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Reprinted with permission from:
The Birds of North America, No. 592, 2001

Bicknell's Thrush

©Tim Laman/VIREO

(Catharus Bicknelli)

CHRISTOPHER C. RIMMER, KENT P. MCFARLAND, WALTER G. ELLISON, AND JAMES E. GOETZ

The song is in a minor key, finer, more attenuated, and more under the breath than that of any other thrush. It seemed as if the bird was blowing in a delicate, slender, golden tube, so fine and yet flute-like and resonant the song appeared. At times it was like a musical whisper of great sweetness and power.

—Burroughs 1904: 51

…only a freak ornithologist would think of leaving the trails (on Mt. Mansfield) for more than a few feet. The discouragingly dense tangles in which Bicknell's Thrushes dwell have kept their habits long wrapped in mystery.

—Wallace 1939: 285

Figure 1. Distribution of Bicknell's Thrush. Patchy distribution throughout its range makes exact delineation difficult. See text for details.

The nasal, gyrating song and plaintive calling of Bicknell’s Thrush are familiar to few birders or ornithologists. The species remote, inhospitable montane and maritime forest habitats, its penchant for dusk and dawn activity, and its reclusive behavior underscore its status as one of the least-known breeding birds in North America. It is also among the most rare and, possibly, most threatened. Breeding from the northern Gulf of St. Lawrence and easternmost Nova Scotia southwest to the Catskill Mountains of New York State, Bicknell’s Thrush probably numbers no more than 50,000 individuals across its naturally breeding range. The species inhabits an even more restricted winter range, occurring regularly on only four islands in the Greater Antilles. Habitat loss and degradation at both ends of its migratory spectrum suggest a tenuous conservation status for Bicknell’s Thrush, which is ranked as the Nearactic-Neotropical migrant of highest conservation priority in the Northeast (Rosenberg and Wells 1995, Pashley et al. 2000).

Following its discovery in 1881 by Eugene Bicknell on Slide Mountain in New York’s Catskill range, Robert Ridgway named and described Bicknell’s Thrush in 1882, then classifying it as a sub-species of Gray-cheeked Thrush (Catharus minimus). George Wallace’s (1939) classic natural-history study focused attention on Bicknell’s Thrush, and a careful taxonomic assessment by Henri Ouellet (1993) led to specific recognition in 1995 (Am. Ornithol. Union 1995). Although reliable field identification of Bicknell’s and Gray-cheeked thrushes remains dubious at best, marked morphological, vocal, and biochemical differences between the two taxa support this designation. The ranges are completely allopatric, with Gray-cheeked breeding farther north (Newfoundland to Siberia) and wintering farther south (Panama through northwestern Brazil and Colombia) than Bicknell’s Thrush. The recent elevation of Bicknell’s Thrush to full species status has heightened interest and concern among birders, scientists, land-use planners, and conservationists.

Bicknell’s Thrush is adapted to naturally disturbed habitats. Historically, the species probably selected patches of regenerating forest caused by fir waves, wind throw, ice and snow damage, fire, and insect outbreaks, as well as chronically disturbed, stunted altitudinal and coastal conifer forests (Ouellet 1993, Nixon 1999, Vermont Institute of Natural Science [VINS]). In addition to these natural successional habitats, Bicknell’s Thrush has recently been discovered in areas disturbed by timber harvesting, ski trail and road construction, and other human activities (Ouellet 1993, VINS). Evidence of local declines and extinctions in “traditional” breeding habitats may indicate either a shift in habitat use or increasing populations (Ouellet 1993, 1996), but more likely reflects the species’ opportunistic use of disturbed habitats. Extensive loss and degradation of the primary forests that Bicknell’s Thrush appears to prefer in winter pose the greatest threat to the species’ long-term viability.

Despite detailed studies by Wallace (1939), VINS and others, few concrete data are available by which to assess the conservation status of Bicknell’s Thrush. The species is poorly monitored by traditional sampling methods, and is unusual spacing and mating system makes estimation of breeding densities unreliable at best. Current range-wide population estimates represent little more than educated guesses. Knowledge of the species’ wintering ecology and demography is fragmentary, and is migratory routes and stopover ecology are poorly known. Recent research on the breeding and behavioral ecology of Bicknell’s Thrush has documented a strongly male-biased sex ratio, with 2 to 4 males feeding young at 75% of nests and multiple paternity of most broods. Possible sexual habitat or geographic segregation on wintering grounds may cause differential survivorship of females and promote skewed breeding sex ratio, but firm evidence is lacking. Much work remains to be done on Bicknell’s Thrush at all stages of its annual cycle and in all parts of its range.

SEXUAL BEHAVIOR
Mating System and sex ratio. Mating system unusual and not easily categorized; may be most similar to that of Smith’s Longspur (Calcarius pictus), which has been termed female-defense polygynandry (Briskie 1993), in that both males and females mate with multiple partners, multiple paternity is common, and >1 male often feeds nestlings. In Vermont, >75% of broods sired by multiple males; some males with offspring in 2 nests in the same breeding season. Of 13 broods in 1998 and 1999, 10 with >2 sires, 3 with single father (VINS). Overall, 4-yr mean male: female ratio on 3 Vermont study plots 1.8:1.0 (annual range 1.4-2.8:1.0; VINS). Cause of male-biased sex ratio not known, may relate to ratio at hatching, differential natal dispersal patterns, events on wintering grounds (e.g., differential male and female survival due to winter habitat segregation); needs investigation.

Rimmer, C.C., K.P. McFarland, W.G. Ellison, and J.E. Goetz. 2001. Bicknell’s Thrush (Catharus bicknelli). In The Birds of North America, No 592 (A. Poole and F. Gill, eds.). The Birds of North America, Inc., Philadelphia, PA.