Daily torpor is a means of saving energy by controlled lowering of the metabolic rate (MR) during resting, usually coupled with a decrease in body temperature. We studied nocturnal daily torpor under natural conditions in free-living common swifts Apus apus resting in their nests as a family using two non-invasive approaches. First, we monitored nest temperature (Tnest) in up to 50 occupied nests per breeding season in 2010-2015. Drops in Tnest were the first indication of torpor. Among 16 673 observations, we detected 423 events of substantial drops in Tnest of on average 8.6°C. Second, we measured MR of the families inside nest-boxes prepared for calorimetric measurements during cold periods in the breeding seasons of 2017 and 2018. We measured oxygen consumption and carbon dioxide production using a mobile indirect respirometer and calculated the percentage reduction in MR. During six torpor events observed, MR was gradually reduced by on average 56% from the reference value followed by a decrease in Tnest of on average 7.6°C. By contrast, MR only decreased by about 33% on nights without torpor. Our field data gave an indication of daily torpor, which is used as a strategy for energy saving in free-living common swifts.

Animal Physiology Faculty of Biology Marburg University Marburg Germany

Czech Centre for Phenogenomics Institute of Molecular Genetics of the Czech Academy of Sciences Vestec Czech Republic

Institute of Avian Research 'Vogelwarte Helgoland' Wilhelmshaven Germany

Research Group of Ecology and Behavioural Biology Institute of Biology University of Siegen Siegen Germany

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Cool birds: first evidence of energy-saving nocturnal torpor in free-living common swifts Apus apus resting in their nests

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