Growth in 2 phases

The juvenile falcons of Uccle are celebrating their 5th week on Monday, which means in 7 days they will fledge!

In fact, the growth of a Peregrine falcon takes 6 weeks. During these 42 days, the falcon chick, which weighs between 35 and 40 g at hatch, will reach the average weight of 675 g if it is a male and 1 kg if it is a female! In the meantime, the first white down will be replaced by a second grayish down and then replaced by a full plumage that will allow the young Peregrine to become the best aerial hunter in the world!

Growth is divided into two main phases. During the first 3 weeks in the nest, the chicks are growing, getting bigger. Five days after hatching, they've already doubled in weight! At three weeks, they weigh about 85% of their fledgling weight, or around 575 g for a male and 850 g for a female. On average, falcon chicks therefore gain 35 g per day during their first 3 weeks of life. It is at this moment that they are ideally banded, first because their legs are the perfect size for the positioning of the rings, and then because the sexual dimorphism, in this case the difference in weight between the male and the female, is well marked.

The second phase, which also lasts 3 weeks, is going to be very different. It is no longer a question of getting bigger but of developing a plumage with several thousand feathers. The number of feathers that make up the plumage of a Peregrine Falcon is not exactly known, but it is estimated between 5,000 and 8,000! <<Producing>> so many feathers requires a tremendous amount of energy and that is why during the first three weeks of life, the falcon chick grew so much. Their body had to grow to such a size that it could function as an effective "feather-making machine". From the age of 3 weeks, most of the energy developed by the body of young falcons will therefore be devoted to allowing the growth of these feathers. The remiges (the long wing flight feathers) will grow 2 to 5 mm each day! Gradually, but in the end quickly, the second down will drop and be replaced by a complete plumage that will allow the juvenile falcons, at the age of 6 weeks, to take their first flight and therefore to hunt and therefore to ... live or survive! In the end, the wingspan of a female will reach 110 cm while that of a male will reach around 95 cm. After fledging, falcons, like all birds, no longer grow. They have reached their "normal" height and weight.

This growth in 2 distinct phases is another example of the incredible phenomenon of biological evolution!

The flight of the falcons from Uccle is therefore expected to happen from Monday 24 May onwards. If you are at the beginning of next week at Saint Job square, take your binoculars and camera. It will be a spectacle because being covered with a plumage of 8000 feathers is one thing, knowing how to use is quite another !!!

Fledging at the ULB is expected to begin on May 31, as there is one week of delay between the hatching of the two broods.