Different Designs for Different Jobs : Birds Don’t Just Wing It!
With a few notable exceptions, wings are the primary tool for transport in birds. These highly modified forelimbs are so well adapted for producing lift and enabling flight that wings are barely recognisable as sharing any similarity with our own (until you look at the bones anyway). Even within that we call “wings”, there’s an incredible amount of variation. A robin’s wing only looks like an albatross inasmuch as it’s a fairly flat surface covered in feathers. Otherwise, their shapes are completely different. This, however, makes complete sense when we consider how differently they use their wings. In the same way the sleek, athletic legs of a horse are completely different to the clambering limbs of a monkey, birds wings are adapted for different purposes. Robins bound between trees in short bursts of energy, and so need compact wings capable of flapping quickly to generate force in sudden bursts. Albatrosses, in contrast, need to soar for weeks or even months on end over the sea, and so need large wings to generate a lot of lift without much drag, allowing them to glide efficiently.
The wings of a bird are adapted to the lifestyle they live, meaning that there are as many wing shapes (morphologies) as there are lifestyles (ecologies). In other words, form follows function, and we can almost predict the behaviour of a bird by the shape of its wings alone.
Albatrosses are the world’s most accomplished long-range gliders, spending weeks or months on the wing in search of food. As a result, their wings are adapted to generate lift effectively, while also reducing the drag they produce. This allows them to glide with minimal energetic input and at high speeds to cover as much distance as possible in as little time.
The best wing shape to allow this is a ‘high aspect ratio’ wing; one in which the wing’s span (shoulder to tip) is far greater than its chord (front to back). An albatross wing is, as we might expect, very long and thin. This shape generally generates less drag than wings with a larger chord due to differences in airflow and so-called ‘skin friction’ (caused by air rubbings against the feathers). High aspect ratio wings therefore allow very fast flight, as the bird’s speed isn’t reduced as much by the wing dragging it in the opposite direction.
By these same merits though, albatrosses also suffer from a decrease in manoeuvrability. Flying at high speed, with little drag, and using long, thin wings limits the ability of the bird to make tight turns because of what is called the ‘roll inertia’. This is essentially a measure of how resistant the bird is to changing the direction in which it’s already moving by rolling into a different direction. Imagine you’re (for some reason) running along holding some dumbbell weights with your arms outstretched. Without stopping, you’ll find it easier to change direction if you drop the weights, or bring them closer to your body, both of which would decrease your roll inertia. A bird with a large roll inertia, caused by long wings moving very fast, is going to struggle to change direction rapidly without a large input of energy, meaning that albatrosses are generally seen making slow, seemingly lazy turns.
To reduce roll inertia, some birds take the option to travel more slowly by increasing the drag on their wings through having a lower aspect ratio. In the case of vultures or kites, for example, this comes in the form of wings which are still quite long, but also quite deep from front to back (i.e. a large wing chord). This is particularly useful for these birds, as they often gain altitude by circling within columns of rising air known as thermals, and so need to be able to turn fairly tightly to stay within these columns.
High Speed Pursuits
The long wings of vultures are great for generating lift and staying aloft for long periods of time, but the length of these wings isn’t much good for flying between trees or making the sudden, tight turns required to move through a forest quickly. To do this, birds need smaller wings. This is both because they’re easier to fit in the space between trees, but also because the smaller wingspan reduces the bird’s roll inertia. Birds like Harris’s hawks can therefore rapidly change direction without as much energy input as larger bird might need, making them high speed woodland hunters.
Having shorter wings also reduces the amount of drag produced during flight, which is particularly useful for birds which need to fly fast. Nowhere is this more the case than in the case of the peregrine falcon or the gyrfalcon, two of the fastest animals on the planet. Both these birds have fairly short wings which taper down to a point at the tip, a feature which also optimises airflow over the wing to reduce drag. This, combined with an ability to tuck the wings close in to the body during stoops allows them to reach breakneck speeds as they hunt.
Small, highly tapered, low inertia wings are also characteristic of some of the most unique birds: hummingbirds. The low drag, lightweight wings (inertia reduction coming into play again!) of these animals combine to minimise the muscular effort required to move the wings, and also allow them to be flapped very fast. Over 80 times per second in some species. Despite having small wings for a bird of their size, they can still generate a lot of lift just by virtue of moving them quickly, and so they move a lot of air in a given space of time.
An understanding of why the wings of different birds vary so much, and what abilities they give their owners, allows us to decide how best to design our bio-inspired mechanisms at Animal Dynamics. In learning how animals achieve their remarkable and almost effortless movements, we can apply these principles to create effective and hyper-efficient machines.
By Jonny Page