The very wet and not very warm weather on Saturday meant the ospreys stayed tight on their eggs. Typically in the UK males take a turn for no more than about 35% of daylight hours and the females are usually the only ‘night shift’ performers. On the Kielder nests 37 on Nest 2 does rather more incubating than YA seems to, although with ‘only’ the long distance camera available on Nest 1 it is sometimes difficult to be sure who is on the eggs.
Many of the videos show the start or end of incubation shifts. You can see the osprey gently settling over the eggs and shuffling or rocking slightly. This is to ensure the brood patch covers the eggs properly. Both sexes possess these heavily vascularized areas of skin. During the breeding season the birds lose their down feathers from this area to allow heat to transfer to the eggs. Some clips show the incubating osprey turning the eggs. One day recently Mrs 37 was almost as reliable as a watch, turning the eggs every 20 minutes. This action is to ensure an even distribution of warmth for both the individual egg and the clutch. It is especially important during the first half of incubation as it prevents membranes in the egg from sticking together prematurely. The ospreys adjust their behaviour to take account of the ambient temperature. So if it was very hot the eggs would just be shaded from the sun rather than fully incubated under the brood patch. Not very likely at Kielder though!
When hatching is approaching the incubating osprey will tend to be slightly higher off the eggs. From about 48 hours before hatching, sometimes earlier, ‘pipping’ can be heard from the developing chick and it will start tapping the shell with its beak. An egg tooth, a tiny hardened tip to the upper mandible, is the tool that enables escape into the big wide world! Zooming in a nestcam on an egg it is possible to see the egg tooth chipping through in the hours before the chick hatches.
A new video has just been added which shows Mrs 37 carrying out that vital egg turning.
Sources: Ospreys A Natural and Unnatural History by Alan F Poole; Population Ecology of Raptors by Ian Newton