centimeters in length and weighing approximately 26-30 grams.  Distinguishing physical characteristics include olive-brown upper parts, white underparts (sometimes slightly yellow-tinged), gray cheeks, a spotted breast and some chestnut coloration on the tail and wings. Males and females are physically indistinguishable.

HABITAT: The Bicknell’s thrush is an extreme habitat specialist, found in disturbed subalpine spruce-fir forest in its breeding range, and in high-elevation broadleaf rainforest in its winter range.

RANGE: This species breeds in the high peaks of New England and northern Nova Scotia, wintering in the Caribbean’s Greater Antilles islands.

MIGRATION: For winter, the Bicknell’s thrush migrates from its subalpine breeding habitat in the northeastern United States and southeastern Canada to the tropical broadleaf forests of the Greater Antilles. Both northbound and southbound migration routes are restricted to the area east of the Appalachian Mountains.

BREEDING: The Bicknell’s thrush has an unusual mating system in which both males and females mate with multiple partners, multiple paternity in a single clutch is common, and more than one male cares for nestlings in each brood — a system that benefits offspring because more than one male will bring food to each nest. These thrush generally begin arriving in breeding habitat in late May and begin nesting in early June, with females laying eggs in the summer. Breeding usually occurs annually.

LIFE CYCLE: This bird reaches breeding age at about one year. The oldest known Bicknell’s thrush was 11 years of age, though surveys report an annual mean age between 1.73 and 2.44 years.

FEEDING: Invertebrates are the primary prey for the Bicknell’s thrush, particularly during breeding season. Ants, beetles and butterfly and moth larvae comprise the bulk of adults’ diet, while adult sawflies, wasps, bees and ants are an important prey item for nestlings. This bird may also feed on blueberries, bunchberries and wild grapes later in the summer.

THREATS: These thrushes are gravely threatened by global warming, which is projected to change the distribution of trees that make up their montane-forest breeding habitat. Numerous other environmental stressors are already contributing to the thrush’s decline, including pollution from acid rain, ground-level ozone and mercury; forest pests; logging; and habitat destruction for development for ski resorts, communications infrastructure and wind-energy projects.

POPULATION TREND: Because of this species’ unusual and complex mating system and the dense, often inaccessible habitat it prefers, estimating its overall population size is difficult. However, studies of individual populations have consistently shown decline in the past 10 to 15 years.

Bicknell's thrush photo © Larry Master