The brood patch, the hatching muscle and what’s occurring inside our peregrine eggs

With freezing night-time temperatures there may still be some of you that are concerned about the impact of the cold on our peregrine family. As Nigel rightly pointed out in his recent blog, peregrines thrive in a wide range of climate zones across the globe. In terms of keeping their eggs warm, peregrines, like many other species have evolved a special adaptation – a featherless area on their breast called a brood patch.

Inside the egg, the developing peregrine chick has its head tucked under its wing

In this area the skin becomes thicker and there is an increased blood flow to help transfer body heat from the adult bird to the embryos inside the eggs. The more regular viewers amongst you might have even seen the female lightly plucking her brood patch to help her keep the eggs warm.

Over the next few days it is likely that the female will switch from incubating the eggs to keeping her chicks warm – and I’m sure that thousands of you have been avidly watching the peregrine camera secretly hoping to be the first person to spot the chicks hatch.

Until that moment arrives, I thought it would be interesting to highlight just what is going on inside the eggs. Whilst they are being incubated, a developing peregrine chick has its head tucked under its wing. Chicks also have a large muscle called, rather appropriately, the hatching muscle, which runs from the middle of the neck right to the top of their heads.

When the eggs have been incubated for about 30 days (any-time now) this muscle starts to contract. This makes the chick’s head snap upward and the egg tooth, a hard pointed area on the top of the beak, comes into contact with the eggshell, causing it to crack.

As the egg tooth pushes against the shell it creates a small hole in the surface and cracks then begin to spread across it. A day or two after making the initial hole in the shell, known as a ‘pip’, the chick starts to move around inside the egg. As the chick turns around the egg tooth presses against the inside of the shell, eventually cutting a line right the way around. Once this is complete the chick is able to break out.

The chicks hatch over a period of a couple of days and generally the differences in size between those in a brood is smaller than in most raptor species – however, as we saw last year, one of the chicks may develop less slowly than others as they compete for food.

The chicks, called eyasses, are covered in white down to help ward off the cold. After between three and five weeks the down starts to be replaced with feathers.

At about the same time as the feathers start to appear the chicks will also start walking about and jumping around in the nest. As they grow, they will require more and more food and at this stage the female adult will begin taking a more active role in hunting once again – giving webcam watchers more opportunities to see the chicks in the nest.

At about five to six weeks old the chicks will start taking their first tentative flights – a quite daunting prospect I would imagine when perched so high on the ledge of the university’s Newton building. They will stay close to the nest, however, and remain dependent upon the parents for a couple more months,  meaning that we’ve got plenty of time to watch and enjoy their development between now and the summer.

Erin McDaid
Nottinghamshire Wildlife Trust