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Birds of Europe: Wing loading

Definition

"Wing loading", generally speaking (this can refer to either flying animals, including birds, or airplanes), is the ratio of total mass over levitating surface area:

The total mass is typically measured in g (or kg), the wing surface area in cm² (or m²). Taking into account the Earth's gravitational force, another logical unit is Newton per square meter (or N/m²). We use the unit g/cm² here.

What levels of wing loading can one expect from birds?

Trade-offs

What determines the level of wing loading, i.e., why doesn't every bird species have low wing loading (low total mass AND large wings) so as to be excellent fliers? What compromises are being made and why?

Wing size

1. Agility/maneuverability: Large wings with enormous lift compromise a bird's ability to make quick turns.


2. Need for speed: The optimal wing shape for high-speed flight is different from a wing that produces maximum lift. So the largest wings are not necessarily the fastest. Generally speaking, the faster one wants to fly, the more back-swept the optimal wing shape will be.


3. Diving under water for prey: Wings to be used as flippers tend to be shorter and made of harder material, leading to lower airlift.


4. Need for stealth: Silent wings, as e.g. owls', are made of softer feathers which compromises lift. Corvids, on the other hand, don't care who can hear their wingbeats.


5. High metabolic rates: Birds that need a lot of fuel must be excellent fliers to quickly find sufficient food.


6. Moulting: Worn feathers need to be replaced; while the new feathers grow/develop, airlift is compromised. This can range from minor effects (in partial moults) to rendering species flightless (especially in synchronous complete wing moults).

Body mass

1. Need for power: A bigger bird with the same proportions as a smaller one will, entirely naturally, have a higher level of wing loading (see explanation below).


2. Need for food reserves: Long-distance migrants need fat reserves which increase their wing loading until consumed.


3. Need to take on a lot of food: Scavengers such as vultures may not find food on a daily basis, so when there is food available they will gorge themselves and thereby increase their wing loading drastically, to the point at which they struggle to take off.


4. Egg-laying: Females about to lay eggs carry extra weight.

Why bigger birds (of a particular species) have higher wing loading

In the listing above we mention the "need for power". In order to wield more power, a bird needs to "bulk up".

So when talking about "bigger birds" now, it must be made clear that "bigger" means that everything scales proportionally. We will not be comparing birds of different species, but different birds of the same species!

Let us consider, as an example of this, the need for power in a raptor (bird of prey). A bigger bird can subdue, kill, and carry bigger prey. In most species of raptors, in particular in falcons and goshawks, the female is the dominant provider of food (for herself and for the brood). So it is usually the female that is bigger than the male. In a pair of Brown Falcons, for example, there can be no doubt which of the two is the female (see photo).

Lateral view of a pair of Brown Falcons; male on the right, female on the left

Now to the wing loading! It is a ratio, namely of total mass over wing surface area. As a bird grows, while the proportions (such as the length of the wings compared to the length of the body) remain the same, its wing surface area will grow in two dimensions - length and width. But, at the same time, the volume of its body (also the wings, but that is not counted!) and hence, for equal density, its mass, will grow in three dimensions: length, width and height.

Consider a hypothetical bird that is 50% bigger than its partner (the above pair of Brown Falcons is not far off the mark). Instead of the partner's length, say of 2 units, the bigger bird is 3 units long. The same in width, 3 vs. 2 units. So the wing area of the bigger individual will be 3x3 units, rather than 2x2 (or 9, compared to 4). That is an increase by a bit more than a factor of 2.
In order for the two to have equal wing loading, the bigger bird's mass must then also only increase by a factor of 9/4, but in practice it doesn't. The body will grow in 3 dimensions, so the volume will scale not by a factor of 9/4, but by (3x3x3)/(2x2x2), i.e. 27/8. And that is an increase in volume by a factor of almost 3.5.
This implies that, all dimensions of a bird's body growing in proportion, the wing loading for a 50% increase in size will automatically increase by a factor of (27/8)/(9/4)=1.5 (=50%). These are the 50% coming from the growth in the third dimension. In reality, in the above example of Brown Falcons, the male's wing loading is 0.35 - the female's is 0.45 g/cm².

Hence (since a larger body is slower and harder to move than a small one), a bird, in order to gain power, trades in some of its agility. And that is the reason why a smaller bird will try to evade a bigger one by flying tight bends - that is where lower wing loading really helps.

Measurement/computation

There are two substantially different ways of calculating the levitating area. Either one counts only the surface area of the combined wings (and the area in between the wings) or one also takes into account the uplift created by the body (and the tail...) itself, and therefore counts the surface area of the body together with that of the wings.

An extreme example of this can be found in the literature, in which data of one collected specimen of a Great Bustard (Otis tarda) are used in two different ways. In both cases, the total mass is listed as 8.95 kg. However, one team of authors uses the combined surface area of the two wings (and ONLY the wings), of 5,728 cm², whereas another author used the combined wing surface area, PLUS the surface of the bird's body (which also contributes to the uplift), leading to a total surface area of 15,897 cm². NB: The body contributes almost 2/3 of the total surface area! The first method uses what is called an "isolated anatomical area" (wings only). The second makes use of the "aeronautical planform area". The use of either method makes sense in different contexts. To study wing performance, one will want to use the wing surface area only. In order to determine the total uplift, including the body's surface area, on top of the wing surface area, leads to the best results.

The two vastly different surface areas, for the same total mass, lead to two equally different values for the wing loading, of 1.56 g/cm² (isolated anatomical area) vs. 0.56 g/cm² (aeronautical planform area).

This makes it abundantly clear that, in order to compare the flight capabilities of different species of birds, one must stick to one way of measuring the surface area providing lift. If not explicitly stated otherwise, we use here only surface areas of the two wings combined, i.e. the "isolated anatomical area" method (NOT including the body).

These pages are largely based on our own observations and those of our contributors. The structure of these bird pages is explained HERE. For more salient facts on any bird species please refer to a field guide.