🧬 biology

Hotter temperatures constrain movements in a top avian predator of the African Savanna

Using GPS tracking data from Martial Eagles in Kruger National Park, this study reveals that rising temperatures constrain adult movements to mitigate heat stress rather than to provide shade for chicks, a behavioral shift that has become more frequent over the last 17 years and threatens chick provisioning and broader predator-prey dynamics.

Original authors: Santiago Zuluaga, Megan Murgatroyd, Arjun Amar

Published 2026-09-30

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Santiago Zuluaga, Megan Murgatroyd, Arjun Amar

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 ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed.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

Heat is a silent force that shapes how animals move, hunt, and survive. For creatures that rely on the air to travel, the atmosphere offers a double-edged sword. On one hand, rising temperatures create rising columns of warm air, known as thermals, which allow large birds to soar effortlessly for hours without flapping their wings. This is a free ride that saves energy. On the other hand, extreme heat poses a direct physical danger. Animals must avoid overheating, which can lead to exhaustion or death, often by seeking shade or reducing their activity. When a parent bird is raising a young one, these pressures become even more complex. The adult must balance its own need to stay cool with the urgent need to find food for a growing chick. In the vast, sun-drenched savannas of Africa, where temperatures are climbing, scientists are asking a critical question: as the world gets hotter, do these soaring predators fly further to catch the free ride, or do they stay grounded to avoid the burn?

A team of researchers set out to answer this by studying the Martial Eagle, the largest eagle in Africa and a top predator of the savanna. These birds are masters of the sky, with wingspans that can reach nearly two meters, and they hunt a variety of prey, from large reptiles to medium-sized mammals. The scientists focused on five female eagles in South Africa's Kruger National Park, equipping them with small GPS trackers that recorded their location and altitude every hour during the day. They watched these birds over thirteen successful breeding events between 2014 and 2024, tracking exactly how far the mothers flew, how high they climbed, and how much time they spent at the nest while their chicks grew from hatchlings to nearly full size. The goal was to see how the daily maximum temperature influenced these movements.

The results revealed a clear and worrying pattern. Contrary to the idea that hotter days would help these eagles fly further by providing better lift, the opposite happened. As the temperature rose, the eagles moved less. They took shorter steps, flew lower, and traveled shorter distances from their nests. The data showed that this change was not a straight line; there was a specific tipping point for each bird, depending on the age of its chick. For mothers with older, larger chicks that needed more food, this tipping point arrived at a lower temperature, around 26°C to 28°C. Once the air got this warm, the eagles began to pull back their activity. For mothers with younger chicks, they could tolerate slightly higher temperatures before they started to limit their movements, but the trend was the same: heat made them stay close to home.

The researchers also looked at why the birds were behaving this way. They considered three possibilities. First, the birds might be staying put to cool down themselves. Second, they might be staying at the nest to shade their chicks from the sun. Third, they might be flying more to take advantage of the better air currents. The evidence strongly supported the first idea: the eagles were limiting their movements to protect themselves from overheating. The data showed that as temperatures climbed, the time the mothers spent at the nest actually decreased, ruling out the idea that they were staying behind to provide shade for their young. Instead, the birds were reducing their own activity to avoid the physical stress of generating too much body heat while flying. This suggests that the energy saved by soaring on hot thermals is not enough to outweigh the risk of overheating when the birds are active.

This behavior has a ripple effect that goes beyond just the eagle's comfort. The study found that the number of days exceeding these critical temperature thresholds has increased significantly over the last seventeen years. In the hottest months of the breeding season, more than half of the days now reach temperatures high enough to force the eagles to restrict their movements. This means that for a growing portion of the year, these top predators are physically constrained from hunting as far or as frequently as they might need to. While the eagles might switch to hunting different types of prey, such as reptiles that are more active in the heat, this shift could alter the nutritional quality of the food provided to the chicks. If the diet changes or the frequency of feeding drops, the survival and development of the next generation could be at risk.

Ultimately, this research paints a picture of a top predator struggling to adapt to a warming world. The Martial Eagle is not just a symbol of the African sky; it is a barometer for the health of the entire savanna ecosystem. When the heat becomes too intense, even the most powerful flyers are forced to the ground, and the delicate balance of who eats whom begins to shift. As global temperatures continue to rise, these constraints on movement could disrupt the food web in ways that are only just beginning to be understood, threatening the stability of the very landscapes these magnificent birds call home.

Problem Statement
In the context of global climate warming, exposure to high temperatures (thermal risk) is a critical factor influencing the movement behavior of predators and prey, with broad implications for ecosystem dynamics. While lethal heat effects are well-documented, sub-lethal effects—such as reduced foraging efficiency or altered breeding success due to behavioral adjustments—remain less understood. For soaring birds, there is a competing theoretical framework: while high temperatures may force animals to reduce activity to avoid heat stress (the "landscape of heat" hypothesis), they may also improve the "energy landscape" by enhancing thermal uplift, potentially reducing the energetic costs of flight. This study addresses the gap in understanding how rising temperatures affect the movement behavior of resident, breeding raptors, specifically focusing on the trade-offs between thermoregulation, parental care, and foraging efficiency.

Methodology
The researchers analyzed GPS tracking data from five adult female Martial Eagles (Polemaetus bellicosus) in Kruger National Park, South Africa, covering 13 successful breeding events between 2014 and 2024. The study focused on the chick-rearing period (days 1–100 post-hatch), a time of peak energy demand.

Data Collection: Eagles were fitted with GPS trackers (weighing <2% of body mass) recording fixes at least hourly during daylight. Data were regularized to ensure 60-minute intervals to reduce temporal autocorrelation.
Variables: Five daily movement metrics were derived: mean step length, maximum distance from the nest, mean flight height, relative time spent at the nest, and relative movement frequency.
Environmental Data: Daily maximum temperatures ( $T_{max}$ ) were obtained from a weather station approximately 128 km from the nests.
Statistical Analysis: Mixed-effect models (LMM and GLMM) were employed to test the relationship between movement metrics and daily $T_{max}$ , controlling for chick age. The authors tested for both linear and quadratic (second-order polynomial) relationships. They also calculated temperature thresholds where behavioral changes occurred and analyzed long-term trends (2008–2024) in the proportion of days exceeding these thresholds.

Key Contributions and Results
The study provides empirical evidence that higher temperatures constrain the movements of a top avian predator, largely supporting the "landscape of heat" hypothesis over the "improved energy landscape" hypothesis.

Quadratic Relationships: For four of the five movement metrics (mean step length, maximum nest distance, mean height, and relative movement frequency), the relationship with temperature was quadratic. Movement increased or remained stable at moderate temperatures but was significantly constrained (reduced step lengths, lower flight heights, and less time moving) as temperatures exceeded specific thresholds.
Temperature Thresholds: The thresholds at which negative behavioral responses emerged varied by chick age and metric:
- Mean Step Length: 26.4°C – 29.7°C.
- Mean Height: 26.3°C – 28.9°C.
- Maximum Nest Distance: 28.5°C – 33.1°C.
- Relative Movement Frequency: ~27°C.
Interaction with Chick Age: The behavioral response was plastic and modulated by internal motivation. Females tending older chicks (which have higher energy demands) began to constrain their movements at lower temperatures (~26–28°C) compared to those with younger chicks. Conversely, flight height constraints began at lower temperatures for females with younger chicks.
Rejection of Alternative Hypotheses:
- Hypothesis 2 (Shading): The study found no support for the idea that eagles reduce movement to shade chicks. Instead, time spent at the nest decreased as temperatures rose, suggesting that brooding for warmth is the primary driver of nest attendance, which relaxes as chicks thermoregulate.
- Hypothesis 3 (Energy Landscape): Contrary to the prediction that warmer temperatures would increase movement via improved thermals, eagles moved less frequently and flew lower in hotter conditions, indicating a shift toward perch-hunting and reduced activity to minimize heat gain.
Long-term Trends: Analysis of 17 years of weather data (2008–2024) revealed a significant increase in the proportion of "hot" days (exceeding the behavioral thresholds) during the breeding season. In peak months (September–November), more than 50% of days now exceed these critical thresholds, a substantial increase from previous decades.

Significance and Claims
The authors claim that this study represents a critical first step in understanding how rising temperatures affect top avian predator-prey systems. The primary significance lies in demonstrating that behavioral thermoregulation in breeding eagles involves a trade-off between self-maintenance and parental provisioning.

The paper posits that as global temperatures rise, the increasing frequency of days exceeding these thermal thresholds will likely force eagles to curtail foraging movements more frequently. This could lead to:

Dietary Shifts: A potential shift from mammalian/avian prey to reptilian prey (which are more active in heat), potentially altering the nutritional quality of food provided to chicks (e.g., reduced essential fatty acids).
Fitness Consequences: Potential impacts on chick development and post-fledging survival due to altered food provisioning rates or quality.
Ecosystem Disruption: As top predators, changes in Martial Eagle behavior could initiate trophic cascades, altering predator-prey dynamics and ecosystem functioning across African savannas.

The authors emphasize that these findings highlight the necessity of exploring quadratic relationships in temperature-behavior studies, as linear models may obscure the critical thresholds where behavioral constraints begin.

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