Merlin

Bicknell's Thrush Life History

Habitat

Habitat Forests

Bicknell’s Thrush nests in disturbed, often regenerating or stunted forests with some undergrowth, usually at higher elevations. Such habitats can be the product of damage by ice, fire, or storms or of human activities, such as timber harvest or trail construction. Through much of the breeding range, balsam fir is a key tree species, with red spruce, black spruce, white birch, and mountain-ash also typically present. Such habitats occur only in higher elevations in New York (above 3,600 feet) but occur much lower in the northern parts of range, such as New Brunswick (as low as 1,500 feet). Bicknell’s also breeds in spruce-fir forests on the coast of New Brunswick and Nova Scotia, at much lower elevations. In most breeding locations, the ground is mossy, the trees are small and grow close together, and there are plenty of fallen trees, dead trees, and snags. Observing birds in this dense environment can be difficult, but Bicknell’s does favor areas with edges for nesting. Migrating birds seek out structurally similar habitats, in both spring and fall, especially overgrown woodlots with abundant invertebrates or fruit. In the Greater Antilles, wintering birds inhabit wet montane forests.

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Food

Food Insects

Bicknell’s Thrushes eat mostly insects and other arthropods, especially ants and beetles, as well as small fruit when available. Other prey items include flies, bugs, aphids, snails, spiders, moths, slugs, grasshoppers, and even small salamanders. They feed mostly on or near the ground, scratching with their feet to uncover prey or patiently watching and listening, much as American Robins do. When feeding young, they forage at all levels in the forest, including in undergrowth and trees, taking whatever insects (and larvae) are available by gleaning, hover-gleaning, and even flycatching. They feed young mostly insects and other arthropods.

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Nesting

Nest Placement

Nest Shrub

Females apparently select the nest site and build the nest, usually near the trunk of a tree in dense stands of short or stunted (“krummholz”) balsam fir, often near a gap or edge in the forest. Nests average about 5 feet off the ground.

Nest Description

Cup nest made of balsam fir or spruce twigs and moss, sometimes with grasses, sedges, ferns, leaves, bark, hair, or lichen as well. Lined with grasses or horsehair fungus. Nest dimensions average about 4.8 inches across and 3.4 inches tall, with interior cup 2.7 inches across and 1.8 inches deep.

Nesting Facts
Clutch Size:3-4 eggs
Number of Broods:1-2 broods
Egg Length:0.8-0.9 in (2.1-2.3 cm)
Egg Width:0.6-0.7 in (1.6-1.75 cm)
Incubation Period:9-14 days
Nestling Period:9-13 days
Egg Description:

Bluish green with light brown speckling.

Condition at Hatching:

Naked and helpless.

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Behavior

Behavior Ground Forager

On the breeding grounds, males do not appear to defend classic territories but rather sing from a large home range that may overlap with ranges of other males. Males do chase other males on occasion but not to the extent that other territorial male songbirds do. Females, however, regularly chase away other females from their smaller home range, which is centered on the nest area, especially when building the nest and laying eggs. Males sometimes pursue females in flight, sometimes singing, and droop and flutter their wings when perched, singing quietly, presumably as prelude to copulation. However, male home ranges often include multiple active nests. This unusual approach to territoriality is mirrored in an unusual mating system. Both males and females mate with multiple partners, and most broods contain young from several fathers. Males often feed nestlings in two nests at once, and often more than one male feeds young in any given nest. Males often feed nestlings that are not their own offspring. Traditional pair bonds between males and females are not apparent in this unusual arrangement. Indeed, no other bird species in North America has a similar mating system except possibly Smith’s Longspur. On the wintering grounds, both males and females hold discrete territories that overlap very little or not at all (males tend to winter at higher elevations than females), and they vigorously defend these against other Bicknell’s Thrushes.

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Conservation

Conservation Red Watch List

Bicknell’s Thrush was recognized as a full species only in 1995, so long-term population trends from the North American Breeding Bird Survey are not available. Canadian surveys have shown steep declines in populations since the 1960s. Partners in Flight estimates a global breeding population of 110,500, rates the species a 17 out of 20 on the Continental Concern Score, and includes Bicknell's Thrush on the Red Watch List, the group’s highest level of conservation concern. The reasons for declining populations are not well understood but may include airborne pollutants that harm the species’ high-elevation forest habitats and contribute to high levels of mercury in this species. Bicknell’s Thrush has disappeared from numerous historical breeding locations, partly because of warming temperatures. Some predictive studies of global climate change indicate the loss of more than half of Bicknell’s breeding habitat by 2038 and the loss of more than 90% by 2100.

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Credits

Lutmerding, J. A. and A. S. Love. Longevity records of North American birds. Version 2015.2. Patuxent Wildlife Research Center, Bird Banding Laboratory 2015.

Partners in Flight (2017). Avian Conservation Assessment Database. 2017.

Sibley, D. A. (2014). The Sibley Guide to Birds, second edition. Alfred A. Knopf, New York, USA.

Townsend, Jason M., Kent P. McFarland, Christopher C. Rimmer, Walter G. Ellison and James E. Goetz. (2015). Bicknell's Thrush (Catharus bicknelli), version 2.0. In The Birds of North America (P. G. Rodewald, editor). Cornell Lab of Ornithology, Ithaca, New York, USA.

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