Monty Python and the Holy Grail: Airspeed Velocity of an Unladen Swallow

The ground and wind speed would be equal on days without wind, but this rarely happens.

You’ll be more familiar with these terms if you fly a lot. Of course, if you do fly, you will know that there are different types of measurements for airspeed. Here, we will use true airspeed – the speed of a craft, or bird, relative to the air it’s flying through.

We need to know the Strouhal number to determine a bird’s airspeed. When applied to animals, this metric is typically used to estimating the swimming speed of a fish or other marine animal but can be applied to any animal moving through a fluid (and an air column, while gaseous, is still a fluid).

For an animal or insect in flight, the Strouhal number is determined by the frequency of wing strokes multiplied by the amplitude of the wing, divided by the animal’s forward speed through the air. The amplitude is the distance covered by the wing in one beat, while the frequency is the number of times the bird beats its wings each second.

In that situation, it is the ratio of the animal’s forward speed to the frequency of its tail movement. Because natural selection is likely to tune animals for high propulsive efficiency, the range of Strouhal numbers for animals in water tends to fall within a narrow band.

However, Graham K. Taylor et al discovered that the Strouhal number for birds and other flying animals also converges on the same narrow range (which usually peaks within the interval of 0.2 to 0.4), although only when cruising. From this, they concluded that the Strouhal number “seems to be a general principle of oscillatory lift-based propulsion.”

Taylor advised inverting the midway Strouhal number to obtain an approximate airspeed (about 0.3, the average between 0.2 and 0.4). In other words, the airspeed is roughly three times the sum of the frequency and amplitude. This effectively gives us the “cruising speed” of any creature that flies or swims.

Regarding a swallow and its “airspeed velocity”, we first need to know which bird we are talking about. Hold on tight, and this will get a little pedantic.

You have been warned.

Calculating the airspeed velocity of the swallow

First, we must determine which bird best fits those discussed in the movie.

The first problem is getting our hands on the correct birds in question.

Only two species of swallow—the West African swallow (Hirundo domicella) and the South African swallow (Hirundo spilodera), sometimes known as the South African cave Swallow—are named after Africa. This is a little strange, as 47 of the 74 swallow species can be found on the continent.

Since the question in the film is attempting to compare two species of swallow most commonly geographically restricted to either Europe or Africa, we will need to find the species that best fit these requirements.

Thankfully, the South African swallow potentially fits this requirement.

This bird has a relatively small range and usually migrates only as far North as Zaire. So, we’ll run with that for now.

However, we could also use the West African swallow.

All well and good, but almost no data recorded actually indicates the airspeed of either type of African swallow. Rather than make blind guesses about the African swallow, it may be better to look into the capabilities of the European, or barn, swallow, for which extensive study data exists.

According to research from the Avian Demography Unit of the University of Capetown, the European swallow has an average length of 4.8 inches (12.2 cm) and an average mass of about 0.7 ounces (20 grams). However, this is around seven times smaller than the “5-ounce” (about 142-gram) bird mentioned in the film.

This is a huge problem, as the largest swallow species, like the North American “Purple Martin,” rarely exceeds 2 ounces (about 55 grams) in weight. However, that swallow never visits Europe or Africa, so that’s out. It does have a large range across the Americas, however.

So, what are the largest European or African swallows with limited ranges to their respective continents?

Well, we have two potential candidates: –

• The “Mosque Swallow” in Africa. This bird has a limited range to Africa and is the largest swallow species on the continent. Adults can grow up to 9.4-inches (24 cm) long and tend to have a maximum weight of 1.94-ounces (55 grams).
• The “Barn Swallow” in Europe. This is one of the most widely distributed swallow species in the world. It has a typical length of 7.1 inches (18 cm), a wingspan of up to 13.4 inches (34 cm), and a weight of up to 0.8 ounces (25 grams).

Based on this, we have to give the Monty Python crew a slapped wrist for suggesting a swallow species that doesn’t exist. However, since the film is based in the Middle Ages, are there any now-extinct swallows that could fit the bill (pun intended)?

A cursory look at some of the known extinct birds over the last few hundred years shows that no swallow types of the size needed to appear to have existed. So that’s out.

So, where are we with this?

The largest swallow that could be chosen is the African mosque swallow. So, using that bird as our proxy for the swallow in question, let’s test the other claims made in the film.

Using the information mentioned in the videos above, we will estimate that “a swallow” flaps its wings 43 times every second to maintain the necessary airspeed velocity. Since the guard is referring to an African swallow, not a European swallow (“Oh, yeah, an African swallow maybe, but not a European swallow. That’s my point.”), how does this stack up with reality?

Examining research by zoologist C. J. Pennycuick, we can find relevant data on the European swallow in a journal article entitled ‘Predicting Wingbeat Frequency and Wavelength of Birds’. While the European swallow wasn’t examined explicitly in his study, a swallow of the same average weight was.

This 0.7-ounce (20-gram) swallow was found to flap its wings about 12 times every second, with an amplitude of 4.8 inches (20 cm) each time.

So, it appears the guys at Monty Python are way off here. We can give them a little credit, considering that the swallow observed in this research wasn’t precisely what King Arthur was talking about. 

Now that we have the number of beats per second of the swallow’s wings and the amplitude, we can estimate the airspeed. Each beat of the swallow’s wings carries it some distance forward.

According to various studies, there is no way to know precisely how far the average European swallow’s wing flap propels it, but we can estimate about 2.46 feet (0.75 meters) per beat. When we take the number of beats per second (12) and multiply it by the distance per beat (0.75), we get a value for the velocity of 29.5 feet per second (9 meters per second).

Translating this into miles per hour, we see that the airspeed velocity of a European swallow is 20.1 mph. This number also lines up with the measured velocities of real birds, which means we’re on the right track for the mathematics.

So, answering the main question here, the “airspeed velocity” of an unladen swallow (European) is something like 20.1 miles per hour (32.4 kph) or 29.5 feet per second (9 meters per second). However, some have been clocked at 46 mph (74 kph) in the past!

But, the guards admit that an African swallow could manage the feat. So what about the airspeed velocity of the Mosque swallow?

Sadly there is limited, if any, relevant data available for this swallow, so we’ll have to scale up the results from the Barn Swallow.

The mosque swallow is about 2.2 times larger, so, all things being equal, that would give us an airspeed velocity of about 40.2 miles per hour (64.7 kph) or 59 feet per second (18 meters per second).

But this is a very rough estimate.

But what if the swallow was laden by carrying a coconut to Europe?

Could the swallow carry a coconut?

Now that we understand the bird’s possible airspeed, let’s assume that, theoretically, a European swallow could find a coconut in its normal habitat. Would it even then be possible for a swallow weighing 0.7 ounces (20 grams) to pick up a coconut and transport it to Europe?

For reference, the coconut referenced in the film is 1 pound, which equates to 16 ounces, or 453.59 grams (almost half a kilogram). Coconuts typically weigh 1 and 1/2 pounds (680 grams).

As seen in the video, assuming a large coconut is a fairly absurd proposition. There’s no way that a bird as small as a swallow could carry a coconut that was more than double its weight. However, we may be able to make the math work here. 

Since we’re already dealing with an absurd scenario, let’s imagine that the swallow found the world’s smallest coconut ever recorded, weighing in at 0.14-ounce (3.847-gram), according to the India Book of Records. Again, since the swallow postulated in the scenario is about twice the size of even the largest living swallow, we can scale down the coconut too.

So then, the question becomes, can a 0.7 ounces (20-gram) swallow carry a 0.14-ounce (3.847-gram) coconut?

Well, maybe.

A 0.7-ounce (20-gram) swallow carrying just one-fifth of its weight seems more probable than a 0.7-ounce (20-gram) swallow carrying a coconut more than sixteen times its weight. However, a lot would have to line up to make this unlikely scenario happen.

Some sources claim that a small swallow can carry about 1 ounce and still fly. If true, a single European Swallow is woefully ill-equipped to carry a coconut, even a small one.

Now, if we were to trade out the European Swallow with a Mosque swallow, the chances of this becoming more realistic are increased considerably, about 2.2 times. However, even the largest Mosque Swallow would likely struggle with a typical coconut, which is more than 8 times its weight.

If the payload was a similar record-breaking small coconut, it might be possible; however, assuming a “small swallow” able to carry 1 ounce was scaled up to the size of a Mosque swallow.

Of course, you’d need to train the swallow to ignore its instincts and fly over far longer ranges than it is typically used to. But here is where it gets interesting.

The Monty Python clip does suggest two swallows working together to carry a single coconut over a long distance. If both of these birds were Mosque Swallows, and the coconut was as small as the one mentioned above, this might be possible.

Despite the incredible improbability of the scenario, it would theoretically be possible for several large European swallows or a pair of mosque swallows to somehow come across a tiny coconut and carry it to the destination where King Arthur would have found it.

As it turns out, Monty Python has a little bit of truth in it, but only just barely.

Chances are there are a few other angles to this problem as well. If anyone out there wants to find an African Mosque swallow and measure its airspeed with absolute certainty, we could probably answer the original question more accurately.

Even better, if anyone out there wants to train a swallow, or a pair of swallows, to carry a small coconut, you likely would have a viral video on your hands. Happy bird watching!