Patterns of Bird Flight

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Patterns of Bird Flight
Introduction

To counteract the effect of gravity, birds exploit a fluid dynamic force called lift (Anon. 2009). Lift is created by asymmetrical wings moving through the air which produces a higher air pressure beneath the wing than above it (Knox et al. 2005). The shape of the wing heavily dictates the sort of flight the bird is proficient in, and therefore the environment in which they live (Knox et al. 2005). For example, certain wing shapes perform better in clustered environments and are more manoeuvrable, while migratory birds are less manoeuvrable and rely on gliding ability. Hence, the structure of the wing and its function are interrelated (Knox et al. 2005).

The ratio of wing span to wing width is a significant consideration in bird flight; this is termed aspect ratio (Anon. 2009). Lift is exercised over a larger area and less drag is generated, when wings are longer relative to their width (Anon. 2009). High aspect ratios imply long and slender wings and low values indicate short and broad wings (Hedesntr et al. 2009). Another feature of bird flight is wing loading, which is the relationship of body mass to wing surface area (Anon. 2009). Some birds have small wings relative to their body mass (high wing loading); whereas others have proportionately large wings (low wing loading) (Biewener 2003).

Bird Type Aspect ratio Bird Gliding frequency Gliding distance
Willie Wagtail 2:1 1.

2.
3. Low
Nil
low Short
None
short
Pelican 4:1 1.
2.
3. High
Medium
High Long
Very long
Long

Table 1 : Gliding ability in birds with different aspect ratios

Gliding speed
Bird Type Weight Bird 1 2 3
Silver Gull 290 Slow Medium Medium
Pelican 5400 Very Fast Very Fast Fast
Willie Wagtail 18 Slow Medium Slow

Table 2 : Gliding speed in birds of different weights

Aerial locomotion can be separated into unpowered flight (gliding) and energy-intensive muscle powered flight (Knox et al. 2005). Simply, gliding is flying without wing beats. Some birds we observed were only capable of gliding for a few metres, others could glide for a quite a distance, speed also varied. The largest bird observed, the Pelican, has a high wing loading of 180. A high wing loading enabled a greater gliding speed and the Pelican could stay aloft for relatively long periods of time (Knox et al. 2005). Small birds such as the Willie wagtail have very low wing loadings, and glided relatively slowly in short bursts before they fell, because in small bird surface friction and induced drag is very high (Anon. 2009). High aspect ratio wings, which normally exhibit low wing loading are ideally suited to slower, long distance gliding (Knox et al. 2005). Conversely, low aspect ratio wings are typical of explosive flight and demonstrate gliding inefficiency (Biewener 2003).

Bird Type Weight Vertical take-off Flapping wings Run-up (+ length)

Magpie Lark 85 Yes Yes 0m
Silver Gull 290 Yes Yes 0m
Pelican 5400 No Yes 4m
Willie Wagtail 18 Yes Yes 0m

Table 3: Take-off in birds of different weights

To produce the airstream necessary for list, a bird first needs to beat its wings (Anon. 2009). A small jump was only required for the magpie lark and wagtail to takeoff. Therefore lift can be realised from zero air speed (Biewener 2003). For larger birds such as the Pelican this is was not possible, and instead a run up was needed to create sufficient airflow for takeoff (Biewener 2003). Low aspect ratios and comparatively light body masses and also low wing loadings in both the Magpie Lark and the Wagtail, allowed for swift vertical acceleration and rapid takeoff.

The high weight and wing loading of the Pelican meant that it was considerably more difficult for it to generate sufficient lift, and could be seen ‘running’ across the surface of the water using its webbed feet for approximately 4 metres in order to reach the minimum air speed needed to become airborne. These leg movements were coordinated with the wing beats.

Wings Beat/min
Bird Type Weight Bird 1 2 3
Willie Wagtail 18 240 240 240
Silver Gull 290 60 60 60
Pelican 5400 25 25 25
Welcome Swallow 15 360 360 360

Table 4: Rate of wing beat in birds of different weights

Birds beat their wings to produce an airstream around them, which consequently creates lift and forward velocity (Anon. 2009). Drag is reduced and the angle of attack decreases, producing lift with each downstroke (Anon. 2009). This powered flight necessitates strong muscular contraction (Anon. 2009)(Knox et al. 2005).
The low mass and wing loading of the Swallows and Wagtails meant they had to beat their wings rapidly to stay aloft and move forward, as drag is relative to their mass (Anon. 2009). The wagtails were almost able to hover as they were able to produce large amounts of lift on both the downstrokes and upstrokes (Knox et al. 2005). Rapid flight by large heavy birds such as Pelicans is stalled by both weight and drag (Anon. 2009). By specifically orientating their wing, they were able to generate lift on both upstrokes and downstrokes; this is done by creating a supplementary airstream over them to keep them in the air (Anon. 2009). Fundamentally, larger birds oscillate their limbs at much lower frequencies than smaller ones (Pennycuick 1990).

Wing Loading Bird Type Flight Speed Manoeuvrability Habitat
High Pelican Fast Low Open water
High Pied Cormorant Medium Low Coastal/Water
Medium Silver Gull Medium Medium Coastal/Water
Medium Lorikeet Fast High Arboreal
Low Willie Wagtail Fast Very High Terrestrial
Low Welcome Swallow Very Fast Very High Terrestrial

Table 5: Manoeuvrability of birds with different wing loadings

Birds with low aspect ratios and low wing loading exhibited exceptionally high manoeuvrability. The Welcome Swallow and Wagtail especially require a fast flying pace and high manoeuvrability, as they are insectivorous and are prone to predation due to their small size. Similarly, the Lorikeet is extremely successful in a forested environment because wing shape permits tight turning amongst vegetation.

As explained before, although wings with high aspect ratios are best for gliding due to the large amount of lift produced, birds such as Pelicans are unable to occupy structurally complex environments due to low manoeuvrability (McAllister 1988), and as such thrive on open water or in coastal regions where environmental impedance is minimal. Apparently, high wing loading has an equally adverse effect on airborne agility.

Summary

We observed several species displaying different body and wing characteristics, including mass, aspect ratio and wing loading. These all influenced gliding ability, speed and manoeuvrability. High wing loading and aspect ratio is typical of heavier gliding birds that are less likely to inhabit complex environments due to their lack of dexterity in the air. Conversely, smaller birds exhibiting a significantly lower aspect ratio and wing loading demonstrated a higher manoeuvrability but were disadvantaged in gliding.

References

Anonymous. 2009. BIOL1131 Plant and Animal Biology, Section A : Animal Biology, Laboratory Manual, Schools of Animal Biology and Plant Biology. The University of Western Australia.

Knox, B., Ladiges, P., Evans, B. & Saint, R. 2005, Biology: An Australian Focus Third Edition, McGraw Hill, Sydney.

Hedesntr, A., Johasson, C., Spedding, G. (2009) Bird or bat: comparing airframe design and flight performance.

Biewener, A. (2003). Animal locomotion. Oxford University Press

Pennycuick, C. J. (1990). PREDICTING WINGBEAT FREQUENCY AND WAVELENGTH OF BIRDS

McAllister, J. (1988). COMMENTS ON THE WING SHAPE OF THE HYPOTHETICAL PROAVIAN.

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