THE INTERTARSAL JOINT OF THE HARRIER‐HAWKS POLYBOROIDES SPP. AND THE CRANE HAWK GERANOSPIZA CAERULESCENS

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Abstract
SUMMARY: The Harrier‐Hawks Polyboroides spp. and Crane Hawk Geranospiza caerulescens have legs capable of flexing backward as well as forward at the intertarsal joint. This is evidently adaptive to their habit of extracting prey from crevices using the feet. The structure and properties of the intertarsal joint have been investigated using an alcoholic specimen of P. typus and skeletons of P. typus and G. caerulescens; specimens of Circus cayaneus (alcoholic) and C. aeruginosus (skeleton) were used for comparison. Increased mobility of the joint extends to medial‐lateral swing as well as flexion; the faculty for backward flexion appears to be somewhat offset by some loss of forward flexion. The tibiotarsus, tarsometatarsus and the ligaments and menisci of the intertarsal joint are described and figured. No significant differences in musculature were found. The most obvious difference between Circus on the one hand, and Polyboroides and Geranospiza on the other is the extreme narrowness of the joint in the latter two genera. This is probably the most important factor permitting increased joint mobility. The possible significance of other structural features observed is also discussed, and related adaptations of the distal tarsometatarsus and foot are pointed out. It is concluded that the resemblances of the hind limbs of these two genera are not strong evidence of phylogenetic relationship.