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Volume 86, 1959
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Flightless Birds

A browse through the first edition of the “Origin” (1859) uncovers a host of topics relevant to problems of New Zealand biology, past and present. Darwin's first chapter, “Variation Under Domestication” includes his observations on the contrasting proportions of wings and legs in wild and domestic ducks. With domestication, greater body weight, and less need for flight, a domestic duck's legs have become heavier, its wings lighter than those of a wild duck. Darwin in several places referred to the somewhat mysterious laws of “correlation of growth” that have been much clarified by workers on “functional morphology” such as S. Brody34 on domestic animals, and D. M. S. Watson35 on fossil lineages. The application of principles of functional morphology—of the correlations in growth rates, and of quantitative dependence of the growth of one part on the growth of

[Footnote] 34 Brodie, S., 1945. Bioenergetics and Growth, New York.

[Footnote] 35 Watson, D. M. S., 1949. Pp. 45–63 in Genetics, Paleontology and Evolution, Princeton.

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another, have been put to good use for the benefit of man in the development of domestic meat-producing animals by research workers such as John Hammond in the United Kingdom and C. P. McMeekan in New Zealand, but the same principles have seldom been applied to the interpretation of differences between wild animals. Darwin's domestic duck (he referred in particular to the Aylesbury36), heavier in body and stouter in leg than its wild progenitor, can be compared with our New Zealand Takahe, which shows similar contrasts with the Pukeko, its closest relative, presumably closely similar to the stock from which the Takahe sprang37. The Pukeko is an adaptable moderate-sized waterfowl with adequate powers of flight, widely distributed in the Southern Hemisphere. Notornis differs chiefly in its large size, stout legs and neck, reduced wings, and oversized beak. Increase in size has been a common tendency among herbivorous vertebrates, and has a simple physiological explanation. The food requirements of an animal can be broken up into a “maintenance ration”, required to maintain life, and a “production ration”, which provides the energy for all activity, locomotion, reproduction,

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Fig. 1.—The importance of flight has generally kept birds average size row, except in Darwin's domestic ducks, in the protection of the farm-yard, and flightless birds, surviving in the specially protected conditions of primitive New Zealand.

[Footnote] 36 Origin: 9, 117.

[Footnote] 37 Fleming, C. A., 1951. Some general reflections on Notornis. Notornis,, 4 (5): 103–6.

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growth, etc. The maintenance ration is related to the weight, but is not proportional to it, increasing roughly as the animal's surface area—i.e., as the ⅔ power of the weight. But the production ration is directly proportional to the energy used, and thus to the weight. Thus food consumption consists of a maintenance ration varying as the square of a linear dimension plus a production ration varying with its cube. For the same amount of activity, a larger animal has a greater thermodynamic efficiency than a small one. Darwin's Aylesbury duck and New Zealand Notornis have this physiological advantage over wild ducks and pukekos, provided that other factors allow their survival.

But flight is more difficult in large birds than in small ones, since they must fly faster to avoid crashing. Efficiency of a wing is a function of its area (proportional to the square of a linear dimension) but the load to be carried is proportional to the cube of a linear dimension, so that a bird twice the linear size of another would have to support eight times the weight of its smaller relative with only four times the wing area. A swamp-hen about twice the size of a pukeko, as Notornis is, would have much less efficient wings unless wings and muscles changed in compensation. This was Haldane's argument that a conventional “angel” would need a 4ft keel on its sternum to support the large muscles necessary for flight. But energy used in flight is a debit against the production ration of a bird, and is a more expensive item in a large bird than in a small one. This is the chief reason why large birds have become flightless when flight was no longer necessary for survival38. The importance of flight has generally kept birds' average size low, except in Darwin's domestic ducks, in the protection of the farm-yard, and New Zealand flightless birds, surviving in the specially protected conditions of primitive New Zealand, freed from the need to escape from carnivorous predators.

Large size also accounts for the robust legs of farm ducks and Notornis, for the strength of a supporting column is related to its cross-section (square of linear dimension) and must increase disproportionately to keep up with a cubic increase in weight. Similarly, the capacity of a column to support weight varies inversely as the square of its length (Euler's principle) so that leg bones of heavy animals tend to be shorter than those of smaller related species.

I have made this long digression on the excuse of Darwin's farm duck to emphasize that intensive directed research on domestic animals can still provide many analogies for the interpretation of wild ones, as Darwin well knew.

[Footnote] 38 Romer, A. S., 1945. Vertebrate Paleontology, Chicago: Univ. Press.