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Two Chicks in 2020

Posted: Friday 29th May 2020 by Emyr MWT

We have two chicks in 2020, one egg didn't hatch.

Here is the chick information:


  • 1st CHICK HATCHED - 26th MAY at 16:30 (Egg No 2: 36 days 14 hours 50 mins)
  • 2nd CHICK HATCHED - 28th MAY at 12:34 (Egg No 3: 35 days 13 hours 38 minutes)

Time between chick one and two: 1 day, 20 hours, 4 minutes.

Both young chicks are doing fantastically well, helped by the good weather and attentive parents. Lovely also to see Idris (being allowed to) feed his own chicks.

And here is the 2020 hatching video, two and a half days condensed into two and a half minutes:


Unhatched Egg

We know, 100%, that it was Egg 1 that failed to hatch. As I write, it is 43 days old, now beyond even the most optimistic of hatching window predictions.

Egg 1 - far left


So why didn't it hatch?

We know that approximately 10% of all bird's eggs, across all species, fail to hatch. A huge sweeping generalisation I know, but it's roughly one in 10.

Here are the stats for both Dyfi and Glaslyn nests spanning a cumulative 25 years in all:

Unhatched Eggs

  • Dyfi - 7% (2 out of 28 over 10 years)
  • Glaslyn - 6% (3 out of 49 over 15 years)*

(* 2015 data excluded - lots of hormonal eggs that year with no breeding male until very late on)

So at 7% and 6%, the two Welsh Dyfi and Glaslyn nests respectively seem to be beating the odds, on average.


All the eggs for the Dyfi nest including the two that did not hatch

*DNH = Did Not Hatch


There are two main reasons a wild bird's egg does not hatch:

Not Fertilised

The fact that Telyn laid her first egg in record time - just 10.6 days after mating with Idris for the first time (see table above), might give us a clue here.

Did enough time pass for the egg to be properly fertilised?

Telyn lays her first egg on 17th April, it has failed to hatch


Well, fertilisation failure is relatively uncommon in wild birds. Research studies have shown only around 1-2% of blue and great tit unhatched eggs were due to fertilisation failure. Similar results have been found in other species, including the yellow-shouldered Amazon parrot!

Yes, 10.6 days is quick for this nest, but Telyn's mother, Maya, has been recorded plopping eggs out in a week or so at her Rutland nest - and they still hatch. Fertilisation failure seems unlikely to me.

Prenatal Mortality

The vast majority of eggs that fail to hatch do so due to prenatal mortality; that is, the developing chick dies in the egg.

We know that chick mortality risk typically declines through each stage of the development stage; if something does go wrong, it generally happens at the very start of development - in the first day or two.

Indeed, this is exactly what happened to our only other DNH in 2016. We had this egg analysed in a lab and the verdict was "This egg WAS fertilised but development was arrested at a very early stage.”

Glesni and Monty's 2016 unhatched egg

Did Telyn's Delayed Incubation strategy have anything to do with it? Probably not. If this strategy had a habit of resulting in addled eggs, it would be quickly selected out of the population as a whole.

Talking of Delayed Incubation - where the first egg - (or two eggs for an osprey) is intentionally left un-incubated so that the hatching times are closer together than when the eggs were laid, here's an interesting statistic from this year:


  • 69.0 Hours between 2nd and 3rd egg laid
  • 44.0 Hours between chick 2 and 3 hatching


This is a foreshortening of eggsactly 25 hours, more than one full day, a time reduction of 36% compared to the hatching sequence. Hatching closer together in time generally (for ospreys) results in a slightly increased nest productivity.

Hatching spans in days for 10 seasons

There are other possibilities such as a double-shell egg (very rare), but the most likely cause here is prenatal mortality.

Did the fact that Idris is a new male this year influence anything?  Might he have interfered somehow with Telyn's strategy and incubated, unintentionally triggering the chick development clock? We will never know, but unlikely.

Idris shows us his nictitating eye membrane as he incubates


And Finally

We have two beautiful chicks and we have some good news for them: they have an inflated chance of survival now there are only two of them. The Goldilocks weather we've been having is also set to continue for some time yet.

Lady Luck may not have been on the side of the chick in Egg 1, but she has certainly been shining on our surviving two chicks. Nature and luck are indelibly intertwined.

Egg 1 will probably stay in the nest for the next few weeks, possibly being pushed out to the sides as the chicks grow or nesting material covers it. If it is still there in five weeks, and if we can ring and swab the chicks this year, we will remove the egg.

Will we send it to be analysed again? Probably not. If we can retrieve it we may put it in a display case so that visitors to DOP can see it when they come; we have a license to do so. We'll see.

Lastly, a huge thank you to all of you that have donated to DOP. It is our only form of income this year. Without you, we very literally would not have made it this far this season. There may be just one of us left at DOP, for now, but we hope you are enjoying the show...

Diolch yn fawr. 

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