The main advantage of flight, as a means of locomotion, is its speed, which is much faster than the alternatives of swimming, walking, or running. Whether by flapping or gliding, flight facilitates rapid long-distance travel, and allows birds to accommodate migration as a twice-yearly event within their annual schedules. Nevertheless, birds vary greatly in the lengths and types of journeys that they can undertake, depending on their body size, wing shape, muscle power, and other aspects. These various features influence the speed and mode of flight and the amount of fat that birds can carry as an internal body reserve to fuel their journeys.

How flight speed is measured?

The flight speeds of birds usually been calculated using a vehicle like a car or an airplane traveling alongside the bird, or it can also be tracked by using radar the movements of particular flocks or individuals. Radar measures are the most accurate, and can be applied directly to birds on migration. They can also be corrected to allow for wind effects (as measured by the use of air balloons) and flight altitude (as measured by the radar). The such correction makes different values more comparable, for both wind and altitude can affect the speed of flight. 

Flight speed variation capture

The flight speed of a bird tends to increase with rising altitude as the air density decreases and the bird then has to fly faster in order to generate the same forces required to support its weight and propel itself forward at lower altitudes. In addition to measured values, theoretical flight speeds can be calculated from aerodynamic principles on the basis of body mass, wingspan, and wing area. These various sources of information, whether measured or estimated, all indicate that larger birds generally fly faster than small ones, although body and wing structure has an additional influence.

Flight speed and adult birds

This relationship between flight speed and body weight arises because each unit of wing area has to carry more weight in a large bird since the ratio of surface area to volume is less. Therefore, the air has to flow faster over the underwing surface to provide the necessary lift, which the bird achieves by flying faster. 

As a rough guide, 

10 g birds cruise in flapping flight at around 32 km per hour, 

20 g birds at around 40 km per hour, 

100 g birds at around 43 km per hour 

and 1,000 g birds at around 54 km per hour. 

In practice, however, species of similar weight vary considerably in their actual flight speeds, according to body and wing shape and other features.

Wing role in flight speed

Wings vary between long and thin or short and broad according mainly to the needs of everyday life. Largely for reasons of wing design, terns, harriers, and owls fly slower than expected from their body weights, while pigeons, ducks, and auks fly faster.

In addition, individual birds can vary their flight speed and wingbeat frequency according to prevailing conditions and intent, and some can also switch between different types of flight, such as flapping and gliding. The result is that some species can change their flight speeds in still air by more than threefold, with wind effects adding yet further variation. 

Power in flight and speed

The power requirement changes accordingly as birds change their speed and motion of flight. The power is generated by the flight muscles, which are in turn fuelled by stored carbohydrates (glycogen) and fatty acids delivered to the muscles by the bloodstream. Within species, power requirements do not increase in direct proportion to the speed of flight. All birds have a certain speed at which the power consumed for flapping flight is minimal, and if they fly either slower or faster than this speed it requires more power. This is the reason why birds migrating won’t usually fly at the maximum speed even if they are capable.

Flight for migration

Birds are likely to fly either at ‘minimal power speed’, which gives the minimal rate of fuel use and hence maximum time airborne, or at the somewhat faster ‘maximum range speed’, which gives the longest distance on a given amount of fuel. These two speeds span the most useful range of flight speeds for migration, and measured flight speeds from birds in the field are generally fairly close to their estimated maximum speed range (small birds fly slightly faster than expected than large birds which are slightly slower). On migration, birds are likely to fly substantially faster than the maximum range speed only in special circumstances, such as when countering a headwind or fleeing a pursuing falcon.

Power requirements in relation to body weight

Using aerodynamics, relationships between speed and its power requirement have been calculated relative to a wide range of bird body weights. The power (P) needed for flapping flight at maximum range speed increases with body mass (M) around according to the proportion P = M 1.17. 

Since, larger birds have lower metabolic rates, flight at maximum speed is more costly in the case of larger birds. The power needed by a bird for flapping flight is often expressed as a function of its basal metabolic rate (BMR), which is the lowest rate of energy consumption that an inactive living bird can normally achieve. As expected from the above relationship, the power required to fly at minimum power speed or maximum range speed is a smaller multiple of BMR in smaller birds than in larger birds.

The upshot is that small birds have more power available to them than large birds, relative to that required to fly. This in turn means that smaller species can carry relatively more fuel for migration. If there is an increase in body weight it drastically affects the extra fuel birds can carry and as a result, it affects non-stop flight range.

The proportionate weight of fuel that can be carried by a bird at maximum range speed has been estimated to decrease linearly with increasing body size, down to none at about 6 kg. Birds with fat-free bodies and weights 6 kg, also carrying fuel reserves, fly at speeds slower than their maximum speed range. Species with fat-free body weights less than 

750 g will double their weight through fuel deposition and still have sufficient power to fly at maximum range speed.

Effects of migratory fattening

The heavy fuel load carried by long-distance migrants provides the necessary energy, but it also obliges the birds to fly faster if they are to achieve maximum range. Birds doubling their lean body mass through fuel deposition (as some do) should theoretically increase their maximum range speed at the start of the flight by about 1.4 times. However, according to calculations, at the start of the flight, the power output by the muscles must be increased almost 2.8 times to achieve this.

During a long flight, in which body mass declines through fuel consumption, the bird would be expected to gradually reduce its cruising speed (and power output) so as to achieve the maximum possible range. Reduction of flight speed has been confirmed in radio-tracked Brent Geese during migration. However, few birds are likely to manage to fly at their maximum range speed throughout a long flight, especially with a heavy fuel load, which would require exceptionally hard work and might put big demands on the heart and lungs. To begin with, they are therefore more likely to fly at the slower minimum power speed.

Cutting the costs of flight

Interesting facts about V-formation

Some large birds also use this method of flying to cut their fuel costs by flying in line or V-formation. This is usually among geese, swans, gulls, cranes, pelicans, cormorants, and others. Each bird flies behind and one in front and one in the side, benefiting from its slipstream, gaining lift and decreased drag. This is possible because each bird sheds vortices from its wingtips which gives a lift to the one behind. Individuals flying in V-formation have been estimated to save 12–20 percent on energy costs compared with birds flying alone. But the bird leading has no such advantage in power saving, and they frequently relinquish their position by pulling out and joining the line further back. Other birds migrate in flocks of varying shapes, which may again save on energy costs.

Some large birds reduce their fuel costs by soaring and gliding on outstretched wings, making use of updrafts to climb and remain aloft, and gliding, losing height, from one place of the updraft to another. Updrafts are mostly produced by horizontal winds deflected over cliffs and slopes and also by thermals (series of rising air caused by heating of the ground unevenly). For some large bird species, soaring–gliding flight affords massive energy savings, as their flight costs are only 5–25 percent of the requirements estimated for continuous flapping. The heavier the bird, it became more difficult to create lift for flapping flight by relying only on muscle power, and the greater the energy saved from switching from flapping to soaring. This may be the reason, the soaring flight is seen predominantly in large birds, such as pelicans and eagles.

Flapping and Soaring flight

Flapping flight

Whether a bird flaps or soars influences many aspects of its migration. In a continuous flapping flight, a bird must generate the forces which sustain its weight against gravity and provide the thrust required for forward propulsion against friction and further drag. The power for both the forward thrust and the lift is produced by the breast muscles, and directional control ability is provided by the wings and predominantly by the tail. Provided with sufficient fuel, some birds migrate by flapping flight can travel for hours or days on end. They can cross water or other adverse terrain and can fly by night as well as by day, and at high as well as at low altitudes.

In moving between their wintering and breeding places, birds often travel directly, prefer the shortest routes and concentrate to some extent through mountain passes or along coasts that deviate slightly from their main direction. As flapping flight uses a lot of fuel, however, such species must normally lay down substantial body reserves, especially for traveling overseas or other inhospitable areas where they cannot feed. Continuous flapping produces heat, which helps birds without the need to burn extra fuel to keep them warm and fly at high latitudes and altitudes. In hot conditions, however, heat production from continued flapping can result in the need for evaporative cooling (panting), which can prevent overheating, but only at the expense of increased water loss and dehydration risk.

Soaring flight

The lift in the soaring flight of birds works by, forward motion from the gravity and rising air currents, this still-wing mode of flying

requires considerably less internally generated energy than continuous flapping. Many soaring species use a mix of gliding and powered flight, with intermediate costs, but seek to maximize the contribution from gliding and resort increasingly to flapping as thermal conditions deteriorate.

Soaring land birds also tend to concentrate in spectacular numbers along narrow land bridges (such as Panama), or at narrow sea-crossings (such as Gibraltar or the Bosphorus), and thereby avoid spending long periods over water. They usually take long roundabout routes over the land to make most journeys over land and minimize the distance traveled by expensive flapping flights. 

Despite the greater distances, the total energy consumption by the birds is thereby much reduced. Moreover, their travel routes are determined primarily by geography and landscape, soaring birds take the same traditional narrow-front ‘corridor’ migration routes year after year, but may use different routes in autumn and spring. And because they usually travel low enough to be seen by ground-based observers armed with binoculars, they can be counted and studied in ways that other higher-flying or night-flying birds cannot. 

Birds with soaring flight

Systematic counts of migrating raptors and other soaring birds have been made annually over many years at a number of migration watch sites, including Falsterbo in Sweden.

Compared with small soaring species, such as sparrowhawks, large eagles (with greater wing-loading) have to start later in the day, when thermals are strong enough to lift them;

Sea birds and soaring flight

It is not only land birds that travel by soaring–gliding flight. Many seabirds make use of up currents formed either as the wind is deflected off waves, or as a wave of ‘swell’ displaces air upwards as it moves over the sea surface. Some seabirds use ‘dynamic soaring’, which partly depends on wind speed slowed by the sea surface. The bird first climbs into the wind, then turns to glide with the wind, gaining distance while losing height. After creating a low turn in the trough of a wave, it begins to cycle again. 

Summary

Summarising this, migrating birds that travel by flapping flight have high fuel needs but can normally 

(a) migrate day or night; 

(b) use direct and straight routes; 

(c) travel large areas of the inhospitable substrate without halting; 

(d) primarily use fat stores to migrate long distances without feeding and, if required, without resting; 

(e) migrate on a broad front rather than a well-defined, narrow route; and 

(f) travel at high altitudes. In contrast, land-bird migrants that travel mainly by soaring–gliding have greatly reduced fuel needs,

but normally 

(a) migrate mostly in the warmest part of the day and not at night; 

(b) avoid large water bodies, and often take roundabout routes that offer good wind and thermal conditions, thereby increasing the total distance covered; 

(c) migrate along well-defined constant routes, some of which are taken year after year by birds from large areas; 

Because soaring migrants require less internal energy per distance covered, they can progress more rapidly on long migrations and can travel with smaller fat reserves than flapping migrants.