Here are the 2025 egg laying times - almost exactly three days apart:

🥚 13:48 - 9^th April (earliest ever at this nest)

🥚🥚 13:37 - 12^th April 

🥚🥚🥚 13:52 - 15^th April

Studying osprey incubation over the last 20 years in Wales has shown us two, mostly distinct strategies for incubating eggs, chiefly coordinated by the female.

A Delayed Incubation Strategy involves not incubating the first and sometimes second egg 'properly', or not at all, until the full clutch is laid. This is opposed to a more conventional strategy where incubation starts immediately after the first egg is laid.

A female osprey needs around three days to produce an egg - this is a serious biological investment, as it is for birds in general as they are all 'oviparous' (producing young by means of eggs).

The chart below shows average times to produce an egg for all our females going back to Nora in 2011 at the Dyfi nest. Arguably 2013 and 2014 data sets should be omitted from this chart due to the shennanigans during those years with a second nest being too close, but are included here for completeness.

Delayed Incubation

Ospreys sometimes use a delayed incubation strategy to synchronise hatching among their eggs, improving the survival chances of the entire clutch. 

Three chicks from three eggs, all things being equal, would hatch at roughly the same cadence as the eggs were produced - so a six-day spread for three chicks. 

By delaying full incubation until all eggs are laid, the eggs begin developing at roughly the same time, leading to more uniform hatching, in essence 'cheating' the laws of biology.

This reduces competition among nestlings, as they are closer in size and strength, ensuring that later-hatched chicks aren't outcompeted for food by larger siblings.

This strategy is particularly beneficial in environments where food availability or weather (that's us!) can be unpredictable, as it maximises the likelihood that all chicks receive adequate parental care and resources. Additionally, synchronized hatching can reduce the overall time the nest is vulnerable to predators or storms, as the fledging period is more compact.

We see far fewer Bobby Bachs (runts) with this strategy.

This chart shows the average hatching spans of all the three-chick broods we've had at the Dyfi.

It's a bit more nuanced than this, but we can essentially split these females into two camps - those that employ delayed incubation strategies vs those that don't. 

Nora and Telyn are delayers, Glesni not. 

This resulted in Nora and Telyn foreshortening a potential six-day spread right down to 2.1 days and 2.7 days respectively - that is a greater than 50% reduction in hatching cadence. Glesni only got this down to 4.5 days - and this can probably explained away by the fact Egg 2 and Egg 3 need slightly shorter incubation times anyway, as eggs get progressively smaller in size, the first one always being the largest.

Alright then, if this delayed strategy is so successfully, why don't all females do it, why doesn't Natural Selection completely kill off the least successful strategy?

Because it probably isn't less successful - both strategies are almost certainly as productive as each other; we just haven't got the data to show this. By the very fact that one strategy hasn't out-competed the other, means both are roughly equal in nest fecundity (a measure of the reproductive success of an animal) on a population level.

So the last question then - if shortening the hatching span of a clutch brings so many benefits (as noted above), there must be an equally negative force cancelling out the benefits?

Indeed there is.

Delaying incubation of an egg(s) is inherently dangerous. While the embryo inside can exist in a torpor state for a while until incubation begins in earnest, sometimes several days later, that egg is exposed to potentially wildly changing temperatures. If the gamble pays off, great - but if chick development starts too early, or never at all, and before proper incubation begin, it's curtains.

Moreover, not incubating an egg not only exposes it to a wide range of day/night temperatures, but also increased predation pressure.

Every year, for example, we see owls silently landing on the nest while the ospreys are completely unaware they are sitting just inches away from them.

Here's one example:

So, in summary, both strategies are roughy equally (over a long period) as successful as each other.

But Wait...

What if a female is adaptable?

What if a female can pivot between these two strategies year-on-year, based on the environmental factors at play during the time of laying?

Example: A male doesn't return one year and by the time a new bird turns up and both birds go through the ritual pair bonding stuff, it's suddenly mid-May (this happens often in a population). Would a female in that scenario 'choose' one strategy over the other due to these environmental factors?

Presumably a non-delaying strategy would be best in this scenario as it comes with less risks - play the precautionary hand of cards, at least for this year.

Surely this ability to change strategies on the fly would be the 'have cake and eat it' master of all plans, beating the more ridgid and binary 'I'm a life-long delayer (or not) strategy'

I don't think Telyn employed a delay strategy this year. Did warmer temperatures, Idris being home before her (or any other metric) cause her to pivot to a non-delaying strategy in 2025? Is she pivotting between stategies on a year-by-year case basis, optimising fecundity success?

We'll know more in a couple of weeks - hatching expected Saturday 17^th May.

What do you think? Let us know on the YouTube Live Chat or other social media platforms.

Look out for the next Science Sunday soon - all about Hormonal eggs. Eggciting stuff.