Do Owls Use Torpor? Winter Thermoregulation in Free‐Ranging Pearl‐Spotted Owlets and African Scops‐Owls

Abstract

Numerous avian taxa use torpor, which involves pronounced reductions in body temperature (T b ) to below normothermic levels. However, the occurrence of this phenomenon in owls (Strigidae) remains largely unknown. We investigated winter patterns of thermoregulation in the crepuscular 80-g pearlspotted owlet Glaucidium perlatum and the strictly nocturnal 61-g African scops-owl Otus senegalensis by obtaining telemetric measurements of skin temperature (T skin ) from free-ranging individuals in the Kalahari Desert of southern Africa. Pearlspotted owlets remained homeothermic throughout the study period, whereas African scops-owls routinely used shallow torpor, with T skin reduced by 3.3Њ-8.6ЊC (pooled mean, 5.3Њ ‫ע‬ ) below normothermic levels for 3-4 h after sunrise. The 1.1ЊC mean lowest T skin recorded in three African scops-owl individuals was . The thermoregulatory differences be-29.0Њ ‫ע‬ 0.1ЊC tween these two species may be related to their diets and activity patterns. African scops-owls are almost exclusively insectivorous and experience a marked reduction in food availability on cold winter nights. In contrast, pearl-spotted owlets have more flexible activity patterns and include larger or diurnal vertebrate prey in their diet.

It's cool to be dominant: social status alters short-term risks of heat stress

“…T b data were collected at ∼5 min intervals using an automated receiver/data logger (SRX-400, Lotek, Newmarket, ON, Canada) and Yagi antennae. We calibrated the transmitters prior to implantation following Smit and McKechnie (2010). Temperature-sensitive VHF transmitters have been reported to drift (Williams et al, 2009).…”

Torpor in Dark Times: Patterns of Heterothermy Are Associated with the Lunar Cycle in a Nocturnal Bird

“…Skin temperature is widely used to asses thermoregulatory patterns of birds, and although T skin is not exactly equal to T b , it is a robust proxy of nocturnal T b in birds (Brigham et al 2000;McKechnie et al 2007;Smit and McKechnie 2010). The temperature-sensitive radio-transmitters (2.5g, model PD-2T, Holohil Systems, Carp, Ontario, Canada) were pre-calibrated in a temperature-controlled water bath at temperatures between 5° and 45°C, using an NISTtraceable mercury thermometer.…”

Avian torpor or alternative thermoregulatory strategies for overwintering?

“…Globally, non-passerines and particularly nocturnal species are more likely to be heterothermic, and much thermoregulatory research has focused on these species (e.g. Brigham, 1992;Körtner et al, 2001;Lane et al, 2004;McKechnie et al, 2007;Smit and McKechnie, 2010;Doucette et al, 2011Doucette et al, , 2012. However, only a handful of studies have documented the nocturnal energetic strategy of free-ranging passerines, and even fewer have measured core T b .…”

“…For sedentary, insectivorous species, not only does winter present thermal challenges but also the increased energetic demand must be met when prey availability is reduced or unpredictable (Smit and McKechnie, 2010;Doucette et al, 2012). A small body size poses further energetic challenges.…”

“…A small body size poses further energetic challenges. These factors are associated with widespread torpor use by Australia's small mammals (Geiser, 1998(Geiser, , 2004Munn et al, 2010), but although torpor is a thermal strategy for some insectivorous non-passerines (Körtner et al, 2001;Doucette et al, 2012;McKechnie et al, 2007;Smit and McKechnie, 2010), there is currently little quantitative evidence for torpor amongst southern hemisphere passerines. Other adaptations to a harsh and unpredictable climate such as aseasonal and cooperative breeding (Russell, 1989;Astheimer and Buttemer, 2002) suggest that torpor may be likely, together with the fact that many small Australian insectivorous birds are sedentary (Recher and Davis, 1997;Morris and Wooller, 2001).…”