Sunday, 24 February 2013

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.

[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

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