INTRUDER!! Who is it please?

You're in the 360 Observatory, Monty and Glesni are mantling and piping out osprey intruder calls quicker than you can prise your binoculars back off your friend's neck, and suddenly you make out a dot in the distance. It's a bird 300 metres away, high up and moving at 30mph.

Can you tell who it is? It has a leg ring - who is it? QUICK - ID it.

At 300m you're doing well to ID a moving bird as an osprey. Forget male or female, no chance. Leg ring? Impossible, you wouldn't see it at that distance even with a £2,000 pair of binoculars.

An osprey needs to fly very close and preferably be photographed to make out a ring number and even then you're at the mercy of good light, good photographic technique, and a willing participant happy not to hide their legs away under a mass of undercarriage feathers. But what if it doesn't have a ring? Half of all UK ospreys don't.

Recognising FK8 as we did on Wednesday evening with a 4K camera shooting at over double the usual frame rate (50fps) is not the norm, we had a bit of luck too - this bird came very close to the nest. But there are other tricks we can use.

A Tweed Valley three-year-old female flies directly over the nest on her migration back from Portugal to Scotland

Behaviour is a Good Indicator

If it's just Glesni that looks a bit agitated, it's probably Blue 24 getting within range, 200m or less. If it's just Monty that starts to express alarm calls, there's a good chance it's another male in his territory. So the gender of the intruder is often given away by the behaviour of Monty and/or Glesni.

If both Monty AND Glesni chip and scream, it's probably a bird they are unfamiliar with, like FK8. And this would also be true of a returning offspring - neither Monty nor Glesni would recognise a returning chick as their own several years down the line, they would engage with it as they would any other intruder, as a threat to their nest which needs dealing with.

Here's the video of FK8 approaching the Dyfi nest last Wednesday. In just a second or two she was gone but look at Monty and Glesni - they clearly don't recognise this bird as an osprey they are familiar with. These are 'Classic' osprey intruder calls, you only hear these 'chips' when there is another osprey around, not at any other time.

Intruder osprey in 4K definition

Plumage Patterns

Here's a ringed bird - so easy to ID, right?

The plastic Darvic ring is hidden, but we can see the metal BTO ring is on the right leg, so this is a Scottish bird (In England and Wales it's the other way around: Darvic right leg, BTO left).

The good thing about this image is that we can see the whole underwing of one side very well. Those brown plumage dots and splodges throughout the wing feathers, especially the under-arm areas, give us a big clue as to the identity of this bird. These are as individual as fingerprints and can be a great tool to identifying ospreys, but this method does come with a health warning.

No two photographs are ever the same and birds have an annoying habit of extending their wings to varying degrees when in flight, making the underwing patterns look slightly different each time. Great care and time is needed, as well as experience and a magnifying glass. John Wright at Rutland is the master of identifying ospreys this way. However, if you have no record photograph of this bird before, the best underwing photograph in the world is as useful to you as a field full of rabbits is to an osprey. Just like criminal fingerprinting, if your suspect's fingerprints have not been recorded before, you've had it.

This bird is a Scottish male, CX7, that spent several weeks in north Wales last year. He's a 2014 bird from a Loch Doon nest near Galloway Forest Park, Scotland. If we see him again, hopefully his underwing plumage will give him away if his leg ring doesn't, but we'd still need a decent photograph.

Glesni - ring not showing but a great capture of her underwing patterns

Moulting Feathers

A few days ago the male from the mid Wales nest (ON5), Dai Dot's old nest, visited DOP.

Dylan - the resident male at ON5

Other than clearly being a male due to his white underwings and chest, we had no idea who this chap was. A quick phone call to John Williams from Natural Resources Wales who manage the mid Wales nest told us that the male there was not around at the time we took this photograph. John also told us that Dylan has a primary feather missing on the right wing - the last-but-one feather (P9). Dylan was our intruder.

So by looking for moulting feathers we can, and with help from other projects sometimes, identify ospreys as individuals if we know the recent provenance of these birds. Missing feathers can also help us ID the same intruder over several days and sometimes several nests, despite not being able to positively ID the bird as an individual.

Nicks & Nacks

Better still, some birds develop individual nicks or other imperfections in their feathers. Here's a photo of Blue 24 I took last weekend.

Blue 24 with a damaged P8 feather, left wing

The third primary in on the left wing has been snapped off during a crow fight. This kind of feather damage can be one of the best clues of all, as they will stay with the bird until that feather moults, which could be up to 18 months away. So even if we see an osprey at half a mile away this summer, we'll know it's Blue 24 straight away. She will not need to be as close as usual for us to ID her even if we can't see her leg ring (as in this photo) or her underwing patterns.

Darvic Rings

Personal registration plates worn on the legs are, of course, the best way of identifying birds as individuals.

Rutland's White 03(08), better known as Monty's first breeding mate, Nora

But there's an inherent problem with leg rings: you need a heck of a lot of things going for you to make an ID.

Even with the best telescopes, 300 - 350m is the maximum distance you can read a Darvic leg ring of a static bird, much less if you're photographing a bird in flight. You need good light, an unobtrusive view and, preferably, a bird that is perching such as Nora (above) or at least an osprey with its legs dangling and not moving too quickly such as FK8 on Wednesday evening.

Some rings are easier to read than others. Since 2009 onwards British ospreys have been ringed with blue coloured Darvics; these days different European countries have their own osprey ringing colour - Black in Germany, Yellow in Spain, Orange in France and so on.

Rutland's Blue 5F(12) on the Dyfi nest in 2016

So we know that if we see a bird with a Darvic on the right leg, it's a Welsh or English bird. However, care must then be taken to read the ring carefully. Many letters and numbers can look similar at even a short distance. D and O, 0 and O, X and K, 3 and 8 etc.

Next you need to look out for any rings that are upside-down. Rings should be read upwards from the foot up - FK8 is a great example of a ring that was put on upside-down.

Upside-down leg ring

Many of you are familiar with Black 80, the first Welsh osprey from the Glaslyn nest (2006) to return to the UK and breed. Did you know that his ring was sent to us registered as Black 08?

It doesn't matter in most cases. If you can read the ring well enough, you can therefore, by definition, work out that it's the right way up or not. The only problem you'd get, really, is if the ring was palindromic in nature, such as Black 80/08. If both of these rings exist and one is put on upside-down, then you have the problem of two Black 80s or two Black 08s.

Black 80, or Black 08 to be fastidiously correct, being ringed in 2006. He's still breeding in Dumfries and Galloway, Scotland

Face and Eye Markings

Many osprey projects around the world these days have High Definition cameras - one even has 4K cameras!

By zooming in on head and eye features, a permanent record can be made of fine details. Practically all ospreys have iris dots, so this is a good place to start. Head plumage patterns can also be an effective method of identifying birds as individuals.

Dai Dot: Left eye iris dots and two distinctive white feather stripes above his beak

Dyfi Intruders

We get dozens of intruders at the Dyfi, but we are able to identify very few of them. Monty and Glesni do such a great job of fending these other birds away, the vast majority never come close enough to be identified.

Here is a list of all the ringed birds we have positively identified from their ring numbers over the years. Other than Blue 24 featuring every year for the last five years, notice how young all of these birds are. Mature and established breeding ospreys don't tend to bother other nesting ospreys, they are keen to get to and from where they are going, their prospecting days well behind them.

Be Absolutely Sure

Despite all the hurdles that exist in the field to correctly identifying ospreys as individuals, we do sometimes manage it. Patience and experience are key factors, as are a lot of late night scrutinising, sometimes, on a large monitor.

We never report and ID an osprey, however, unless we are 100% sure of who he/she is. We've all been in that position where you want a bird to return so much, practically any blue ring can look as if it's got DH written on it. Unless we're 100% absolutely sure, we won't call it.

We'll let you know when we see Einion for the first time in the UK, we just haven't seen him yet ☺

Alwyn: 12 years of spotting ospreys full time for a living - he doesn't miss many!