Abstract
In his original paper, Mr. Alexander, while examining curious suggestions such as that the super-inflation of the internal air-sacs might account for flotation, did not mention the possibility of, and made no allowance for, increased displacement due to the fluffing out of the feathers. Therefore his calculation regarding the amount of water displaced was invalid. His argument against the practical value of the suggestion, that it would cause instability, ignores the power of the living bird to control its balance by its peculiarly adapted feet and perhaps also by feather adjustments. Yet this is the one power that his waterproof jacket experiment demonstrated, for although he actually lowered the centre of gravity, the bird became unstable—obviously because its control had been interfered with. We still think that Mr. Alexander's data are insufficient; until he has measured the actual displacement of the swimming duck and found the water displaced to weigh less than the bird itself, it is futile to argue about supplementary forces of ‘levitation’ or any other force. Last week, I watched a grebe at one time swimming high in the water, at another time almost submerged. That suggests an alteration in specific gravity, that is to say, since the bird's weight is constant, in volume; and the raising or adpressing of the feathers, with a corresponding increase or decrease in the air-jacket, seems a possible and natural explanation of the change.
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Bird Flotation. Nature 126, 278 (1930). https://doi.org/10.1038/126278a0
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DOI: https://doi.org/10.1038/126278a0
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