Ringing at Uccle, small male, large females

Uccle's juvenile falcons were ringed this weekend. The family has 2 females and 1 male.

Each of them was weighed and their wing was measured. These two pieces of information make it possible to distinguish males from females.

These are indeed, at equal age, more corpulent than their brothers. They are also distinguished by much stronger legs and feet. When they reach adulthood, so in fact when they leave the nest, the Peregrine males will weigh around 650 g. while the length of their wing will be around 300 mm. Most females will (a little) exceed the kilo, for a "wing length" of 350 mm.

Why such difference? The most supported hypothesis consists in thinking that, considering that it is the female who spends the most time on the nest, or next to the nest, in order to brood the eggs or feed the falcons; that the fact of being " strong ”gives her an advantage in terms of defending her offspring against predators or competitors (other Peregrines). Over the course of evolution, the most corpulent lines of females would therefore have been favored, leading to the situation that we are seeing today.

Very interesting explanation, but then, why are the males not the same size? It would be an advantage to be two strong parents able to effectively protect the nest!? Yes, but! A complementary theory incorporates the "hunting" parameter. Indeed, Peregrines are aerial hunters. They almost exclusively capture birds in flight, in the "open" sky. Their capture method is remarkable, with chases at more than 100 km/h and dives at almost 400 km/h. Unbelievable! But this technique is relatively uniform, constant. Being able to capture a wide variety of species is therefore a definite advantage. And this is where a significant difference between the two partners is interesting. Who says difference in size says difference in agility and strength. And this is the basis for this second theory: the lighter males are more agile and therefore more effective in catching small, very fast-flying passerines such as swifts and swallows. The females, more corpulent and beefier, are all in force and therefore capable of capturing, maintaining, killing, a much heavier prey such as a curlew, a duck or ... a goose. Yes, a goose! Not a goose from Toulouse carried from a backyard, of course, but a wild goose, like a greater white-fronted goose from Siberia, which winters in the polders. And of course, when a female Peregrine catches such a prey, weighing 2 to 3 times her own weight, she is not able to transport it flying, in order to bring it to her chicks! But she is undoubtedly able to capture it, kill it and eat it on the ground.

The difference in size between male and female Peregrine would therefore be the perfect combination between hunting efficiency and defense of the brood. The fruit of tens of thousands of years of natural selection!

Let's come back to Uccle's falcons: the male weighed 682 g and his wing measured 177 mm. The females weighed 941 and 1024 g for a wing of 206 and 207 mm. Each was marked with two rings. One, on the right leg, is made of metal and is engraved with a unique number and the abbreviated address of the Royal Belgian Institute of Natural Sciences which is responsible for the Belgian Ringing Center. The other is black, marked with a code with a white inscription of 3 characters repeated three times on the circumference. This ring identifies the falcon without having to recapture it. This type of marking is used in the case of specific study programs. The goal is to be able to identify the falcon remotely, without recapturing it or finding it dead; by deciphering the code with binoculars, a telescope or ... a camera. The code for the ring of the male is AXN, that of the females is AXH and AXR.

The 2020 ringing operation could not be filmed in order to comply with the corona protection measures in force. To find out how it works, the 2019 banding film is attached to this blog.



Video 1: ringing of the falcons of the brood 2019