Category Archives: Osprey Biology

Osprey Biology

How we identify our birds

Metal and colour Osprey ringsEach bird has a metal ring on the left leg and a plastic colour ring on the right leg. The rings do not hurt the birds nor do they worry them in any way. Rings are only allowed to be put on birds by licensed ringers, which for our project are Michel Beaud, Adrian Aebischer and Roy Dennis. The metal ring on the left leg is provided by the Swiss Ornithological Station (SOS) at Sempach and has a unique number. If someone finds a ringed bird they can inform the SOS who will then tell us that someone has seen the bird.

Osprey rings
Osprey rings.

To be able to read the metal ring you usually need to have captured the bird. However, on the right leg we put a larger plastic colour ring which can be read more easily when the bird if flying or perched, usually with a telescope or a telephoto camera lens. Each bird has a unique code and in 2015 we put blue rings with the white letters PP followed by a number (PP1, PP2, up to PP6) on the right leg. These rings were already placed on the birds in Scotland so that they were identifiable when they were brought into Switzerland, and the numbers were included on their CITES export and import permits.

Note that  blue colour rings are also used for Ospreys in the UK, but these have different combinations of letters and numbers and are placed on the left leg. Therefore, if anyone sees an Osprey with a blue ring on its right leg, it is an Osprey that has been reintroduced to Switzerland. The birds are named after the letters and numbers on their blue plastic ring.

Osprey Biology

What makes an Osprey unique?

Balbuzard pêchant par Michel Jaussi

  • It is the only diurnal raptor that feeds only on fish.
  • Ospreys are the only species belonging to the genus (Pandion) and family (Pandionidae), meaning that they are taxonomically very distinct.
  • Along with owls, they are the only raptors with a reversible outer toe, meaning that they can grasp their prey with two toes in front and two toes behind. Once caught, the fish don’t stand a chance.
  • The global distribution of a same species of raptor in every continent (apart from Antarctica) is exceptional, with only the Peregrine Falcon having a wider distribution amongst diurnal raptors. But yes, taxonomy is the opinion of the scientist. It is possible that what has up to now been considered the same species (but different subspecies) living on different continents may at some stage become separate species.  But even if science decides that there are different species of Osprey on different continents, this doesn’t change the fact that all Ospreys look very similar and have the same ecology throughout the world. They live at the top of the food chain and are excellent indicators of the health of aquatic ecosystems.

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Osprey Biology

Breeding

Ospreys can start breeding at the age of 3 years (except for a few exceptional cases), with the average age of first breeding recorded at 3.8 years in France. The more  territories that are available in the region where the birds were born, the earlier they start to breed. However, if the best nests are already occupied by other Ospreys, young adults prefer to delay breeding rather than go further afield to seek new territories elsewhere. Continue reading Breeding →

Osprey Biology

Remarkable fishers

Osprey rings by Wendy Strahm

Osprey carrying fish by Sylvain Larzillière
Osprey carrying a Chub (Squalius cephalus) in its talons.

Ospreys are the only diurnal raptor to feed almost entirely on live fish, and their fishing skills are legion. Flying or hovering high in the sky, they make spectacular dives into the water once a potential prey is spotted. With luck they will then snatch their fish with their enormous talons, firmly gripping it with their reversible toe. They generally take fish that either live in shallow water or at the top 10-20 cm of the water column, for example chub or roach.  Unlike species like cormorants or grebes, Ospreys do not dive and swim underwater.

At times it seems that the fish is too large for the bird to carry, and there are even reports of an Osprey being unable to release a fish that is too heavy, causing the bird to drown. There are no documented cases of this actually happening, but the story is repeated in many places.  We will believe it when we see it!

A fantastic video of an Osprey fishing can be seen here, showing the North American subspecies, which is not the same as the subspecies found in Europe (see taxonomy).

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Osprey Biology

Where are Ospreys found today?

Osprey distribution
Distribution map and population status of Osprey Pandion haliaetus populations in Europe. Adapted from BirdLife Intl (2004) and modified with data from Schmidt, Dennis & Saurola (2014). The green “R” indicate where Osprey reintroduction projects have or are being undertaken.

The Osprey, a cosmopolitan species subdivided into four sub-species, occurs in the Palearctic, North America, the Caribbean and Australia (see taxonomy). It is a migratory species which can be found on every continent except Antarctica. Nowadays the Palearctic sub-species has an irregular distribution, mainly occurring from Scotland to the west to the Kamchatka Peninsula and Japan to the east. While its European population is fairly large (with an estimate of about 10,000 pairs), this population is quite fragmented and has undergone strong declines and extinctions in the relatively recent past. Continue reading Where are Ospreys found today? →

Osprey Biology

Migration

Osprey and flamingo Banc d'Arguin by Wendy Strahm

Most European Ospreys nest in the north and winter in Africa south of the Sahara (apart from a few Mediterranean populations which are more or less sedentary). A few birds stop before crossing the Mediterranean and winter in Spain or even in the south-west of France, but this is unusual. In Switzerland Osprey migration occurs over a fairly wide period, and small numbers of birds can be seen mainly from mid-March to the end of May in the spring and from mid-August to the end of October in the autumn. Continue reading Migration →

Osprey Biology

Why won’t the Ospreys return to breed in western Switzerland by themselves?

Balbuzard par Pascal RapinThe reason that it is very unlikely (with nature you can never say impossible, but we can say very unlikely) that the Osprey will  start breeding in western Switzerland again without help is due to “philopatry”. Philopatry is the tendency of an individual to stay or to return to the place where they were born in order to breed. Some species, like the Osprey, have a very strong philopatry, which results in a very limited ability to disperse to new areas. This means that when a population disappears, it is exceedingly unlikely that individuals who were not born in this area will recolonise it, even if suitable habitat exists. Continue reading Why won’t the Ospreys return to breed in western Switzerland by themselves? →