Dramatic increases in population size and major extensions of winter range have occurred among midcontinent Lesser Snow Geese (Chen caerulescens caerulescens) in the last few decades. Agricultural landscapes within these range extensions provide winter foods that are distinctly different from those upon which most birds in this population subsisted until about 1970. I examined geographic variation in body and bill morphology of Lesser Snow Geese by comparing geese from: (1) traditionally used coastal brackish marshes contiguous to the Gulf of Mexico (n = 314), (2) agricultural landscapes centered around rice production and occurring up to 160 km inland from the coast (n = 200), and (3) agricultural landscapes with extensive corn production about 1,000 km north of the Gulf of Mexico (n = 125). Analyses of size and shape in body, head, and bill morphology were done on principal components of 10 metric variables. Geese from marsh habitats were largest in body size, those from rice prairies were intermediate but most variable, and those from the Missouri River valley farthest north were smallest. In addition, ''marsh'' geese had thicker bills, ion ger skulls, and longer culmens than "corn" geese. Five nonexclusive hypotheses (phenotypic selection, habitat selection, nutrition, fall migration endurance, and introgressive hybridization with Ross' Geese [Chen rossii]) are proposed to account for this geographic variation in external morphology. The interplay between winter range extension, habitat use, and morphology may have motivated large increases in continental population size and may be coupled with patterns of demographic and morphological change reported on breeding areas.
