In reply to jon:
I've just come on here to post that I spotted a pair of Egyptian Vultures / Percnoptères yesterday when walking the dog about 5 mins drive from the house. I saw them at very close range, but they didn't stay at close range for long when they saw me. I saw them twice very nearby last year, but am still not sure if we fall in the summer resident range or whether they are just resting on their migration.
Almost all birds have evolved to fledge the nest in under a year.*
It must have given competitive advantage in the past over species which took much longer to fledge - eg. The ability to migrate somewhere nicer, with more food when winter comes around.
How can the chick be ready to migrate?
Both parent feed it lots of high quality protein. They don't attempt to raise a large clutch size, generally laying no more than 2 eggs. If food is scarce then only the first chick (laid & hatched first & therefore bigger) survives and is fed.
The chick is an eating machine. It eats, sleeps & grows. Only in the latter stage of the nest does it start to exercise its muscles.
With many species the fledglings weigh more than the parents when they leave the nest - this gives them a fighting chance until they learn to feed themselves - I'm not sure if this is the case with the vultures.
The parents continue to feed the fledged vulture for up to a month after leaving the nest.
Finally, when they migrate they use thermals to gain height and glide lots - It takes a lot less energy than flapping.
Why don't they come back with their parents the next year?
They aren't sexually mature and so flying back to breed would be a waste of time and energy. Sexual maturity takes time and energy, and this couldn't wasted while the chick was in the nest - everything went into muscle, bone & those beautiful black & white feathers.
*I think the Wandering Albatross has the longest fledging period with the chick in the nest for 280 days and since the egg takes a long time to hatch the whole process can go over a year.