How much fish can an osprey lift?

  (Warning: may contain mathematics. Do not try this at home!)

Could YOU lift yourself up to chin level on a horizontal gym bar, with a domestic washing machine tied to your ankles?

No, me neither. 

During the 2012 breeding season, Dyfi osprey “Monty” brought several hundred fish back to his nest. All were of good size and some were very big, including this monster mullet which must have weighed almost as much as himself.[1]

 It set me thinking: how much weight could an osprey like Monty lift? Sometimes, ospreys can snatch their prey at the surface without diving, but not always. In the worst case, he would have to drag it straight up out of the water after capture, so there would be no contribution from aerodynamic lift in forward flight. Instead, the initial lift all depends on one single wing flap to get the fish (and himself) clear of the water. I decided to work out the numbers...

During his nesting work at Cors Dyfi, Monty has been too busy to be measured, which is understandable. I took average figures for (male) wing length and chord of 67.5cm and 45 cm respectively. The displacement lifting power of a bird's wing during takeoff is given by the formulae:-

… where r is the wing length, w is the chord, n is the number of wings, θ is the angle of flap (see below), p is the density of air (taken as 1.225kgm³), and g is the force due to gravity.[2]

When taking off from water, ospreys use a “clap-and-fling” wing motion where the wings are brought up above the back until the tips almost touch, open the trailing edges to form a “V” shape between them, and then throw the wings forward and down until they are horizontal. It is believed [3] that this action generates extra initial lift because of vortex effects at the wingtips. Assuming that this is done on the first flap, I took a figure of 90 deg for the wing travel angle θ.

Plugging all these values into the formula gave an interesting result. It suggests that a single flap of Monty's wings should be able to lift 3.87 kgs straight out of the water! If Monty himself weighs (say) 2 kg soaking wet, that confirms that he can indeed catch and bring away a fish of almost his own body weight.

Sources:

[1] Dyfi Osprey Project: Facebook entry 25/8/12

[2] N.Pilkington, M.Parry, J.Bettles, I.Clarke; Special Topics P3_6 Journ.Phys Vol 10, No 1 (2011)

[3] Weis-Fogh T. Quick estimates of flight fitness in hovering animals, including novel mechanisms for lift production. The Journal of Experimental Biology 59,169-230, 1973

Preliminary Migration Overview

February 2013

The North Atlantic jetstream continues to lurch and wobble on its way, as has been the case since this time last year. At present it is giving unsettled weather over most of the British Isles, and triggering thunderstorms today in southern Italy.

Its counterpart over the Sahara is still further north than usual. A seasonal blocking anti-cyclone east of the Azores continues in place. (The arrows on the chart are NOT surface winds, they are the high-altitude jetstream circulation.)

Conditions over west Africa and western Europe are not favourable for early migrants, with contrary winds at low levels and the westerly and northerly airstreams holding temperatures down.  General visibility over NW Europe is poor.

In the coming weeks, we would like to see a weather picture developing similar to this...

... which was the surface chart last year (9th March), when high pressure over Britain, the western Med, and north Africa enabled birds to make good progress towards "home".  Monty reached Wales on the 2nd April, but it was the situation shown here which enabled him to get moving.


I will blog again in a few weeks time, when the picture should be getting a little clearer.

Bird Vision (1) - "The Impressionist Osprey"

With the ending of the First World War, exhibitions of art began again in Paris - and none was more eagerly awaited than a new collection of works by the acknowledged master of the impressionist style: Claude Monet.

The subject was no surprise: “Les Nymphéas” - water lilies, which the great man had rendered many times. But the canvases were huge – up to twenty feet long – and when the paintings[1] were revealed, there was consternation among the self-appointed experts:

“Name of a holy dog, what is this that it is?” they muttered (in French) behind their hands, “Has le pauvre Claude totally lost the plot, malheureusement?”

The lily flowers were pink, not white as they should have been. In place of Monet's subtle graduations of hue and tone, the new pictures were a riot of purple and blue, with shimmering traceries of indigo and blinding white in the backgrounds. Many were shocked, but some of Monet's fellow artists wondered if their leader had abandoned his exploration of colour, and was now trying to say something about the very nature of light itself. 

They may have been right...

Humans have trichromatic vision: specialist cone-shaped cells in the retina of the eye respond to different wavelengths of light. The widest response is to red and green, with a narrower response in blue, peaking at around 450nm. Perceived response to very short wavelengths (below 400nm) is almost non-existent in people with normal colour vision, because a pigment in the lens of the eye filters out this “near ultra-violet” light.

Birds are different. 

Many have a fourth type of cone receptor with a peak response around 380nm. (It varies between species.) This means that birds really can see reflected or transmitted ultra-violet light, whereas we only notice materials which are fluorescent under UV – a different phenomenon that can be observed (when sober) in most night clubs. Birds use their enhanced colour vision in many ways: some have plumage which shows clear patterning or bands of UV reflectance, invisible to us. Kestrels can spot the urine trails left by rodents. Hummingbirds use UV patterns to identify their favourite flowers. And there's more.

The ultra-violet components in sunlight transmit more effectively through water than longer wavelengths, and have a different angle of scattering from the surface. The ability to see UV could give ospreys a hunting advantage, because they may be able to use their short-wavelength vision to pick out targets with varying luminance in water at some depth. This attribute of vision, called “contrast sensitivity” would be more important to an osprey than to us, because our food is found in trees. (or in Tescos.) Interestingly, it has also been discovered that the absorbance of UV by water is related to the amount of dissolved oxygen in it.[2] This raises the possibility that hunting ospreys – and many other bird species – may be able to assess the quality of a potential feeding site while flying over it, because the water surface would actually be a different “colour” to them!

Claude Monet would have understood that. During the war, he had discovered that his sight was deteriorating, and underwent surgery for cataracts in both eyes. With his  “UV filter” removed, he saw the world of colour in a different way, and developed a new mode of painting to interpret it.

We can never know exactly how birds perceive their environment, but the idea of an osprey hunting through that ever-changing impressionist landscape of pastel shades and iridescent water is an attractive one. I like it.

[1] Now on permanent display in two purpose-built galleries at the Musée de l'Orangerie in Paris.

[2] "Estimation of biochemical oxygen demand based on dissolved organic carbon, UV absorption and fluorescence measurements." Kwang et al, J.Chem, v2013