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Heart rates were measured during free and forced diving on each of two species of aquatic birds: the double-crested cormorant (Phalacrocorax auritus), a true diver, and the Canada goose (Branta candensis), a bottom feeder in shallow water. When they immersed voluntarily they showed no bradycardia, but when the same birds were forcibly held under water there was a rapid drop in heart rate to well below that at rest. This decrease indicates that ther may be a large component of emotional stress in the heart rate records from previous diving studies where restrained animals were forcibly submerged.
Kanwisher et al. (Fri,) studied this question.