32 days at 37,7°C
The length of brooding (or the incubation period) depends on the species. A chicken sits on her eggs for 21 days, a mute swan for 36 days, a long-tailed tit for 13 days and a peregrine falcon for 32 days.
Without a constant temperature, the embryo won’t develop well, but that’s not the only thing necessary for a successful hatching! The environment needs to be of a certain humidity and the eggs must be turned regularly.
In case of the peregrine falcons, the brooding temperature must be between 37,3°C and 37,7°C. The temperature may go down a little, for instance when the falcons leave the nest to chase a predator. The drop in temperature is mostly harmless because the chick will produce more and more heat itself. Every rise in temperature however can be fatal. If the temperature reaches 40°C, the chick will die immediately.
The relative humidity must be around 50% so that the egg loses about 15% of its weight at the end of the brooding. If the little falcon is too big, he fails to turn around in the egg and can’t pierce the shell. In which case he will die of suffocation.
Turning the eggs makes sure that the membranes that let oxygen pass start sticking to each other. The egg yolk (on which the embryo is positioned) must remain in the middle of the egg. Two elastic bands keep the egg yolk in the right position. But if the egg isn’t turned regularly, those bands will stretch out and the egg yolk might stick to the bottom of the egg. And then the embryo may die.
It’s hard being an egg!