Delayed Incubation in Ospreys

To Delay or Not to Delay

It would be advantageous for most birds to lay all the eggs in a clutch together, one straight after another, so that down the line the chicks hatch at around the same time. Of course, they can't do this as it takes time for the female to produce an egg.

For ospreys this time period is around three days; there's no cheating the laws of biology. They can, however, cheat the laws of hatching.

So a three-egg clutch will take around six days to lay. You would therefore expect the chicks to hatch six days apart - but that almost never happens.

Each subsequent egg is slightly smaller than the one before it, so by definition those eggs will take less time to hatch. This would foreshorten a six-day hatching sequence down by a day or so.

But Telyn's three chicks hatch in less than half this time: just 2.6 days on average. Nora (2011 & 2012) was the same - 2.1 days for her.

A reproductive strategy called Delayed Incubation explains this. Freshly laid eggs can spend a lot of time in the zone of suspended development, not requiring much incubation at all - and indeed that is precisely what we see Telyn do each year. She only 'properly' incubates her eggs once the third one is laid.

Hatching Synchronisation, as it is called, has many benefits in terms of nest productivity and the survival of the chicks, the most obvious being chicks of roughly similar size and strength in a brood (just imagine the comments on an aggression video in which is Bobby Bach was five days behind his big sister/brother!).

Not all female ospreys employ Delayed Incubation - Glesni, for example, did not… There are downsides too: increased predation pressures and an increased risk that temperatures get so low that suspended development becomes ceased development, to name but two.

But on a population level, both strategies exist in parallel. Presumably outcomes are roughly equally successful between both strategies, otherwise one would eventually be selected out by Natural Selection.