Over the years, the question of inbreeding in ospreys has often cropped up. The recent arrival of a son of the Glaslyn ospreys at the north Wales nest has caused some raised eyebrows, but it really shouldn't.

Blue 80 is a male chick hatched at the Glaslyn nest in 2012. In 2014 he was spotted back in the UK for the first time as a two-year-old, causing havoc at the Dyfi nest! He had returned to the UK as an adult and was prospecting for a nest site and a mate. Blue 80's father has not returned to the Glaslyn nest in 2015 and on 16th April, his son Blue 80 replaced him as the male of the pair.

Blue 80 on the Dyfi nest in 2014 - the fledgling (left) is Deri.

Blue 80 has been mating with his biological mother since his arrival - what better time to try and explain inbreeding in ospreys and answer a few questions...

The Human Thing

When we think of animals, we tend to see them and think of them through human eyes. Of course we do - that's what we are! I'm sure a boa constrictor looks at life through a boa constrictor's eyes.

This is not usually a bad thing to do, or a wrong thing to do, but now and again a situation will crop up in nature where related traits shared between humans and another species are so far apart, so different, they really should not be looked at in the same (human) way. Inbreeding is one of these situations. There's a bit of science coming up, but let's try and keep things relatively simple.

Inbreeding in many bird species (especially those that can fly) is very common, particularly in abundant species and among species with large geographical ranges. It is not a bad thing, if it were, the species would have died out by now or be in severe difficulties. It's all to do with the genes..

Blue 80 returns to his ancestral nest on 16th April, 2015 - he's almost three years old.

The Bathroom Game

Imagine you're in your bathroom with a thimble, which is almost full of water. The wash basin is also full of water, as is the bath. You also have a pot of blue fountain pen ink.

This is your task: you have to put a drop of ink in only one of the three, but the water has to remain colourless. Which one do you choose - the bath full of water, the wash basin, or the thimble?

The answer is easy of course. Put a drop of ink in the bath and swish the water around a bit and you'd never know that there was any ink in there at all. You could probably pull off the same trick with the wash basin, but you most certainly could not with the thimble.

Genes: The Numbers Game

Inbreeding tolerances in wild populations is all to do with genes and genetic diversity. Some species have a very large genetic diversity, some have a very low amount of diversity.

Ospreys have been around for a very long time, at least 15 million years. Modern humans on the other hand have barely been around for 1% of that time - 150,000 years. Add to that at least one genetic bottleneck that occurred around 70,000 years ago when, based on back engineering genetic analysis of modern day humans, Homo sapien numbers fell to a population as low as 1000 breeding males, and the same amount of females. That's around the population of a small village like Machynlleth. Everyone alive today, all 7 billion of us, is descended from this small population of modern humans. So that's 7,000,000,000 individuals from just 1,000 ancestral pairs.

Even though global human populations look different on the outside, we are remarkably similar genetically; there simply hasn't been enough time for us to develop any significant degree of true genetic diversity.

It's Time to Leave Son
(but come right back once you're old enough)

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 to 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 an astonishingly large genetic diversity like the osprey does, it really doesn't matter. In fact, the osprey has 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 (right) returns to his natal nest as a three-year-old in 2015. His mother (left) is still the breeding female.

So what advantages are there to an osprey to come back to the same colony of birds that it came from?

• By the very fact that the natal nest has been successful in the past, surely means that it is very likely to be situated in a good habitat for ospreys with enough food available
   
• There is a better chance of finding a mate if all the other osprey offspring play the philopatry game and return
   
• There may be old osprey nests available if their owners fail to return - ospreys love to inherit homes rather than build their own (don't we all!)
   
• Ospreys are specialists (only eat fish), so only certain breeding habitats will do, those that are close to water. A genetic predisposition to return to a natal colony almost guarantees finding the right habitat

So it seems that the benefits of returning close to one's place of birth to breed outweighs any negative effects of the inevitable and resulting genetic inbreeding that would take place. And there is a very good reason for this - ospreys can afford to do so. With their huge diversity of genetic makeup, not only is it not harmful to have a moderate amount of inbreeding in a population, it can actually be advantageous. The osprey's social ecology and population dynamics, including their tendency to return to natal nests/colonies (the high degree of philopatry) as adults, tells us this.

The bathroom ink game now comes into its own. The ink is the level of inbreeding tolerance and the container of water is the amount of genetic diversity of the species. Humans are the thimble in this analogy, there really isn't that much scope for too many close-relation breeding attempts before things start to break down, the genetic diversity (water volume) simply isn't large enough for the water to remain colourless.

The osprey is the bath. Drop after drop after drop of ink, so long as you give the water a swish now and again (breeding between distantly related birds), barely does the water remain anything other than colourless. In a nutshell, the greater the genetic diversity of a species (amount of water), the greater the level of inbreeding tolerance it can handle (amount of ink you can put in).

Too much? Every species can tolerate a bit of inbreeding, some more than others. It's a matter of genetic affordability.

Sure, too much inbreeding in ospreys would eventually take it's toll as it would in any species, but this seldom happens in wild populations. Inbreeding in ospreys is a lot more common than you think. It happened at a Cumbria nest recently (mother and son) and at the Loch of the Lowes nest in Scotland a few years ago (mother and son).

Common as Muck

Think of inbreeding in ospreys not through human eyes as some sort of stigmatised, incestuous taboo, only to be spoken about with a raised eyebrow and the rolling of the eyes. Think of it as a natural, highly successful reproductive strategy that has served ospreys well over many millions of years; long, long before our simian ancestors ever thought of leaving the tree canopy and standing upright.

There is one last advantage of inbreeding in a species that 'can afford it', like the osprey. The hugely successful Super-osprey, for example, the Glaslyn male 11(98), sired 26 chicks to fledging age in just 11 years, five of these have since returned to the UK and have produced 33 'grand-chicks' to date. A phenomenal reproductive success. Half of his genes are now in Blue 80; his legacy, and his inheritance, live on.

If you get a chance to go and see the new Glaslyn male, Blue 80, don't hesitate. They have a fantastic new visitor centre in 2015 and crystal-clear live HD live pictures on large TV screens. You can follow the Glaslyn ospreys here and on Facebook.

The King is (most probably) dead, long live the King...

Blue 80 (left) brings a fish back to his new mate, his mother.