Just as their eggs are laid at intervals of one to three days, ospreys hatch a day or two apart. They are covered in down when hatched, but begin to grow new feathers within days. The chicks must rely entirely on their parents for food, and they grow very, very fast- they are three-quarters the size of an adult within a month.
Some sibling rivalry and bullying is normal but extreme violence and eating siblings is not – unlike in owls or eagles. Only if food is in short supply do some chicks fall behind or not survive as ospreys are very tender and attentive parents.
They are almost adult size by five weeks and ready to fly by 7-8 weeks. It is a very fast track growth spurt fuelled by their very high protein diet of fish brought in by dad.
The chicks are often ringed at around five weeks of age. By this time their leg bones are fully formed and the parents have had time to bond with them so that they won’t be abandoned as a result of the disturbance, but they are still too young to flap accidentally out of the nest.
This should be the only time they are ever handled or disturbed by humans at the nest site. Information on size, weight, sex, health etc is often collected during this brief process. This is also when satellite tracking devices can be attached to the birds.
By about six to seven weeks of age, osprey chicks are ready to test their wings for the first time. They often exercise on the edge of the nest and lift off in short hops before taking off properly for the first time. To encourage the chicks to fledge, the adults will bring less and less fish back to the nest– effectively starving them off the nest.
Once capable of flying, the chicks learn how to hunt for themselves, though they will generally stay near their parents for another 30 to 50 days.
The growth rate of osprey chicks is amazing – it only takes 12 to 14 weeks from when they hatch to when they begin their migration back to Africa. By this time they will weigh around 3.5lbs.
Young ospreys typically separate from their parents permanently in the autumn. They migrate separately from their parents, and probably won’t meet them again.
If they survive until their second or third year of life (the odds against which are probably more than 50% due to the hazards of migration and man) the young will spend a good ‘gap year’ or two in Africa and when they are ready to breed they are usually driven to return to the country where they were hatched and attempt to set up their own nest.