favor of the swift' s locomotion is nullified.Now, a claw on the heel of the tarsus, functioning as a toe, would enable the swift to maintain its balance in a downward course, or even on a level surface; but lacking such a handy attribute the swift must pitch forward when caught in such positions.The fact that the swift does pitch forward when on a level surface would seemingly account for this bird' s inability to rise from the ground as do ordinary birds.
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W. H. Bergtold (1926) studied this question.