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

• Published in: Biology Letters
• Main Authors: Wellbrock, Arndt H. J., Eckhardt, Luca R. H., Kelsey, Natalie A., Heldmaier, Gerhard, Rozman, Jan, Witte, Klaudia
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 2022
• Subjects: Apus apus
• Online Access: http://dx.doi.org/10.1098/rsbl.2021.0675; https://royalsocietypublishing.org/doi/pdf/10.1098/rsbl.2021.0675; https://royalsocietypublishing.org/doi/full-xml/10.1098/rsbl.2021.0675

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 ( T nest ) in up to 50 occupied nests per breeding season in 2010–2015. Drops in T nest were the first indication of torpor. Among 16 673 observations, we detected 423 events of substantial drops in T nest 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 T nest 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.