Energetics, Thermoregulation and Nocturnal Hypothermia in Australian Silvereyes

Abstract

We investigated whether the Silvereye, Zosterops lateralis (l&12 g) uxs heterothermy to reduce energy expenditure when confronted with adverse environmental conditions. We also determined the thermal physiology and energetics of this species. Z. later&s entered nocturnal hypothermia over a range of ambient temperatures (T,) from 3-26°C. Below the thermoneutral zone (TNZ), metabolic rate (MR) decreased by up to 50% and this reduction was more pronounced at high T, than at low T,. The reduction of MR at night was accompanied by a reduction of body temperature (Tb) from 40.3 2 0.5 to 36.9 ? O.l"C, a decrease in the difference between T, and T, and a reduction in thermal conductance. Within the TNZ, basal metabolic rate (BMR) was 2.43 t 0.41 mL 0, gg' hh' , while the corresponding day RMR was significantly elevated at 3.26 2 0.42 mL 0, g-' hh'. The ability of Silvereyes to reduce daily energy expenditure by employing nocturnal hypothermia may be one reason why this species and its relatives are able to occupy a wide variety of habitats and climates.

Torpor‐assisted migration: Regular use of shallow torpor reduces rest‐phase energy expenditure for songbirds during migration

“…Therefore, we expected to see more frequent and deeper torpor use at colder T a . However, T a had no effect on frequency or depth of torpor use in any species, consistent with previous studies on shallow torpor in passerines (Maddocks & Geiser, 1997;Wojciechowski & Pinshow, 2009), but contrary to studies on deep torpor in nonpasserine birds (Brigham, 1992;Spence & Tingley, 2021;Wolf et al, 2020) and bats (McGuire et al, 2014) where T b closely tracks T a .…”

“…Torpor has long been recognized as an important thermoregulatory strategy for small mammals in a variety of circumstances throughout the annual cycle (Geiser, 2004 ), but aside from hummingbirds (Eberts et al., 2021 ; Powers et al., 2003 ; Shankar et al., 2020 ; Wolf et al., 2020 ), and a few other specific cases (Brigham, 1992 ; Doucette et al., 2012 ; Merola‐Zwartjes & Ligon, 2000 ), there has been relatively little attention to heterothermy in birds, particularly in passerines (but see Benedetti et al., 2014 ; Maddocks & Geiser, 1997 ; McKechnie & Lovegrove, 2002 ; Wojciechowski & Pinshow, 2009 ). More than half of extant birds are in the order Passeriformes, with more than 6500 species found in all habitats around the globe (Raikow, 1986 ; Schmitt & Edwards, 2022 ).…”

Torpor‐assisted migration: Regular use of shallow torpor reduces rest‐phase energy expenditure for songbirds during migration

“…Therefore, we expected to see more frequent and deeper torpor use at colder T a . However, T a had no effect on frequency or depth of torpor use in any species, consistent with previous studies on shallow torpor in passerines (Maddocks & Geiser, 1997;Wojciechowski & Pinshow, 2009), but contrary to studies on deep torpor in nonpasserine birds (Brigham, 1992;Spence & Tingley, 2021;Wolf et al, 2020) and bats (McGuire et al, 2014) where T b closely tracks T a .…”

“…Torpor has long been recognized as an important thermoregulatory strategy for small mammals in a variety of circumstances throughout the annual cycle (Geiser, 2004 ), but aside from hummingbirds (Eberts et al., 2021 ; Powers et al., 2003 ; Shankar et al., 2020 ; Wolf et al., 2020 ), and a few other specific cases (Brigham, 1992 ; Doucette et al., 2012 ; Merola‐Zwartjes & Ligon, 2000 ), there has been relatively little attention to heterothermy in birds, particularly in passerines (but see Benedetti et al., 2014 ; Maddocks & Geiser, 1997 ; McKechnie & Lovegrove, 2002 ; Wojciechowski & Pinshow, 2009 ). More than half of extant birds are in the order Passeriformes, with more than 6500 species found in all habitats around the globe (Raikow, 1986 ; Schmitt & Edwards, 2022 ).…”

Daily Cycles in Body Temperature in a Songbird Change with Photoperiod and Are Weakly Circadian

“…It is presumably an adaptive mechanism in that during long winter nights, environmental temperature is likely to be lower, and lowering Tb more will reduce energy loss. Hypothermia reduces energy expenditure and increases the chances of surviving winter (Waite, 1991; Maddocks and Geiser, 1997; Clark and Dukas, 2000), and birds with reduced food intake show a lower daily nadir in body temperature (Thouzeau et al, 1999). The greater decrease in starlings’ Tb was not caused by lower environmental temperature since temperature was kept constant during day-night and was the same for all photoperiods.…”

Effect of Sampling Regime on Estimation of Basal Metabolic Rate and Standard Evaporative Water Loss Using Flow‐Through Respirometry

“…The four sampling regimes that we selected from the literature for analysis were representative of a wide array of actual and potential sampling regimes. These were chosen to provide comparison of an essentially continuous regime (20 s/20 min) with a shortinterval, short-minimum-mean regime (3 min/21 min; Doucette and Geiser 2008), an intermediate-interval, long-minimum-mean regime (6 min/60 min; Downs and Brown 2002;Bush et al 2008), and a long-interval, intermediate-minimummean period regime (12 min/36 min; Maddocks and Geiser 1997;Song and Geiser 1997;Holloway and Geiser 2001). We found a significant effect for these sampling regimes on the estimation of BMR, measured as and , and oṅVo Vco 2 2 EWL ( Table 1).…”

Torpor‐assisted migration: Regular use of shallow torpor reduces rest‐phase energy expenditure for songbirds during migration

“…Therefore, we expected to see more frequent and deeper torpor use at colder T a . However, T a had no effect on frequency or depth of torpor use in any species, consistent with previous studies on shallow torpor in passerines (Maddocks & Geiser, 1997;Wojciechowski & Pinshow, 2009), but contrary to studies on deep torpor in nonpasserine birds (Brigham, 1992;Spence & Tingley, 2021;Wolf et al, 2020) and bats (McGuire et al, 2014) where T b closely tracks T a .…”

“…Torpor has long been recognized as an important thermoregulatory strategy for small mammals in a variety of circumstances throughout the annual cycle (Geiser, 2004 ), but aside from hummingbirds (Eberts et al., 2021 ; Powers et al., 2003 ; Shankar et al., 2020 ; Wolf et al., 2020 ), and a few other specific cases (Brigham, 1992 ; Doucette et al., 2012 ; Merola‐Zwartjes & Ligon, 2000 ), there has been relatively little attention to heterothermy in birds, particularly in passerines (but see Benedetti et al., 2014 ; Maddocks & Geiser, 1997 ; McKechnie & Lovegrove, 2002 ; Wojciechowski & Pinshow, 2009 ). More than half of extant birds are in the order Passeriformes, with more than 6500 species found in all habitats around the globe (Raikow, 1986 ; Schmitt & Edwards, 2022 ).…”

Daily Cycles in Body Temperature in a Songbird Change with Photoperiod and Are Weakly Circadian

“…It is presumably an adaptive mechanism in that during long winter nights, environmental temperature is likely to be lower, and lowering Tb more will reduce energy loss. Hypothermia reduces energy expenditure and increases the chances of surviving winter (Waite, 1991; Maddocks and Geiser, 1997; Clark and Dukas, 2000), and birds with reduced food intake show a lower daily nadir in body temperature (Thouzeau et al, 1999). The greater decrease in starlings’ Tb was not caused by lower environmental temperature since temperature was kept constant during day-night and was the same for all photoperiods.…”

Effect of Sampling Regime on Estimation of Basal Metabolic Rate and Standard Evaporative Water Loss Using Flow‐Through Respirometry

“…The four sampling regimes that we selected from the literature for analysis were representative of a wide array of actual and potential sampling regimes. These were chosen to provide comparison of an essentially continuous regime (20 s/20 min) with a shortinterval, short-minimum-mean regime (3 min/21 min; Doucette and Geiser 2008), an intermediate-interval, long-minimum-mean regime (6 min/60 min; Downs and Brown 2002;Bush et al 2008), and a long-interval, intermediate-minimummean period regime (12 min/36 min; Maddocks and Geiser 1997;Song and Geiser 1997;Holloway and Geiser 2001). We found a significant effect for these sampling regimes on the estimation of BMR, measured as and , and oṅVo Vco 2 2 EWL ( Table 1).…”

Torpor‐assisted migration: Regular use of shallow torpor reduces rest‐phase energy expenditure for songbirds during migration

“…Therefore, we expected to see more frequent and deeper torpor use at colder T a . However, T a had no effect on frequency or depth of torpor use in any species, consistent with previous studies on shallow torpor in passerines (Maddocks & Geiser, 1997;Wojciechowski & Pinshow, 2009), but contrary to studies on deep torpor in nonpasserine birds (Brigham, 1992;Spence & Tingley, 2021;Wolf et al, 2020) and bats (McGuire et al, 2014) where T b closely tracks T a .…”

“…Torpor has long been recognized as an important thermoregulatory strategy for small mammals in a variety of circumstances throughout the annual cycle (Geiser, 2004 ), but aside from hummingbirds (Eberts et al., 2021 ; Powers et al., 2003 ; Shankar et al., 2020 ; Wolf et al., 2020 ), and a few other specific cases (Brigham, 1992 ; Doucette et al., 2012 ; Merola‐Zwartjes & Ligon, 2000 ), there has been relatively little attention to heterothermy in birds, particularly in passerines (but see Benedetti et al., 2014 ; Maddocks & Geiser, 1997 ; McKechnie & Lovegrove, 2002 ; Wojciechowski & Pinshow, 2009 ). More than half of extant birds are in the order Passeriformes, with more than 6500 species found in all habitats around the globe (Raikow, 1986 ; Schmitt & Edwards, 2022 ).…”

Daily Cycles in Body Temperature in a Songbird Change with Photoperiod and Are Weakly Circadian

“…It is presumably an adaptive mechanism in that during long winter nights, environmental temperature is likely to be lower, and lowering Tb more will reduce energy loss. Hypothermia reduces energy expenditure and increases the chances of surviving winter (Waite, 1991; Maddocks and Geiser, 1997; Clark and Dukas, 2000), and birds with reduced food intake show a lower daily nadir in body temperature (Thouzeau et al, 1999). The greater decrease in starlings’ Tb was not caused by lower environmental temperature since temperature was kept constant during day-night and was the same for all photoperiods.…”

Effect of Sampling Regime on Estimation of Basal Metabolic Rate and Standard Evaporative Water Loss Using Flow‐Through Respirometry

“…The four sampling regimes that we selected from the literature for analysis were representative of a wide array of actual and potential sampling regimes. These were chosen to provide comparison of an essentially continuous regime (20 s/20 min) with a shortinterval, short-minimum-mean regime (3 min/21 min; Doucette and Geiser 2008), an intermediate-interval, long-minimum-mean regime (6 min/60 min; Downs and Brown 2002;Bush et al 2008), and a long-interval, intermediate-minimummean period regime (12 min/36 min; Maddocks and Geiser 1997;Song and Geiser 1997;Holloway and Geiser 2001). We found a significant effect for these sampling regimes on the estimation of BMR, measured as and , and oṅVo Vco 2 2 EWL ( Table 1).…”