Abstract
Torpor is a controlled reduction of metabolism and body temperature, and its appropriate use allows small birds to adapt to and survive challenging conditions. However, despite its great energy conservation potential, torpor use by passerine birds is understudied although they are small and comprise over half of extant bird species. Here, we first determined whether a free-living, small ∼20 g Australian passerine, the eastern yellow robin (Eopsaltria australis), expresses torpor by measuring skin temperature (Ts) as a proxy for body temperature. Second, we tested if skin temperature fluctuated in relation to ambient temperature (Ta). We found that the Ts of eastern yellow robins fluctuated during winter by 9.1 ± 3.9°C on average (average minimum Ts 30.1 ± 2.3°C), providing the first evidence of torpor expression in this species. Daily minimum Ts decreased with Ta, reducing the estimated metabolic rate by as much as 32%. We hope that our results will encourage further studies to expand our knowledge on the use of torpor in wild passerines. The implications of such studies are important because species with highly flexible energy requirements may have an advantage over strict homeotherms during the current increasing frequency of extreme and unpredictable weather events, driven by changing climate.
Highlights
Endotherms can maintain a high body temperature (Tb) across a range of ambient temperatures (Ta) via appropriate adjustment of internal heat production
We found that the Ts of eastern yellow robins fluctuated during winter by 9.1 ± 3.9◦C on average, providing the first evidence of torpor expression in this species
The aim of our study was to examine the thermal energetics of a small passerine species, the eastern yellow robin (Eopsaltria australis; hereafter “eastern robin”), during winter at a cool temperate climate site in the eastern Australian Northern Tablelands
Summary
Endotherms can maintain a high body temperature (Tb) across a range of ambient temperatures (Ta) via appropriate adjustment of internal heat production. The energetic costs to thermoregulate outside of the thermal neutral zone can be expensive (Mckechnie and Lovegrove, 2002; Angilletta et al, 2010). To deal with these energetic costs, many endothermic species use torpor, a controlled reduction in metabolism and typically Tb (Namekata and Geiser, 2009; Ruf and Geiser, 2015). Torpor Use in a Small Passerine little information on geographical variation is available for birds (Chaplin, 1976; Sharbaugh, 2001). Some potential costs may occur at low Tbs on a cellular level (Nowack et al, 2019), the use of torpor may reduce predation risk by increasing antipredator behavior (but see Amo et al, 2011; Turbill and Stojanovski, 2018), and in small diurnal birds can reduce metabolic demands by as much as 50% (Cooper and Gessaman, 2005) and may increase survival by 58% (Brodin et al, 2017)
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