Over 47,000 people visited the Dyfi Osprey Project in 2011. If all of those people asked just two questions, that’s almost 100,000 enquiries. Of all those thousands of questions we answer every year, one question probably crops up more than any other – how can you tell the difference between Nora and Monty?

Thankfully, there’s an extremely easy way to tell them apart - eye colour.

This osprey is incubating three eggs, but is it Monty or Nora?

Vision is extremely important to birds. Not only do they need excellent vision for flying (those that do, of course), they also need to see where their next meal is coming from. Just like their ancestors, the reptiles, birds have four types of colour receptors in their eyes; humans and most mammals have two. These give birds the ability to perceive not only the visible range, but also the untraviolet part of the spectrum. Birds of prey have an even greater density of these receptors over and above other birds, further increasing their visual acuity.

The colour of a bird’s eye, that is, the iris, is dependent on many things, but is mostly determined by the amounts of various pigments contained in the iris. Despite learning a tremendous amount about bird physiology and anatomy in recent years, curiously, the function of the various pigments, and therefore eye colour, is poorly understood. In many birds of prey, the eye colour changes as the bird matures and can serve as a means of determining an individual's age. Adult ospreys have brilliant yellow eyes, but they are not born with this colour eye.

Nora feeing young Ceulan, who has very different colour eyes

Ospreys, when they hatch, have dark blue eyes. After a few days, the eye colour starts to change to a deep orange colour and they then remain like this for their migration to Africa. When ospreys return to the UK to breed at the age of two or three, however, the eyes are suddenly a bright yellow colour! Is the reason for the eye colour change to denote the reaching of sexual maturity?

Ceulan (right) and his/her sibling started life with blue-colour eyes

Nobody really knows. This is what we do know, however – Monty has retained that deep orange/amber eye colour into adulthood. Yes, his eyes have become slightly brighter over the years, but not by much. We know of only one other osprey that has retained the juvenile dark eye – Henry.

Instantly recognisable - Monty has those piercing orange eyes, extremely unusual for adult ospreys

Henry was a Scottish osprey born in 1998 on the Black Isle. He went on to become one of the famous birds of the Loch Garten nest and bred there between 2004 and 2007, siring 6 fledglings. So this begs the question - why does the eye colour of a very small amount of adult ospreys not change to bright yellow? We simply don’t know.

If Monty was related closely to Henry, might there be a genetic reason? If our theory is right, and Monty is the single male chick that fledged the Welshpool nest in 2004, we know exactly who his parents are. His father was a first generation Rutland bird, translocated from a Scottish nest in 1997, his mother a bird ringed as a chick at her Perthshire nest in 2001 (not 2000 as sometimes reported). Unfortunately, we can’t find a direct genetic link between both Henry and Monty, not saying that there isn’t one, we just don’t understand enough about the whole osprey family tree to know.

Here’s a short video highlighting the differences between the eye colours of our Dyfi family:

It should also be pointed out, of course, that Nora has a leg ring and Monty doesn’t, so it’s pretty straightforward to tell them apart if both their legs are on show. Having an atypical eye colour for an adult osprey has clearly not negatively affected Monty nor Henry in any way – both excellent fishermen and raised families of their own. Will their offspring have orange eyes as adults? This would certainly confirm any genetic connection. Time will hopefully reveal the answer to this question and many more.

So, the incubating osprey you saw five minutes ago at the top of this blog? You now have no excuse not to know which osprey this is!