Seasonal variation in great tit (Parus major) energy requirements: reallocation versus increased demand

Abstract

Understanding how birds annually allocate energy to cope with changing environmental conditions and physiological states is a fundamental question in avian ecology. The two main hypotheses to explain annual patterns in energy use are "reallocation" and "increased demand". The reallocation hypothesis suggests equal energetic costs in winter and breeding seasons, while the increased demand suggests that energy demand should be highest during breeding. Under the standard aerobic capacity model of endothermy, birds are expected to adjust the mass and/or metabolic intensity of their bodies in ways that are consistent with expected cold- and/or activity-induced costs. Here, we look for metabolic signatures of reallocation versus increased demands in the energy requirements of a small, resident passerine of a temperate-zone (great tit, Parus major). To do so, we measured whole-body and mass-independent basal (BMR), summit (Msum), and field (FMR) metabolic rates during late winter and during the chick-rearing period (breeding). We also assessed whether, and to what extent, metabolic rates conform to the predictions of the aerobic capacity model of endothermy. We found that great tits showed no substantial differences in energy expenditure between winter and the breeding season, providing support for the reallocation hypothesis. Only mass-independent Msum showed seasonal variation, with significantly higher values (~4%) in winter compared to the breeding season. Our results also lend support to the predictions of the aerobic capacity model for the evolution of endothermy, as we found that whole-body BMR and Msum were positively related. We argue that both energy reallocation and the limited increase in mass-independent Msum are consistent with the relatively mild winter temperatures recorded during our study period. Our results confirm that both BMR and Msum are flexible traits that vary in ways that are consistent with expected cold- and/or activity-induced costs.

Metabolic adjustments to winter severity in two geographically separated great tit (Parus major) populations

“…It is January February March Gontrode 2022 -6.5 ± 1.8 °C 9.3 ± 2.9 °C Zvenigorod 2021 -6.6 ± 7.1 °C -11.6 ± 7.0 °C -2.5 ± 6.8 °C 2022 -6.1 ± 3.7 °C -1.9 ± 3.4 °C -2.9 ± 5.5 °C 2023 -5.5 ± 7.6 °C -5.0 ± 4.4 °C -0.1 ± 5.0 °C The assessment of winter BMR in Gontrode was conducted at night using open flowthrough respirometry (Lighton, 2018). To measure BMR, the oxygen consumption (VO2) of 40 individuals in a fasted state was monitored, following the methods described in Pacioni et al (2023). Before and after the respirometry measurement, the body masses of the birds were taken to the nearest 0.1g.…”

“…Ambient air was delivered by two pumps at a flow rate of 400 ml/min. The chambers were maintained at 25°C, which falls within the birds' thermoneutral zone (Bech & Mariussen, 2022;Pacioni et al, 2023), determined according to the procedures outlined by van de Ven et al (2013). The birds were measured in cycles, along with several baselines, with the timing and duration of measurements varying depending on the number of birds present during each session.…”

“…Following the metabolic measurements, the birds were returned to their cages and provided with water and food ad libitum. Additional details regarding calibration and the respirometry setup can be found in Pacioni et al (2023).…”

“…During the day, individual measurements of winter Msum were conducted in Gontrode on 36 individuals. The maximum cold-induced oxygen consumption (VO2) in a heliox atmosphere (79% helium, 21% oxygen) was used as the indicator for Msum, following the methodology outlined in Pacioni et al (2023). The sliding cold exposure method of Swanson et al (1996) was used.…”

“…The bird was then placed in a warm room with access to water and food. Additional details regarding calibration and the respirometry setup can be found in Pacioni et al (2023).…”