This article is reproduced from the Dyfi Osprey Project's Facebook series 'Seven Things You Never Knew About Ospreys' that ran June 3rd - June 9th, 2013.

Seven Things You Never Knew About Ospreys

Part 1 - Populations and Distribution

Despite being rare in the UK due to centuries of persecution, the osprey is one of the most widely distributed raptor species, second only to the peregrine falcon. You will find ospreys on every continent on earth except Antarctica. They are superbly adapted to living and breeding in extreme environments from the freezing cold temperatures of the Arctic circle to scorching hot equatorial temperatures.

Ospreys are not closely related to other birds of prey, having had no common ancestor to another bird species for at least 15 million years. That's twice as long as when humans and chimpanzees both shared an ancestor.

Despite their huge global range and millions of years as a single lineage, ospreys look pretty much the same the world over. Some are shorter, some are paler, but the overall template of what an osprey looks like has remained unchanged for millions of years. There are four main 'sub-species' of osprey recognised based on where they breed.

1. Pandion Haliaetus Haliaetus - Palearctic: Europe all the way to eastern Russia (to the left and above Ceulan's head on the map below). Most of these migrate directly south during the non-breeding winter although some small populations that breed in sufficiently hot countries are non-migratory; those ospreys that breed around the Red Sea, for example.

2. Pandion Haliaetus Carolinensis - North America: Tends to have larger, darker body and a paler breast and head than 1. These too migrate south in the winter but some birds breeding in hotter areas like Florida, California, and Mexico will stay put all year round.

3. Pandion Haliaetus Ridgwayi - Caribbean Islands: Very pale head and breast compared with 1, and only a weak 'eye mask'. These do not migrate.

4. Pandion Haliaetus Cristatus - Australia and Tasmania: These do not migrate and some people consider these to be different enough from other osprey populations to be a separate species (Eastern Osprey).

The worldwide population of osprey is very difficult to estimate but is in the many tens of thousands of individuals. In the UK, however, there are only around 260 pairs - around half the population of golden eagles. Like the eagle, most of the ospreys are in Scotland with just a handful of pairs in England and Wales (around 10) and none in Ireland.

World osprey distribution

Part 2 - Evolutionary Adaptations

There are over 300 species of birds of prey in the world, but just one of these has evolved to eat fish only - the osprey. So over the last 15 million years they have evolved some pretty cool adaptations to make them superbly suited to a life of catching and eating fish. Here are 10 of them:

1. Osprey legs (tarsi) are reticulate (scaly). Think of a goalkeeper who has padded gloves for better gripping of the ball. Same with ospreys and scaly legs to better hold on to its prey.

2. Most raptors have toes which vary in size, ospreys don't. All four of its toes are the same length.

3. Most raptors have talons with groves in them, running along their length. Ospreys don't - their talons are perfectly round.

4. Unlike any other raptor (excluding owls), ospreys can reverse their outer toe. Normally, they have three toes pointing forward and one back when perching. But when grabbing a slippery fish, they will hold it with two toes pointing forward, two back. This toe arrangement has a posh Latin name: Zygodactyl.

5. The osprey has large, scaly feet with sharp 'spicules' (little spines) beneath the talons for better grip of slippery fish.

6. Ospreys have a specialised intestine with very strong enzymes to break down and absorb their fish meal - even the bones. Ospreys very rarely cough up undigested pellets like some other birds of prey do.

7. Ospreys have a strongly developed uropygial gland (oil gland). It is situated at the osprey's rump by the tail feathers and the osprey spreads the oil produced around its feathers by rubbing it and then preening. It has both a waterproofing function (handy for ospreys!) and an anti-parasitic function.

8. An osprey's nostril is long and slit-like (most birds of prey have round nostrils) and can be closed during underwater dives so that water doesn't enter the nasal cavities (a bit like nose-plugs in the swimming pool!)

9. Osprey are born with black eyes, then they change to blue, and then amber-brown. By the time British ospreys return from Africa as two year olds, they have changed again! Adult ospreys have yellow eyes (apart from Monty because he's special - he still has amber-coloured eyes).

10. Here's one for Mum and Dad's quiz night.. All the bird of prey species in the UK have yellow legs and feet - apart from one which has white legs and feet. The osprey!

See how many of these Evolutionary Adaptations you can spot in this video of Nora last year.. (best in HD)

Part 3 - Eyes

Ospreys, along with other birds of prey, are famous for their eye sight. We've all heard the expressions "eagle-eyed" or "eyes like a hawk". Being at the top of the food chain, ospreys need fantastic eye sight and over evolutionary time they have evolved some of the best eye designs in the animal kingdom, using the raw materials found in nature to be able to detect prey. Here are 10 things you may not know about osprey eyes:

1. Ospreys, along with most birds, have the largest eyes relative to their size of all animals.

2. Ospreys have two outer eyelids but these are not used for blinking like ours are. Ospreys sleep by closing the outer eyelids, usually the lower lid that rises up to cover the whole eye.

3. Ospreys also have a third eyelid called a 'nictitating membrane'. This is a transparent membrane that sweeps horizontally across the eye like a windscreen wiper and helps the eyes remain clean and moist. Ospreys often close their nictitating membrane when they are feeding their chicks to protect their eyes from possible damage from overenthusiastic bobbleheads!

4. Ospreys have four types of colour receptors in the eye (we have three). These help give ospreys the ability to perceive ultraviolet light as well as the kind of light that we see, 'visible light'.

5. Ospreys have a very high density of receptors in their eyes that give them much better 'visual acuity' than us. Visual acuity is clearness or sharpness of vision - a bit like being able to see the lowest letters of a (Snellen) chart at the opticians from much further away that we can.

6. Ospreys have binocular vision like we do, having both eyes point forward. However, they have better peripheral vision that we do - being able to see to the sides more.

7. Ospreys have dark feathers in front of the eye which help to reduce the glare from the water surface when they are hunting for fish.

8. Ospreys have the ability to calculate the 'refractive index' of the water they are hunting over and estimate the exact location of the fish, including how deep it is. Due to this refractive index, the deeper the fish is, the more they 'appear' to be in a slightly different position. No problem for ospreys!

9. It is thought that ospreys can 'see' magnetic fields and use these as a navigational guide when migrating. An osprey's right eye contains specialised proteins that perceives the earth's magnetic field, providing the osprey with greater directional information.

10. Monty has deep amber eyes and spots on the iris of both his eyes. We have no idea why Monty does not have yellow eyes which in normal for adult ospreys. We've asked everybody from geneticists to biologists to professors of vision. If you know, please let us know!

Monty, Nora and Ceulan from 2012 (best in HD)

Part 4 - Fishing

Perhaps the most enduring part of an osprey's ecology that makes them so interesting is their diet - fish. Ospreys and humans have fished the same lakes, rivers, and estuaries for tens of thousands of years around the world. Over evolutionary time, we have quite literally grown up with them. Here are 10 things you may not know about ospreys and fishing:

1. Ospreys have two main ways of catching fish. The wet method: Soaring or hovering above the water's surface from between 10 - 30m, the osprey will dive head and talon first into the water at great speed after identifying a suitable fish prey. After plunging up to a metre under the water, the osprey will surface, position its wings above the waterline and with great strength, become airborne from a half-submerged position and from a stationary position.

2. The dry method. Rather than hunting from altitude, an osprey will fly almost parallel to the waterline and pluck any surface feeding fish straight out of the water getting only its legs wet. This method is usually used in warmer countries where the fish live near the water's surface.

3. Ospreys will catch anything from tiny minnows weighing only a few grams to fish that weigh up to the size of the osprey itself - up to 2Kg or 4½ pounds. Most, however, are around a pound in weight or less.

4. Because ospreys live on all continents on earth except Antarctica, they have a huge variation of fish species they catch. The Old English name for osprey is 'Mullet Hawk' which gives us a pretty good idea what their main fish prey species in the UK (and actually, where they nested) was before they were persecuted to extinction by 1916.

5. Ospreys won't catch a fish at every attempt. Young fledglings have a very poor success rate, around 5% to start with. However, as they acquire more experience, this success rate shoots up considerably. A recent study (2010) of ospreys fishing on an estuary in the northern Iberian Peninsula showed an average success rate of 69%. Young birds caught fish around two times in every five attempts (40%) whilst more experienced adults caught a fish nine times out of ten attempts (92%)!

6. In the last few years, ospreys breeding in Scotland have started to catch mackerel where historically they had not. This could either be as a consequence of more ospreys choosing nest sites nearer the sea, or a change in the feeding behaviour of the ospreys. Or both!

7. Every year at the Dyfi we check carefully the areas underneath the osprey nest and various perches. We find loads of 'opercula'. The operculum is a hard flap covering and protecting the gills of a fish, it's made out of fused bones. They look a bit like your Granddad's big toe nail! This is the only part of the fish that the osprey can't eat.

Opercula

8. Every year, we record what species and size of fish the Dyfi ospreys (mainly Monty of course) catch. Roughly speaking, around half are grey mullet, 20% are founder and 20% are trout. The other 10% consist of other fish like sea bass, garfish, and perch.

9. Nora had a great dislike of flounders (or to be more scientific - had a least preference for flounders). There may be a good reason for this. Of all the fish caught by Monty, flounders not only have the highest water content and the least calories, but they are also very tough and take more calories to eat. Think of all those neck muscles straining to tear bits off. This is very hard to prove scientifically but is based on our (anecdotal) observations. A bit like the celery story - it takes more calories to eat celery and to digest it than there are in it!

10. In 2011, Monty caught a 'twaite shad' - an extremely rare fish in UK waters belonging to the Herring family. So rare in fact, you need a license to catch one. Monty, now has all the relevant paperwork in plaice.

Part 5 - Migration

For thousands of years people have been confounded by bird migration. Where on earth did many of the birds they saw breeding in the summer go during the winter months? It's only in the last few year that we have started to understand where on earth they do actually go. There is still so much to learn.

1. Most osprey populations around the world migrate. They breed in the northern countries across Europe, Russia, Canada, and the US and migrate thousands of miles south for the winter.

2. So the biggest question of all: Why? Quite simply, in case it gets too cold and freezes over. The Thames, The Volga, The Rhine, The Danube, Hudson Bay, Niagara Falls - all freeze over during severe winter weather and when they do, the osprey would vanish almost overnight.

3. There are other disadvantages too. Fish tend to live at a slightly deeper zone when it's colder and there is a lot less hours of daylight to try and catch them. In fact, in some of the most northerly osprey nesting sites for a few weeks, the sun doesn't rise above the horizon.

4. Ok, so why not stay in the warmer countries and breed there? Quite simple again, more dangers and more predators. Loads of ground predators, avian predators, soaring temperatures that are chick killers in the summer and then in the winter, monsoon rains. (Convinced Dulas died in a tropical storm event)

5. Some ospreys in warmer areas (Mexico, Caribbean, UAE etc) don't migrate however, why? Think of migration as a reproductive strategy. It's a trade off between the disadvantages of migration (getting lost, blown off course, drowning, energy required) and the advantages of migration and better reproduction success. Obviously, for these sedentary osprey populations, the benefits of not having to migrate outweigh of the downsides of a warmer environment.

6. Not all British ospreys go to Africa. Some only go to Spain, Portugal, and even France. In January this year, Spain's Ministry of Agriculture, Fisheries and Environment reported a population of 77 ospreys in Andalusia and that this population was stable.

7. British ospreys tend to migrate to west Africa. Mali, Senegal, Gambia, Mauritania, Guinea Bissau, those type of countries. In breeding populations, the females tend to go first, followed by the fledglings and finally the father, although this is not always the case. Males tend to arrive slightly earlier than females the following March/April but again, this is not always the case.

8. Scandinavian ospreys and eastern European birds tend to migrate to south Africa and East Africa. Russian birds tend to migrate to Asian countries. Canadian and North American ospreys tend to migrate to South American countries. Basically, northern ospreys tend to migrate directly south of where they breed.

9. You can see all four of the Dyfi birds' migration to Africa: Einion, Leri, Dulas and Ceulan, on Google Earth.

10. Last year's chick, Ceulan, holds the record for one of the fastest migrations of a UK osprey that we know of. He left the Dyfi on Monday (Sept 3rd) - he was in Senegal on the Saturday the following week - over 3,000 miles in just 12 days, and five of those over desert. He was 97 days old starting his migration!

Part 6 - Ospreys as Individuals

Welcome to this penultimate offering in the 'Seven Things You Never Knew About Ospreys' series. Over the last few years, osprey ringing and tracking have given us a wealth of information about these birds and a fascinating insight into their lives. Here are 10 stories of some of the best known and loved British birds:

1. EJ - Resident female at Loch Garten, she first bred there in 2004 with Henry. She has had several relationships, both marital and extra-marital and has had several of her eggs kicked out of the nest by other males. Her boyfriends include Orange VS, Blue XD, Red 8T, and her current partner Odin. And these are just the ones we know of!

2. Lady – Resident female at Loch of the Lowes since 1991. As most females do not breed until their third year, she must be at least 25 years old (she is unringed, so we do not know her exact age). She has hatched 63 chicks to date and 49 of those have successfully fledged. Her 50th will be in a few weeks time hopefully.

3. 03(97) – The first of the Scottish translocated ospreys to breed at Rutland and father of Nora. He's still going and is the current breeding male at Rutland Site B. He's successfully raised 27 chicks to fledging to date with three different females. He is Glesni's Grandfather.

4. 11(98) – Scotland to Rutland translocated male who has been breeding on the Glaslyn nest in Wales since 2004 (image below). To date he has fledged 21 chicks with the same female, two of which are the breeding males at Kielder Water (White YA and Yellow 37) and another is the Threave male in Scotland, Black 80. Another two of his offspring have been spotted as adults in the UK, male White YC (2007) landed on the Dyfi nest in 2011 and female White 91 (2009) landed on the Loch of the Lowes nest in 2012.

5. Logie – First osprey to be fitted with new generation GPS satellite transmitter in 2007. Her movements were followed during 2008 on the BBC Radio 4 programme “World on the Move”. Successfully raised two chicks that year, but departed on her Autumn migration in bad weather and was not seen again.

6. Henry – Resident male at Loch Garten from 2003 until 2007 and named because he carried an orange leg ring HV. He was famous for his regular late arrivals, by which time EJ had usually mated with another male and produced eggs, which Henry would then kick out of the nest. He also had orange eyes like Monty.

7. Rothiemurchus – Male that was hatched in 2009 on the Rothiemurchus Estate in Scotland and was satellite tracked. He's still going and has travelled great distances around Scotland in his short life to date looking for a nest site. Last week he visited the Loch of the Lowes nest!

8. 08(97) – The first of the Scottish translocated ospreys to return to Rutland in 1999, but he did not breed until 2007 when he paired up with Green 5N in Manton Bay. The pair moved nest to another Rutland site in 2009 after a failed breeding season due to constant interference from another male osprey. They successfully raised chicks there in 2009 and 2010, Glesni being one of the chicks from the 2010 nest. Sadly 08(97) disappeared in suspicious circumstances in 2011.

9. SSK - A juvenile female satellite tagged in 2002 in Scotland. SSK undertook her first ever migration passing directly over the Dyfi and continued over towards the Bay of Biscay before being blown off course by strong easterly winds. She was heading into the middle of the Atlantic before the weather this time came to her rescue with a change in wind direction. She finally made landfall in southern Portugal having flown over 1,500 non-stop in 64 hours. Not surprisingly perhaps, she stayed in Portugal that winter! SSK returned to Scotland three years later and successfully raised chicks.

10. Unringed osprey on a beach in the Yucatán Peninsula, Mexico in 1991. This was the first osprey I ever saw in my life. In my shorts and T shirt I ran after the bird which was carrying a fish for several hundred yards pointing wildly and shouting "OSPREY, OSPREY!" at all the tourists sunbathing on the beach. My girlfriend later said she had never been so humiliated in all her life and failed to speak to me for the rest of the day. It was the start of a wonderful love affair, and the end of another!

Glaslyn male 11(98) - he later lost his ochre (orange) leg ring sometime during the winter of 2006/7

Part 7 - Threats & Complacency

References to ospreys can be found in literature going back thousands of years. Greek playwright Aristophanes wrote about them in a comedy "The Birds", Shakespeare mentioned them in a play and ospreys are even mentioned in the old testament.

"I think he'll be to Rome
As is the osprey to the fish, who takes it
By sovereignty of nature."

~ William Shakespeare 1608

Since Shakespeare penned those words four centuries ago however, ospreys have been victim of a systematic attack by humans. Here are 10 threats to ospreys that have reduced their numbers dramatically in some areas of the world and caused their extinction in others.

1. Religion - In the middle ages Britain was a Roman Catholic country and no meat could be eaten on Fridays, it was illegal. So, people ate fish instead and fishponds and 'stewponds' became a commodity. Ospreys eat fish on a Friday too so they were hunted to protect the weekly catch.

2. Financial incentives - Ospreys, along with other birds of prey, were classed as vermin during the middle ages in Britain and a bounty was placed upon their head. Fish-eating birds fared worst and the osprey had the highest 'cash-for-corpse' reward of all - 4 pence.

3. Egg collectors - A favourite 'pastime' of British men from the 18th century onwards, it still goes on today, illegally. Only a few weeks ago an attempt was made at the Threave Castle nest in Scotland where Black 80, the 2006 Glaslyn osprey is breeding. Thankfully, this attempt was thwarted by a canoeist and the first of the eggs hatched yesterday.

4. Extinction - By 1916 the last osprey had been persecuted to extinction in the UK, the last pair lost their final battle at their nest site in Loch Loyne, Inverness. A naturalist of the day, Mr William Dunbar wrote in a letter to egg collector John Wolley in 1916, ‘I am afraid that Mr St John, yourself and your humble servant, have finally done for the Ospreys’.

5. Shooting - Shooting is a serious threat to ospreys and it still goes on around the world including Britain. One of the most unfortunate osprey casualties was a juvenile nicknamed Ossie in 2004. He was found shot by a Wiltshire river keeper in early September and taken to the Hawk Conservancy Trust for rehabilitation. After a few weeks he was released, complete with satellite transmitter. In early October he was shot again in Spain, this time fatally, on the first day of the hunting season.

6. Poisoning - Any chemical that is released into the environment finds its way into the food chain. Those animals at the top of the food chain suffer the most. In the 1960's and 70's in the USA, DDT - a pesticide widely used in agricultural and forest production -  caused osprey eggshells to thin and eventually crush under the weight of an incubating female. DDT interferes with calcium production necessary for eggshell strength and is now banned, although not everywhere. Osprey populations in the US plummeted at the same time.

7. Habitat oss - Big birds of prey like the osprey like to inherit nests or displace other birds to acquire nests rather than build their own. Osprey nests are sometimes many decades old or even older. Due to their persecution and extinction in many parts of the world, the old ancestral nests have been destroyed or developed over. That's why we built a nest platform on Cors Dyfi in 2007 - they replicate ancestral nests of ospreys that have now long gone.

8. Baler twine - This string, used widely in agriculture, is a serious threat to ospreys as they will collect it as nesting material and become entangled in it. Ospreys die each year in the UK and around the world after getting caught up in baler twine. Roy Dennis had to cut one of his favourite ospreys free from it in 1982, Red Z. Thankfully, she survived and continued to breed until 1997.

9. Fishing line - Hooks and floats can be swallowed and cause death. Nylon line around a foot can cause blood flow to be cut off and the foot to become useless. The female at Loch Garten in 1993 was lost mid-season when she was last seen trailing a big ball of fishing line. Her mate Ollie was forced to raise their two young chicks alone, and he succeeded! Our own Ceulan died in fishing nets, of course.

10. Complacency - Possibly the greatest of all threats in the UK now.

Some of these threats are now gone and will hopefully never return. Some are as prolific as ever. There are still only around 10 pairs of ospreys in England and Wales combined, it's one of our rarest birds of any description. We must continue to protect and conserve what little we have and try and overturn the damage caused by our ancestors in the past. We must not be complacent, we can't afford to be. We must not let our children inherit what we did.

Natural England recently issued licenses to destroy buzzard nests to protect stocks of non-native birds so that people can blast them out of the sky for 'sport'. Only a handful, but that doesn't matter. It could be more next time, and more the time after. The time after it could be another species. Thin end of the wedge and all that. A friend of mine rang me last week to say that the hen harrier has now become extinct in England because of persecution, what an utter travesty. We simply cannot be complacent and stop fighting.

~~~~~~~

A final thought: Exactly one year ago to the very minute I'm writing this on June 9th last year, I was peering out through a telescope at Monty and Nora's nest from the osprey hide - all the cameras were down. Nora was trying to protect her two remaining chicks in what was the worst summer storm ever recorded. Miraculously, little Ceulan survived, just. And a few hours later at around 4pm we gave the little guy some help as he was too weak to stand up in the nest and call for food. His sibling had already died and was laying next to him.

If you're reading this in the UK, there is a Wildlife Trust where you are doing the same thing as we do here at Montgomeryshire Wildlife Trust - Protecting Wildlife for the Future. Ring them, volunteer for them, join them. Whether your thing is otters, oak woodlands, orchids or ospreys, it doesn't matter, they all need looking after. They all need your help.

We hope you have enjoyed 'Seven Things You Never Knew About Ospreys' and that even the most chronic 'ospreyholic' (you know who you are!) learnt at least one thing, If you did, it was worth it. Remember, the more we learn the more we can protect and the more we protect that more we will learn.

Ceulan