Egg Incubation in Raptors

In the update on the Ospreys, I mentioned that last year the hen osprey failed to sit on her first egg. Typically, Ospreys will sit on her first egg, and while it may take two or three days then lay the second and so on until a clutch of three and rarely four are laid. As they are incubated for the same time, they will hatch at different times. Well that is what typically happens.

But as this young bird doesn't seem to have read the text books, last year she failed to incubate her first egg when laid. While that chick hatched, only two birds fledged as one died. It is likely that it was this chick from the first egg that failed to thrive.

It raises an interesting question; as this pair are an isolated young pair, could it be that without others to learn from they made mistakes?

While it is good news that the Osprey population is growing and expanding, it is where isolated pairs, like this pair in Cumbria, where the birds seem to lack the skills that they would learn from others. There was a similar situation with the Red Kites when a pair tried to nest a year before they were old enough to breed and the result was a failed season.

Thus, with these Ospreys it may well be that they will do better this year, having had the experience from last year.

The comment that Ms Tree asks is interesting as, the smaller the egg the faster it looses heat. So as a good rule of thumb, the smaller the egg is then the hen has to keep the egg warm straight from laying. With Raptors in particular they will space out laying to cope with any possible scarcity of food. As simply as the chicks grow their food demands increase, thus if the chicks were all the same size the parents just could not catch enough fish to feed a full clutch of pre fledged chicks and feed themselves. This is one of the reasons why Ospreys migrate from Africa to northern latitudes to breed. As the longer daylight hours mean that they have more time to hunt. Experienced Ospreys, will often adjust the time they sit on eggs to space out the hatching to match the food supply.

Personally I am just happy to see the birds have returned and are trying to breed again. I have also been informed that local to me, an Osprey was seen, it was likely to have been on route to Scotland.