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Birds have developed the ability to maintain flight at various altitudes. The focus of this essay was placed specifically on flight at high altitudes. The characteristics of birds which maintain flight at high altitudes were found to be enhanced gaseous exchange efficiency, higher O2 affinity haemoglobin, and increase O2 diffusion to muscle fibers as a result of an increase in capillary-fiber ratio in a high-altitude hypoxic environment. It has also been seen that birds found at high altitudes have larger wingspans thereby reducing the energetic costs of flight in low-density air.
Key words: Altitude, hemoglobin, hypoxic, wingspan
Though not all birds are capable of flight, most birds are commonly recognized by flight. Birds are found all over the surface of the Earth (Bicudo et al, 2010) in a variety of environments which they are adapted to. Here we will explore the physiology of high altitude bird flight. The main focus will be placed on migrating birds due to the large amount of research performed on the species and migrating birds are found at a huge range of altitudes.
As stated by Bicudo et al (2010), "High altitude experienced by birds that reside or breed in high mountains or by migrating birds that must cross such ranges" are considered to be high elevation specialist's.
Factors associated with high altitude causes this environment to be especially challenging for avian flight, these factors includes "high ambient wind speeds, low air temperatures, low oxygen availability and low air density" (Altshuler and Dudley, 2006). These factors may be detrimental in a birds' flight performance specifically on bird biomechanics (lift and drag) of avian flight (Altshuler and Dudley, 2006). A consequence of the factors may result in energetics cost being higher in the severely hypoxic and cold environment (Butler and Bishop, 2000).
The objective of this essay is to discuss the many features of birds which appear to be important for high altitude flight (Scott, 2011).
Environmental factors at high altitude
As stated by Altshuler and Dudley (2006), a gradient in altitude indicates a change in numerous environmental parameters. As altitudes increase, temperature and humidity decrease, however, day length or solar angle of incidence remains the same (Altshuler and Dudley, 2006).
An altitude increase, there is a systematic decrease O2 partial pressure (Bicudo et al., 2010). This is one of the most importance changes taking place with regards to respiratory and metabolism due to the reduced availability of oxygen. As partial pressure reduces, there is an inversely increase in the gaseous diffusion coefficient. Furthermore, wind speed increases with altitude, studies propose that birds can minimize their energy costs of flights through the use of wind assistance (Bicudo et al., 2010).
As altitude increases, there is also a noticeable decrease with in water content. This decrease in water content in cool air at high elevations may result in desiccation (Bicudo et al., 2010). Thus high oxygen demands for flight are at odds with a decreased O2 availability, air density and lift (Altshuler and Dudley, 2006).
Physiology of flight at high altitude
Flight requires a high endurance capacity, for this reason, it relies on oxidative metabolism for energy (Bicudo et al. 2010). The pectoralis muscles of a bird is considered the "flight motor" of a bird and constitute up to 35% of the total body mass (Bicudo et al. 2010). Long distance migrants "flight motor" muscles possess rapidly oxidative glycolytic muscle fibers and short distance migrants "flight motor" muscles possess muscle fibers with a much lower oxidative capacity as well as fast-acting glycolytic fibers (Bicudo et al. 2010). It should be noted that muscle capillary-per-fiber number is higher in highly aerobic pectoral muscles and less aerobic leg muscles for high altitude birds (Bicudo et al. 2010).
Flight, as a form of locomotion, is considered to be one of the most strenuous and energetically costly and having a large influence on daily energy budgets of birds (Lee et al., 2008). Thus reduction metabolic costs and power expenditure of flight is one of the greatest selective forces of bird morphology (Lee et al., 2008).
Lee et al. (2008) observed that "alterations in wing morphology could be beneficial to high altitude fliers if these changes help offset the detrimental effects of high altitude environments on flight performance". It is believed that a larger wingspan and a lower wing loading value reduce power requirements during high altitude flight (Lee et al., 2008). As body size in birds increase there will be a greater requirement for lift to sustain flight, this is only true if other ecological determinants of wing morphology are absent. Thus wing span will increase as body size increases due to lift being proportional to wing area (Lee et al., 2008).
A second school of thought maintains that larger wings are less selectively beneficial only in migrants compared to residents, reasoning being that altitude is only experienced for a small portion of their annual cycle (Lee et al., 2008). The opposing view of this is that "larger wings are presumably more energetically expensive to grow and maintain, so the detriments to having excessively large wings at low elevations may exceed potential benefits during the relatively short migration period" (Lee et al., 2008). The migratory bird, the bar-headed geese (Anser indicus), have a residual wingspan greater than that of all other species residing a lower altitudes.
Wing beat frequency is constrained to the O2 partial pressure and metabolite supply of oxygen to the muscles, efficiency of muscle contraction (Lee et al., 2008), and forces exerted on the different elements of the wing (Lee et al., 2008).
"Aerodynamic significance is the general trend of increasing wind speed with altitude" (Bicudo et al., 2010), flight can be hindered or sustained by high wind speeds depending on the direction of flight and wind (Bicudo et al., 2010). Migrant birds which generally have longer and more pointed wings have less resistance around the wing when moving through the air (Bicudo et al., 2010).
Integration of physiological systems at high altitude
Circulation at high altitude
It is thought that at high altitudes, animals exposed to the chronic hypoxia may induce pulmonary arterial hypertension, this being the result of vasoconstriction of the pulmonary arterioles, causing increased resistance in the pulmonary circulation and leading to right ventricular hypertrophy (Sillau and Montalvo, 1981). However, this varies considerably between species and within individuals of a given species such as birds (Sillau and Montalvo, 1981). At high altitude, the cardiovascular system of birds provides a efficient supply of oxygen to the tissues by changes in the cardiac output and distribution of blood in the body (Ostadal and Kolar, 2007).
The ability for birds to maintain high altitudinal flight in air with a low O2 partial pressure is largely due to the capacity for O2 to diffuse from the blood into the mitochondria in various tissues at a higher rate (Scott, 2011). This is accomplished through the haemoglobin has an increased base-line affinity for oxygen and higher ratio of capillary surface area to muscle-fibre surface area in flight muscle (Scott, 2011).
A combination of a tight mesh of capillaries surrounds the avian muscle fibres and the high degree of branching between vessels and smaller aerobic fibres (Scott, 2011). "Although these distinctive characteristics of birds should enhance hypoxia tolerance by improving the overall capacity for O2 transport, being avian is not in itself sufficient for flight at high altitudes" (Scott, 2011).
An increase in capillary to-fibre ratio developed in accordance to a decrease in fibre diameter in the high altitude (Mathieu-Costello and Agey, 1997). This reduction in fibre size is important in increasing the surface for the volume of mitochondria (Mathieu-Costello et al., 1996) and therefore a shorter O2 diffusion time (Bicudo et al., 2010).
A experiment preformed on low-elevation pigeons kept at 3800 m altitude for 5 months, it was found that there was an "increased contribution of tortuosity and branching to capillary length" in pectoralis muscles (Mathieu-Costello et al., 1998). Resulting in "capillary length and surface area/ml mitochondria in the muscle fibres was 17 and 30-40% greater, respectively" (Mathieu-Costello et al., 1998).
It was also found that small birds residing at high altitudes have an significant increase in both capillary number and branching to capillary length in comparison to birds living at sea level (Mathieu-Costello et al., 1998) such as the Tibetan chicken (Gallus gallus).
Gas exchange at high altitude
Gas transfer in the lungs
The unidirectional flow of air within the convoluted and tubular arrangement of the gas exchange components of the parabronchi and the high O2 affinity of avian hemoglobin enable birds to efficiently transfer gas in the lungs (Bicudo et al., 2010). During ventilation the relative orientation of air flow and blood occurs in a parallel or "concurrent" fashion. As stated by Bicudo et al. (2010), 'the result of these adaptations is that the blood leaving the gas exchange interface can have nearly the same O2 partial pressure as inspired air, indicating that O2 delivery in birds is not limited by the pulmonary system". Despite the reduced availability of oxygen at these altitudes, though this adaptation birds are able to increase oxygen consumption by up to 10 to 20 times (Lee et al., 2008).
An increase in the ventilation of the lungs is an important response of the respiratory system to hypoxia (Scott, 2011). The magnitude of this response is controlled by the "partial pressures of O2 and CO2 and the pH of arterial blood" (Scott, 2011). An example of this is in bar-headed geese (Anser indicus) which fly over the Himalayan mountains on its migratory flight are able to sustain flight at altitudes that would render most mammals comatose.
It should be noted that the appearance of this form of ventilation in some birds can be due to being phylogenetically constrained or through acclimation or conditioning (Bicudo et al., 2010).
Corpuscular volume in the blood of bird species adapted to high altitude is the same as that of birds at a lower altitude. This suggests that birds haemoglobin has an increased base-line affinity for oxygen (Carpenter, 1973).
Birds have an ability to maintain plasma volume of their blood during migrations, despite a significant reduction in body mass due to water loss (Butler and Bishop, 2000). Consequently, reduced "plasma volume would result in a increase in viscosity of the blood" (Butler and Bishop, 2000). This reduction in volume would increase the stress placed on the heart and thereby resulting in "inadequate cardiac output and supply of oxygen to the metabolizing tissues" (Butler and Bishop, 2000).
Energy expenditure at high altitude
According to Bergmann's and Allen's rule, the higher the altitude and the greater the body size, the smaller the limb length (Bicudo et al., 2010). However, changes in body and wing size according to these rules would also influence the power requirements for flight (Bicudo et al., 2010). As previously stated longer wingspan at high altitudes reflect the morphological response to the increased energetic demands of flight, alleviating the power requirements cost of flying in thin air (Lee et al., 2008).
Physical factors affecting flight
At low air density a higher input of power is generated for hovering flight. The increased need for power, to maintain flight, results in an increase in O2 consumption (Bicudo et al., 2010). Birds are able to maintain higher flight speeds for the same power output (Bicudo et al., 2010). This is achieved by reducing the lift caused by low air density and reducing the drag (Bicudo et al., 2010). As Pennycuick put forward, with increasing altitude the lift:drag ratio increases (Butler and Bishop, 2000). Thus as speed increases, the power required to maintain it increases with height (Butler and Bishop, 2000).
Body rhythms and energetic at high altitude
With the increase in altitude and decrease in temperature and humidity, birds must maintain the steady state of rate of heat gain and the rate of heat loss (Bicudo et al. 2010). With the external environment being colder than the birds' body temperature, birds are in a constant state of heat loss (Bicudo et al. 2010). Metabolic heat production is the largest heat source at high altitude flight and can increase up to 10-fold above the resting levels (Bicudo et al. 2010). It should be noted that heat production increases in proportion to the reduced ambient temperature to avoid overheating. Factors such as and wind conditions and transfer of heat by radiation and by conduction further influence temperature regulation (Bicudo et al. 2010).Evaporative heat loss plays a very small role in temperature regulation.
Osmotic balance at high altitude
It has been generally understood that water loss is reduced at high altitudes because of cold ambient temperatures (Bicudo et al., 2010). However, desiccation may occur from the reduced water at high elevations (Bicudo et al., 2010). This is magnified by increased ventilation which is required in hypoxia. Thus altitude itself does not in actual fact influence water vapour content, but temperature does (Bicudo et al., 2010).
Birds flying at high altitudes are faced with the challenge associated with the body temperature regulation and maintenance of water loss (Bicudo et al., 2010). Excretory water loss is a small fraction of the total water (10%) and is considered to be independent of ambient temperature and humidity (Bicudo et al., 2010).
Acquiring energy for flight at high altitude
"Migrating birds carry large quantities of fat to fuel their long flights". Birds obtain this fat from pre-migratory fattening and may result in a two-fold increase in body mass (Bicudo et al., 2010). The 'fat' is broken down through a process of lipid oxidation. Lipid oxidation in flight muscles is essential for supporting long-duration flight, however, lipid oxidation requires an increased amount of O2 to produce a given amount of ATP (Scott, 2011).
Birds which have stopover sites can alternate between periods of feeding and travel thereby maintaining their fat reserves (Bicudo et al., 2010). However, it should be noted that mass is gained, there is a decrease in an individual's migration speed and as a result, a decrease in migration success (Bicudo et al., 2010).
In conclusion, it can stated that birds that reside or migrate at high altitudes are physiologically adapted in all aspects of flight. Their various adaptations allow them to maintain flight in conditions which would render other species comatose.