Posted: Sunday 12th May 2019 by Emyr MWT

Aeron returned to his natal nest as an adult this week.

Brother to two sisters in 2017, Menai and Eitha, Aeron like many of his brothers before him was a fairly independent chap.

Here he is on his first ever flight on 11th July 2017 as he almost crashes into his Dad, trying to land besides him.


And here he is 22 months later returning as an adult, his gatecrashing skills still pretty evident.

Aeron holds the Dyfi record for youngest male to migrate - 83.8 days, a record he shares with his brother Gwynant who has also been re-sighted as an adult back in Wales.


Migration ages (with averages) for all Dyfi chicks

So both of Aeron's sisters stayed around enjoying Dad's hospitality for 10 more days after their big brother left on his travels. There's an outside chance Aeron may well have reached Senegal/Gambia (Ceulan took just 11 days) by the time his sisters decided to leave Wales at the end of August in 2017.

The Welsh Story

Here's a list of all Welsh chicks that have positively been re-sighted as adults.


Highlights

  • 16 birds in all - 13 males and three females.
  • Seven of these returnees have successfully bred producing 76 chicks between them. Another bird has had an unsuccessful breeding attempt (Merin in Denmark 2018, not positively identified yet this year) and a ninth, Blue W6, is breeding for the first time in Kielder Forest right now.
  • Average age of first breeding is 4.4 years and all of the birds on this list are, as far as we know, still alive.


Here are a couple of questions we've been asked regarding Aeron's return:

Why don't you put up a nest for all these returning Dyfi birds?

We have. Montgomeryshire Wildlife Trust have other osprey platforms in the county. Numerous factors have to be taken into account before an osprey platform is erected, including legal implications, so it's not as easy as just sticking one up wherever you may think would be a good place.

Another MWT platform being erected in 2017

 

Stakeholders such as angling associations, foresters, land owners, national parks, regulatory and licensing authorities - including local councils for permissions and proximity of any public footpaths - all have to be engaged with. Large swathes of land in Wales are designated in some way, so there are issues there as well.

We should also remember, of course, ospreys are very capable of building their own nests, they've been doing so since our ancestors were living in trees next to them.

 

Does Monty recognise his own son?

Almost certainly, no.

When an animal is old enough to leave its parents and become independent, it has to decide how far away to travel from 'home'. This is called natal dispersal. When an animal is an adult and is old enough to breed, it has to make a decision as to how far away to set up a breeding site. This is called breeding dispersal.

Dispersal is of critical importance for all sorts of ecological parameters for an animal, including genetic dispersal. Surely, it is best for an animal to travel far away from home to breed, so that it doesn't encounter any close relatives, especially if that species (like the osprey) has no way of telling whether other adults are related to it (kin-recognition)?

Well, not always.

If you have a large genetic diversity like the osprey does, it really doesn't matter. In fact, British ospreys have an extremely restricted breeding dispersal - individuals, especially males, tend to come back to the same nest and colony of birds that they were raised from originally. The posh word for this is 'philopatry'. (The reason that the Scottish osprey population is only expanding by around two to three miles every year is exactly due to this high degree of philopatry.)

Blue 80 - a 2012 Glasyn offspring - was sighted back on the Dyfi as a two-year old in 2014. He later moved on to his natal nest and mated with his own mother.

Blue 80 on the larch perch in 2014

 

Case in point: All eight breeding nests in the Rutland colony have males that were born in that same colony.

So from an evolutionary point, there is no advantage to an osprey recognising its own children, therefore, kin-recognition has not evolved. This is true of most (but not all) birds.

After all, how would such a recognition mechanism work?

Monty may not see an offspring for five years or more since that bird migrated. It will have moulted into adult feathers by then and have another two or three complete moults under its belt. Aeron looks very different to the 82-day old fledgling when Monty last saw him two years since. Monty would be unable to smell any offspring too - birds have poor olfactory (smell) abilities.

So Monty, and Telyn for that matter, would see Aeron as a regular intruder, a bird of nuisance value only that is to be seen off. Monty has invested his time and efforts (successfully) into Aeron already, it's now time to nurture and protect his latest investment fund.

Tegid was also ushered away when he visited on 7th April this year.


Glesni

Glesni was with us for five years, 2013 - 2017 - five very successful years.

Aeron now represents the final jigsaw piece of her legacy. Remarkably, we've now spotted one offspring of Monty and Glesni from every one of these five years.

Returned birds in bold

Measuring success in conservation is not always easy and it certainly isn't a quick process. By today we have spotted five out of a possible 11 birds that Glesni raised - that's 45%. An astonishing return rate.

What a bird, what an amazing mother she was.

Glesni


And Finally...

So all these birds coming back - but have you spotted something of a discrepancy here? A mismatch?

Four out of Glesni's five offspring to have returned are males (80%) and 13 out of 16 all-Wales birds to have returned are males (81%). What's going on? Where are all the girls?

Well, they're out there somewhere.

Ospreys have a pretty constant 50:50 sex ratio of chicks produced and the same is true for birds surviving into adulthood. So to have over 80% of birds returning being males simply can't be representative of the whole picture.

They're out there - they have to be.

If there are 13 males out there, there must be, statistically, around the same amount of females. We know of just three, so another 10 Welsh females would be expected to be roaming around or even breeding somewhere. This is normal as males are more nomadic, the females less so being more inconspicuous, especially once they've started breeding.

Think of Clarach breeding deep in a Scottish forest miles away from anywhere. She doesn't do much fishing and is practically tethered to her Aberfoyle nest. We only know of her because she has chosen to nest in one of the 5% of UK osprey nests with a camera on it.

Clarach nesting deep in the Queen Elizabeth Forest, Scotland

 

"That we know of" is the usual caveat we have to use when speaking about percentages of returning birds, but we have to make some realistic statistical extrapolations here. These 16 Welsh birds "that we know of" must surely be more like 26 in the real world once you factor in the extra 10 females that we haven't seen? And there could be more males out there too of course. The figure is probably more than 30 birds.

This year the 'Welsh colony' is 16 years old and we're starting to see some interesting trends. The next 16 years are going to be crucial to the growth and progression of the Welsh population, especially as the various UK colonies will - eventually - start to merge together: Scotland-Northumberland-Cumbria-Rutland-Wales-Dorset.

By 2035 - another 16 years - there will be more counties in the UK with ospreys breeding in them than not. Hopefully.

Here's our boy visiting his natal nest a few days ago on the 7th May.

The race is on... who will be the first Welsh osprey to return and breed in Wales? Keep Calm and Look Up.

 

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