Linking Breeding and Wintering Grounds of Bicknell's Thrushes Using Stable Isotope Analyses of Feathers

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[edit] Author(s)

• Hobson, K. A.
• McFarland, K. P.
• Wassenaar, L. I.
• Rimmer, C. C.
• Goetz, J. E.

[edit] Journal

Auk 118.1 (2001): 16-23.

[edit] Keywords

monarch butterflies migrant songbirds migratory birds natal origins ratios

[edit] Abstract

Previous studies have shown that natural abundance of deuterium (D), and to a lesser extent C-13, in feathers of migrant songbirds in North America can be used to infer geographic origins of molt. We used that approach to investigate whether Bicknell's Thrush (Catharus bicknelli) populations could be segregated on the breeding grounds at six sites in eastern North America to evaluate contributions from the breeding areas to wintering populations. Further, we tested our isotopic model using feathers from hatching-year migrant Swainson's Thrush (Catharus ustulatus) and Gray-cheeked Thrush (Catharus minimus) moving through southern Manitoba and Ontario and found that deltaD values were consistent with northerly boreal forest natal sites in northwestern and central Canada. Despite a strong latitudinal gradient in deltaD of average growing season precipitation over the breeding range, we found considerable overlap in deltaD values of Bicknell's Thrush among all breeding sites. No strong pattern in delta C-13 values of feathers with latitude was apparent. Nevertheless, the more negative deltaD values of feathers from wintering birds in the Dominican Republic compared to breeding area samples suggested that birds from more northerly locations in North America contributed to this population of wintering birds. High variance in feather deltaD values within breeding populations also suggested high natal dispersal, considerable movement of birds among island habitats on the breeding grounds, or both. Even higher variances in feather deltaD values from small study areas on the wintering grounds, compared to similar-sized areas in the breeding range, suggest that many different breeding populations mix in winter habitat.

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