🌿 ecology

Air temperature limits aerobic capacity and scope in active birds

This study demonstrates that air temperature significantly limits the aerobic capacity and scope of active zebra finches by causing overheating and reduced metabolic performance at temperatures well below their resting thermal tolerance, highlighting a critical sub-lethal fitness cost of climate change for active birds.

Original authors: Engert, E. R., Nilsson, A., Andreasson, F., Nord, A., Nilsson, J.-A.

Published 2026-09-12

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CC BY 4.0

Original authors: Engert, E. R., Nilsson, A., Andreasson, F., Nord, A., Nilsson, J.-A.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

The air around us is not just a backdrop for life; it is a constant, invisible force that shapes how animals move, eat, and survive. For birds, which are warm-blooded creatures that generate their own body heat, the temperature of the air is a critical factor in their daily existence. When the air is cool, these birds must work harder to stay warm, burning energy to maintain their internal heat. When the air is warm, they face the opposite problem: they must work to cool down. They do this primarily by panting or spreading moisture across their bodies, a process that evaporates water and carries heat away. Scientists have long understood these limits for birds that are sitting still, resting in a nest or on a branch. However, the real world rarely offers such stillness. Birds must fly, hunt, and care for their young, activities that generate massive amounts of internal heat. A crucial question has remained unanswered: how does the ability to cool down change when a bird is working hard, and at what point does the heat of the day stop a bird from being able to fly or feed its young?

To answer this, researchers turned to the zebra finch, a small, common bird often kept in captivity. They wanted to see what happens inside a bird when it is forced to exercise in air that ranges from cool to quite warm. In a controlled laboratory setting, they placed individual birds into a sealed, transparent wheel. This was not a toy, but a precise scientific instrument designed to keep the bird moving. The wheel spun slowly at first, then sped up, encouraging the bird to hop and flap its wings to keep its balance, much like a hamster running on a wheel. The researchers could control the air temperature inside the chamber, testing the birds at temperatures from 12 degrees Celsius up to 36 degrees Celsius. While the birds ran, the scientists measured exactly how much oxygen they consumed, how much water they lost through their breath and skin, and how hot their bodies became. They also measured the same birds when they were resting to compare the two states.

The results revealed a surprising and limiting reality. When the birds were resting, they could handle air temperatures up to nearly 40 degrees Celsius before their bodies began to overheat. But when they were exercising, the situation changed dramatically. The birds started to overheat at temperatures that were perfectly comfortable for a resting bird. At the warmest temperature tested, 36 degrees Celsius, the birds' bodies became dangerously hot, reaching 45.1 degrees Celsius, a level that is near the upper survival limit for this species. The heat generated by their muscles during exercise was simply too much for their bodies to get rid of, even though they were panting and losing water at a rate eight times higher than when they were resting. Despite this frantic effort to cool down, the water loss was not enough to offset the heat being produced.

Because the birds could not shed heat fast enough, their bodies were forced to slow down. The researchers found that the maximum amount of energy the birds could produce dropped by 18 percent at the warmest temperatures compared to cooler conditions. Their overall capacity for activity, known as aerobic scope, shrank by 22 percent. In simpler terms, the heat of the day was physically capping how hard the birds could work. The birds did not simply get tired in the usual sense; their physiology hit a wall where they could no longer balance the heat they were making with the heat they could lose. This suggests that in the wild, as global temperatures rise, birds may be forced to reduce their activity levels during the hottest parts of the day. They might not be able to feed their young as often or fly as far, not because they lack energy, but because the air is too hot to let them release the heat their bodies create. This study provides a clear mechanism for why heat stress can lead to sub-lethal costs for wildlife, showing that the limit to a bird's endurance is often set by its ability to cool down, not just by its ability to generate power.

Technical Summary: Air Temperature Limits Aerobic Capacity and Scope in Active Birds

Problem Statement
Current understanding of thermal tolerance limits in endotherms is predominantly derived from studies on resting animals. However, this reliance on resting data may yield unrealistic predictions regarding how air temperature ( $T_a$ ) affects animals during active states, such as foraging or parental provisioning. While heat waves can cause direct mortality, prolonged heat exposure often leads to sublethal fitness costs, including missed opportunities and reduced parental investment. Although circumstantial evidence suggests that environmental temperature constrains parental effort in birds, there is a lack of experimental data directly linking air temperature, heat dissipation capacity, and aerobic capacity during sustained activity. Specifically, it remains unclear at what temperature thresholds active birds reach their thermal limits, how these thresholds compare to resting limits, and the physiological mechanisms driving these constraints.

Methodology
The study utilized zebra finches (Taeniopygia guttata) to quantify metabolic heat production (MHP), evaporative water loss (EWL), and body temperature ( $T_b$ ) under varying thermal conditions.

Subjects: Twenty-eight male zebra finches were acclimated to indoor housing (22 ºC) and implanted with temperature-sensitive PIT tags.
Resting Measurements: Resting metabolic rate (RMR) and basal metabolic rate (BMR) were measured using flow-through respirometry in a climate-controlled cabinet. Birds were subjected to a heat ramp protocol (10–40 ºC) to determine the thermoneutral zone (TNZ) and critical thermal limits.
Active Measurements: To simulate sustained activity, birds were induced to exercise continuously on a hermetically-sealed hop-flutter wheel. Experiments were conducted at four air temperatures: 12, 22, 32, and 36 ºC.
Data Collection: During exercise, the study simultaneously measured:
- Metabolic rate (via $O_2$ consumption and $CO_2$ production).
- Body temperature (via implanted PIT tags).
- Evaporative water loss (via water vapor pressure analysis).
- Endurance (time until exhaustion).
Analysis: The researchers employed "broken-stick" mixed models to identify inflection points for resting parameters and linear mixed effects models (LMMs) to analyze the impact of temperature on active physiological parameters.

Key Results

Thermal Limits: In resting birds, hyperthermia occurred only at the highest measured temperature (40 ºC), with the upper critical temperature (UCT) for evaporative water loss identified at 36.8 ºC. In contrast, active birds began to overheat at significantly lower temperatures.
Hyperthermia in Activity: During exercise, body temperature ( $T_b$ ) increased at 32 ºC and 36 ºC, reaching a mean of 45.1 ºC at the highest temperature. This is near the critical upper body temperature limit for the species. Conversely, in cooler temperatures (12 ºC), $T_b$ decreased during exercise.
Reduction in Aerobic Capacity: Maximum metabolic rate (MMR) decreased by 18% at 36 ºC compared to 12 ºC. Consequently, aerobic scope (MMR relative to BMR) was reduced by 22% at the warmest temperature (dropping from 6.0 × BMR at 12 ºC to 4.7 × BMR at 36 ºC).
Evaporative Cooling Constraints: While evaporative water loss (EWL) increased 8-fold during exercise at 36 ºC, this increase was insufficient to compensate for the substantial heat generated by activity. The evaporative cooling efficiency (EHL/MHP) during exercise was considerably lower (0.25–0.43) than maximum values observed in resting birds (up to 1.4).
Endurance: While endurance was not significantly different across temperatures in the statistical model, the mean endurance was numerically highest at 22 ºC (22.3 min) and lowest at 36 ºC (14.6 min), suggesting a potential negative effect of extreme temperatures on performance duration.

Significance and Claims
The authors claim that their study provides a mechanistic explanation for how heat limits aerobic capacity and scope in active birds. The primary finding is that active birds reach their thermal tolerance limits at air temperatures well below those that constrain resting birds. The study demonstrates that the capacity for evaporative heat dissipation is the limiting factor; when the heat produced by activity exceeds the physiological capacity for evaporative cooling, birds become hyperthermic and are forced to reduce their metabolic rate.

The paper posits that these physiological constraints offer a potential mechanism for observed reductions in parental investment and reproductive success in wild birds during hot weather. By showing that aerobic scope is constrained by heat dissipation even in "mild" temperatures (relative to resting limits), the results support the Heat Dissipation Limit (HDL) theory, which suggests energy expenditure is capped by the ability to lose heat. The authors conclude that increased air temperatures can lead to sublethal fitness costs by limiting the energy available for activity, though they note that their experimental conditions (low humidity, no solar radiation) may differ from field conditions.

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