Turdus Migratorius)
ECOGEOGRAPHIC VARIATION IN THE AMERICAN ROBIN (TURDUS MIGRATORIUS) JOHN W. ALDRICHa'3 AND FRANCESC. JAMES2 'Departmentof VertebrateZoology, National Museum of Natural History,Smithsonian Institution, Washington,D.C. 20560 USA, and 2Departmentof BiologicalScience, Florida State University, Tallahassee, Florida 32306 USA AnSTRACT.--Fromanalysis of externalmeasurements of 949 museumstudy skins of Amer- ican Robins(Turdus migratorius) taken in the breeding seasonthroughout North America,we estimatedthe extentof geographicvariation in size,shape, and colorof plumageon the basis of 39 ecoregions.After we defined seven univariate size and six scale-freeshape variables, we studiedtheir variationand the extentof their allometriccovariation. Because wing length is highlycorrelated with sixother reasonable measures of generalsize but is simpleand easy to visualize,we usedwing length as a measureof generalsize. The smallestrobins occur in the warm, humid easternUnited States,and the largestoccur in the high, cool, and dry Rocky Mountains,northern Great Plains,and northern deserts. Wing shape changesallometrically with size, becoming more pointed in larger birds, but wing shapein the Arctic is more pointed than would be predictedby this trend. Another exceptionto the allometriccovariation of size and shapeis that leg length as a proportion of generalsize is smallin Mexicoand large in the cool,humid forestsalong the Pacificcoast. Robinsin the coolmaritime forests of Newfoundlandhave unusually long tarsi. This variation in leg length is accompaniedby
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