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Darwin Correspondence Project

From Robert Shaw   28 November 1876

Glassaugh, Portsoy, | Banffshire. N.B.

28 November 76.

Dear Sir

I have come upon on extract from one of your works, describing the soaring of the Condor, but so far as I know you have not tried to account for it.1 This I have done in the accompanying letter which first appeared in “Land & Water”.2 I find that a good many inhabitants of Salt & fresh water have the same mode of progression I have supposed and I think almost proved the kites to possess. Under the wing of the common Indian kite3 there lie upon his sides just in the course of the air jet certain long stiff smooth and perfectly flat feathers as if on purpose to alviate any obstruction. If my conjecture be discarded what other so feasible can take its place?

I read very lately in a book of Natural history that the vulture finds the carcase by sight. I have positive proof that it is by scent.4

Yours faithfully | Robert Shaw | Major General

P.S. The common Indian kite has an orifice in the humerus large enough to admit a large quill—to the best of my recollection.


Soaring of Eagles.


At page 265 of Mr Buckland’s ‘Log-Book of a Fisherman,’5 he writes as follows:— ‘The object of this beautiful mechanism is, probably, to distribute the power of the wing-muscles when in the act of flying. Towards the upper end of the bone, there is a good-sized hole; this hole is placed there in order to admit air, and, though I believe the fact is disputed by some, yet I believe that the wing-bones of the eagle and other birds are connected with the lungs, so that the bird can fill his skeleton with warm air, when he wants to fly, and thus become an animated balloon.’

Now, one of the four things that puzzled King Solomon (see Proverbs xxx. 19) was ‘the way of an eagle in the air,’ and it has puzzled a good many men since. I remember in the book of a naturalist attached to a scientific expedition to South America his remarking with wonder the continuous soaring flight on a calm day of one of the Falconidæ (if you admit the kite into that noble family).6 He watched it for hours sailing in different directions without a single motion of the wings, the primary feathers, which the slightest stroke must close, being visibly apart in certain points of view, and therefore presumably for the whole time. How is this to be accounted for? A screw steamer sails without, to the astonished savage man’s eye, any visible means of progression; but how is it that a bird in the air does so, to the bewilderment both of King Solomon and, so far as I know, of the living scientific world? I have long had a theory on the subject, which I will state now for the first time, and whether or not it will solve the mystery, you and your readers (as the case may be) shall judge.

Though fond of books of natural history, I am no naturalist, and never dissected either bird or beast, except cooked and on the dinner table. Nevertheless, many years ago I observed in the wing of some large soaring bird, I think an albatross, caught off the Cape of Good Hope, the orifice mentioned as existing in the principal wing bone of the eagle, and this I take to be, not for the admission of air, as Mr B. conjectures, but for the forcible expulsion of it as the invisible m⁠⟨⁠ea⁠⟩⁠ns of progression while soaring for miles continuously, as all observant travellers must have witnessed. This may appear quite fanciful, yet there are analogies both in nature and in mechanics, I will not say in proof, but in support of the surmise.

Among the aquatic insects there is a creature which naturalists assert progresses by squirting water from the hinder extremity.7 Similarly, I believe, the Falconidæ, especially the kites, which are the longest-winged of the family, sail along as if by a miracle, by the forcible expulsion from the inner end of the humerus of the inhaled air expanded by the heat of the body, which I believe to be an ‘animated balloon,’ by not only the bones but the quills being filled with heated air.

Many mechanical examples might be given of analogous action. For instance, no one will doubt that the recoil of firearms, both great and small, would be less violent could they be discharged into a vacuum, and a filled syringe would sink vertically less rapidly in water, if we could by any contrivance make it gradually discharge its contents at the same time. I take it that heated air driven from behind the upper arm of a bird in flight would act like a spiral spring equally at both ends, and all the more effectually that it would impinge on a fluid body more dense than itself.

I will not pursue the argument, ⁠⟨⁠    ⁠⟩⁠8 and shall be glad if you or any of your readers can show something more feasible to account for what really looks, till accounted for in some way, like a miraculous violation of the laws of force and gravitation. A practised naturalist may perhaps find confirmation of my suggestion in the valvular mechanism of the larger Falconidæ.

I do not believe ‘eagle’ is the proper translation in Proverbs. It ought, I am certain, to be kite. There are several species of eagle on the Neilgherry Hills, where I have been,9 but it is not so much their habit to soar as it is with the kites. Similarly, ‘Where the carcase is, there will the eagles be gathered together,’10 is an obvious mistranslation for vultures.—

Yours obediently, | Robert Shaw.

Glassaugh, Portsoy.


The letter appeared in Land and Water, 8 January 1876, p. 8.
The black kite, Milvus migrans.
In Journal of researches 2d ed., pp. 184–6, CD discussed whether carrion birds found their food by scent or by sight.
Frank Buckland; Buckland 1875.
The taxonomic place of kites was controversial in the nineteenth century (see Swainson 1836–7, 1: 318–20, and Newton 1893–6).The nineteenth-century family Falconidae has now been divided into the Accipitridae (eagles, hawks, and kites) and Falconidae (falcons and caracaras). There are several genera of South American kites.
Dragonfly nymphs can move quickly by squirting water from their anuses.
Shaw inked over the words ‘which I advance with diffidence and under correction,’.
The Nilgiri Hills are in southern India. Shaw had served in the Indian Army.
Matthew 24: 28.


Buckland, Francis Trevelyan. 1875. Log-book of a fisherman and zoologist. London: Chapman & Hall.

Journal of researches 2d ed.: Journal of researches into the natural history and geology of the countries visited during the voyage of HMS Beagle round the world, under the command of Capt. FitzRoy RN. 2d edition, corrected, with additions. By Charles Darwin. London: John Murray. 1845.

Newton, Alfred. 1893–6. A dictionary of birds. Assisted by Hans Gadow, with contributions from Richard Lydekker, Charles S. Roy, and Robert W. Shufeldt. 4 parts. London: Adam and Charles Black.

Swainson, William. 1836–7. On the natural history and classification of birds. 2 vols. London: Longman, Rees, Orme, Brown, Green, & Longman; John Taylor.


Encloses printed letter from Land and Water in which he proposes a hypothesis that explains how soaring birds can stay aloft by expelling air from their lungs.

Letter details

Letter no.
Robert James (Robert) Shaw
Charles Robert Darwin
Sent from
Source of text
DAR 177: 153
Physical description
4pp encl

Please cite as

Darwin Correspondence Project, “Letter no. 10690,” accessed on 7 December 2021,

Also published in The Correspondence of Charles Darwin, vol. 24