Energy savings due to the use of shallow body temperature reduction in overwintering Northern Cardinals

Abstract

Background: Northern Cardinals (Cardinalis cardinalis) are able to endure drastic seasonal variations in ambient temperature. Many endotherms in these conditions utilize heterothermy (e.g., torpor) to conserve energy by adjusting their body temperatures according to changing environmental conditions. Previous research shows that cardinals reduce energy expenditure during winter nights. By examining whole-animal function, we asked whether this reduced metabolism was a result of decreased activity or an induced state of torpor. We measured body mass, percent fat content, metabolic rate (V O 2 ) across a range of ambient temperatures and body temperature (T b ) using data loggers during both summer (August to early September) and winter (December to January) conditions. We hypothesized that: (a) daily winter T b fluctuations would reveal an induced torpor, and (b) alterations of insulation would play a significant role in thermoregulation.Results: Although insulation in the form of fat stores was higher in the winter, there was no seasonal difference in whole body conductance. We found no evidence for torpor, but found a slight depression in overall circadian temperature rhythms for the winter animals compared to summer animals, resulting in a predicted 10-16 % savings in daily energy expenditure. Conclusions:Our findings support recent work showing that thermoregulatory mechanisms are not as fixed as previously thought and, while fat deposits may function to increase insulation, they are more likely important for fuel storage. These data identify subtle changes in the homeostatic set point for body temperature and show that these slight alterations can have significant impacts on daily energy expenditure in wild birds.

A partial migrant relies upon a range-wide cue set but uses population-specific weighting for migratory timing

“…Of the environmental factors we studied, temperature was the factor that had the least influence on departure probability. However, in our full and partial migrant populations (residual) temperatures lower than average increased the probability for departure decisions as they likely contain local environmental information about diminishing food resources and increasing thermoregulatory costs [ 74 ] (Table 1 ).…”

A songbird adjusts its heart rate and body temperature in response to season and fluctuating daily conditions

“…For example, corresponding values from greylag geese (Anser anser) show an f H reduction of 22% with an additional decrease of 1°C in mean daily winter T b [26]. Similarly, in northern cardinals (Cardinalis cardinalis) daily energy savings of 10-16% were predicted when T b was reduced by 1.3°C [64].…”

“…This, coupled with lowered f H , provides evidence that blackbirds go into a deliberate, controlled hypometabolic state at night, especially during the winter [70]. Controlled hypometabolism, which animals can implement through lowered hypothalamus setpoints, reduces the difference between T a and T b and decreases the energy required to achieve the desired T b [64]. By decreasing the difference between T a and T b , resident blackbirds may be able to save considerable amounts of energy on thermoregulation, allowing them to conserve available energetic resources and offset the costs of remaining resident in the breeding area over winter [71,72].…”

Measuring body temperature in birds – the effects of sensor type and placement on estimated temperature and metabolic rate

“…Intraperitoneal measurements have been used extensively, both in arid and hot climates (recent examples include Whitfield et al, 2015;Albright et al, 2017;McKechnie et al, 2017;Oswald et al, 2018;Freeman et al, 2022) and in colder environments (Reinertsen, 1982;Reinertsen and Haftorn, 1986;Schaeffer et al, 2015;Appenroth et al, 2021) and are widely considered as non-stressful and accurate when the animals have recovered from surgery.…”

“…Using wild-caught great tits ( Parus major ), we assessed how four common methods that differ in perceived invasiveness and site of measurement – intraperitoneal, cloacal, subcutaneous and cutaneous – performed in ambient temperatures in thermoneutrality (25°C), close to the lower critical temperature (5°C) and far below thermoneutrality (−15°C), and how these methods affected simultaneous measurements of resting metabolic rate (RMR). Intraperitoneal measurements have been used extensively, in both arid and hot climates (recent examples include Whitfield et al, 2015 ; Albright et al, 2017 ; McKechnie et al, 2017 ; Smith et al, 2017 ; Oswald et al, 2018 ; Freeman et al, 2022 ) and in colder environments ( Reinertsen, 1982 ; Reinertsen and Haftorn, 1986 ; Schaeffer et al, 2015 ; Appenroth et al, 2021 ), and are widely considered as non-stressful and accurate when the animals have recovered from surgery. Cloacal, rectal or colonic temperature is usually recorded as a single-point measurement but has also been recorded continuously (see Thomas and Maclean, 1981 ; Nord and Folkow, 2018 ; Andreasson et al, 2020b ; Wolf et al, 2020 ; Pollock et al, 2021 ).…”