Torpor in free-ranging Tawny Frogmouths (Podargus strigoides)

Abstract

Several small caprimulgiform birds (>80 g) are known to entertorpor, apparently to cope with a fluctuating supply of insectprey. Since the large Australian tawny frogmouth (Podargus strigoides; 381–556 g) is also insectivorous, we investigated its thermoregulatory behaviour and thermal biology to determine whether this species is also heterothermic.In an open woodland at ~1,000 m altitude, we equipped eight free-ranging birds with external temperature-sensitive radio transmitters attached to an elastic harness to measure skin temperature (Tskin). Core body temperature (Tb) was measured in three of these birds fitted with an additional intraperitoneal transmitter. Tskin was closely correlated with Tb, although Tskin was usually several degrees below Tb. During the three coldest months of the year (June–August), shallow torpor with Tb as low as 29.1°C occurred frequently,whereas during spring and summer, torpor was not recorded.Torpor occurred either during the night and/or during the firsthalf of the day. Night torpor bouts were initiated after a shortactivity period around dusk and lasted on average for about 7 h. Torpid birds always aroused before sunrise to either commence a second short foraging period or to fly directly to a day roost tree. After birds roosted, Tb fell again around sunrise, and birds occasionally entered a second dawn torpor bout; however, in most cases, Tb increased rapidly not long after entry, most likely due to passive heating by the sun. We conclude that despite their large body size and energetically conservative hunting strategy, tawny frogmouths, like several related caprimulgiform species, frequently enter shallow torpor when low Ta demands high energetic costs for normothermic thermoregulation and likely reduces insect availability.