Changes in Heart Rate and Respiratory Frequency During Natural Behaviour of Ducks, With Particular Reference to Diving

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Abstract
Heart rate and respiratory frequency were recorded from free-range pochards and tufted ducks by using an implantable radio-transmitter. Particular attention was paid to the changes associated with natural diving behaviour which occurred on an outside pond (4 × 8 × 0·65 m depth). Spontaneous dives, which occurred in the absence of any obvious external stimuli, often occurred in a series and there could be in excess of 40 dives in fairly quick succession. The first dive in a series was heralded by tachycardia and tachypnoea. Upon submersion there was transient bradycardia, but heart rate then increased over a period of 6– 8 s until it was close to the value recorded when the bird was swimming on the pond before the dive. This rate was maintained throughout the dive. Upon surfacing, heart rate increased when lung ventilation commenced, and then decreased steadily until a few seconds before the next dive when it increased. The average duration of all spontaneous dives of a series was 8·4 ± 0·2 (137) s for pochards and 9·9 ± 0·8 (95) s for tufted ducks. The birds also dived in response to food being thrown onto the water (feeding dives). Changes in heart rate associated with these dives were similar to those described for the spontaneous dives. Heart rate did, however, tend to be higher during all stages of the feeding dives, and it often increased at the end of a dive before the bird started to ventilate its lungs. When being chased with a net, the birds had extremely high values of heart rate and respiratory frequency. Immediately they dived to escape, heart rate fell to a rate which was similar to the rate recorded when the unstressed birds were swimming. This rate was maintained throughout the dive. Anticipatory increases in heart rate preceded lung ventilation at the end of the dives. The birds showed a progressive reduction in heart rate when their heads were submerged under water in the laboratory. After 10 s submersion, heart rate was lower than the lowest heart rate recorded during any of the free-range dives.