Raptors use thermals to travel by circling within columns of rising warm air to gain altitude without expending the high metabolic energy required for flapping flight. Once they reach a sufficient height, they glide forward, converting their potential energy into forward momentum. This 'soaring' strategy allows large raptors to cover vast distances during migration with significantly lower energy costs than continuous flapping, which is energetically prohibitive for their body size and wing structure.
Thermal soaring is a specialized form of gliding where birds harvest energy from the environment rather than generating it through muscle contraction. A thermal is a vertical column of warm air that rises because it is less dense than the surrounding cooler air. For a raptor to gain altitude, the upward velocity of the air must exceed the bird's natural sink rate—the speed at which it would fall if it simply glided in still air.
Because thermals are often narrow in horizontal dimension, raptors must fly in tight spirals to remain within the updraft. This circling behavior is the most visible indicator of thermal use. During these circling bouts, the bird's wings are typically outstretched, and active flapping is minimal or absent. Research on the Northern Bald Ibis, a species that uses similar soaring strategies, showed that circling events accounted for 29–33% of the total altitude gain accrued during migratory flights. This demonstrates that thermal soaring is a critical component of energy economy during long-distance travel.
The metabolic cost of flapping flight is several times higher than the resting metabolic rate. In contrast, soaring-gliding is considerably cheaper in terms of energy than flapping flight, costing about two to three times the basal metabolic rate (BMR) in large birds. For smaller raptors like the Eurasian Kestrel, which can use both flapping and hovering, the energy expenditure during directional flight and wind hovering is similar, allowing them to use a mix of strategies. However, for larger raptors like eagles and vultures, the energetic constraints of flapping make soaring the primary mode of long-distance travel.
Raptors also use intermittent flap-gliding, where phases of active flapping are interrupted by short gliding phases. This reduces the effective wing beat frequency by 13–20%, further conserving energy. During thermal circling, the effective wing beat frequency can be reduced by 45–65%, as the bird spends most of its time gliding within the updraft.
| Flight Mode | Relative Energy Cost | Typical Application |
|---|---|---|
| Flapping Flight | Highest (several times BMR) | Short bursts, maneuvering, or when no lift is available |
| Soaring-Gliding | Low (2–3 times BMR) | Long-distance migration and commuting in large raptors |
| Hovering | Similar to directional flight in small raptors | Hunting in species like the Eurasian Kestrel |
Raptors detect thermals primarily using visual cues. They may spot other birds circling in the same thermal, which serves as a visual beacon indicating the presence of rising air. Additionally, raptors rely on their keen eyesight to identify landscape features that often generate thermals, such as sun-heated open fields or rocky outcrops.
Navigating thermals requires precise control. Raptors must balance the need to stay within the narrow column of rising air with the need to maintain forward momentum. They do this by adjusting their bank angle and wing position. If the thermal is weak, the bird may need to flap intermittently to maintain altitude. If the thermal is strong, the bird can glide continuously, gaining altitude rapidly. For example, Golden Eagles have been observed to switch between thermal and orographic lift depending on wind conditions, demonstrating a flexible approach to energy minimization.
Observing thermal soaring in the field is a rewarding way to understand raptor behavior. Here is a concrete checklist for identifying and studying this phenomenon:
When observing thermal soaring, try to estimate the bird's climb rate. In a study of Northern Bald Ibis crossing the Alps, average climb rates during circling bouts ranged from 1.0 to 1.5 m/s, with maximum rates reaching up to 2.5 m/s in strong thermals. By tracking the bird's altitude over time, you can gain a better understanding of the strength of the thermal. For instance, Verreaux's Eagles in flat areas of the Sandveld region have been shown to preferentially use thermal soaring, making these open landscapes prime locations for observation.
To make your observations more useful for conservation and research, log the time, location, and specific behavior of each bird you see. You can contribute your data to citizen science platforms like eBird or local raptor monitoring programs, which help track migration patterns and soaring behavior across regions.
Next Step: Try observing at a local ridge or open field between 10 AM and 2 PM on a sunny day. This window captures the peak thermal activity when the ground has absorbed sufficient solar radiation to generate strong updrafts.
Not all raptors use thermals in the same way. Smaller raptors, such as falcons and kites, can use a mix of flapping and soaring, and may not rely as heavily on thermals for long-distance travel. Additionally, raptors may use orographic lift, which is wind deflected upward by landscape structures like mountain slopes and cliffs, instead of or in addition to thermals. The choice between thermal and orographic soaring depends on the topography and meteorological conditions in a given area. For example, Golden Eagles react to increased wind speed by using more orographic lift and less thermal lift, thereby minimizing energetic expenditures.
It is also important to note that thermal soaring is not the only strategy raptors use to conserve energy. They may also use formation flight, where they fly in echelon or U-shaped formations to reduce drag, or they may adjust their flight speed and altitude to minimize energy expenditure. The specific strategy used depends on the species, the environment, and the individual bird's experience and condition.
