Female ospreys take just under three days to produce an egg.

So, for a three egg clutch there will be a six day time gap between the first and last egg, April 14th - 20th this year for Glesni. This photo shows the 2017 three egg clutch in the order they were laid.

So how come all three chicks invariably hatch within a much less of a time span that six days?

'Intraclutch Hatch Synchronisation' is the long-worded answer!

There's no way of getting around the three-day egg production time, you can't cheat the laws of biology, but what ospreys (and many other birds, including several owl species, surprisingly) do to encourage three (or four) chicks to hatch closer together in time is something known as 'delayed incubation'. So, often the first egg is hardly incubated at all (and the second/third egg too in some raptor species) in order to attain as much hatching synchronisation as possible, so that the chicks hatch at closer intervals than when their eggs were laid.

The size of the eggs female ospreys lay also get progressively smaller as she lays the second, third, fourth. This probably also promotes hatching synchronisation. But why do this?

The less time adults birds spend invested in bringing up chicks the better. Predation pressures are also reduced. Siblicide (one chick, usually the older, eating a smaller chick) doesn't occur in ospreys so this can't be a factor. In a nutshell, natural selection has determined over millions of generations that osprey productivity is higher when the chicks hatch closer together, hence why the trait of delayed incubation has evolved.