Cormorants (Phalacrocoracidae) depend on their hind limbs (legs) to pursue prey underwater, rendering these structures critical for their survival. Assemblages of three or more species can be found in a variety of regions including the coasts of western USA (California), central Chile, and western South Africa. Cormorants all hunt via pursuit diving underwater while remaining close to shore, yet are able to coexist; an occurrence that is not well understood. Variation in feeding ecology within cormorant assemblages was expected to be reflected in the legs. Leg structure was examined by measurement of five leg bone characteristics from skeletal specimens of 10 species from assemblages within the aforementioned regions. Principle component analysis identified the femur and tarsometatarsus as having the highest degrees of variation. The femur was found to have the shortest length, relative to total leg length, in putative shoaling fish specialists. The tarsometatarsus was found to have the shortest length, relative to total leg length, in putative benthic fish specialists. Mantel's Test revealed that leg structures of members of the same putative feeding type were distributed closer together in Euclidean space (mean distance: 2.84) than leg structures of members of distinct putative feeding types (3.27); however, this was not significant (r = 0.138, t = 0.086, P = 0.233) indicating that similarity among assemblages is not statistically well-supported. Findings support niche differentiation within the studied regions.
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Vol. 39 • No. 2