“Moths include many major pests of varied habits—defoliators, miners, cut-worms, grain and fabric pests. The larvae form an inexhaustible supply of insect food to almost all species of insectivorous birds, and even many species of birds that when mature feed almost, if not quite, entirely on grain and seeds are when in the nest fed very largely on caterpillars by the parent birds.”
Obviously, then, in India birds comparatively rarely attack butterflies; but they devour millions of caterpillars. It is the same in other parts of the world.
Mr. G. A. K. Marshall, in the course of five years’ observation in South Africa, recorded eight cases of birds capturing butterflies.
Similarly Mr. Banta points out in various issues of Nature, in 1912, that all the evidence available shows that in North America birds very rarely capture butterflies. Field naturalists scarcely ever witness a butterfly chased by a bird. Of 40,000 stomachs of birds examined very few were found to contain remains of butterflies.
In 1911 the butterflies of the species Eugonia californica were so numerous that “the ground was often blackened with them, and great swarms of them filled the air from morning to evening.” Yet of the birds in the locality where those butterflies were most numerous, only five out of forty-five species were found by direct observation and stomach examination to eat the eugonia, and the only bird that fed off them copiously was the brewer blackbird (Euphagus cyanocephalus) which is almost omnivorous, and eats insects of all kinds, even if they be what Darwinians call warningly coloured!
Now, modern theorists, as a rule, ignore facts such as these, and this certainly is the wisest course they can pursue, unless they are ready to give up these theories or make themselves look foolish.
However, I am glad to be able to record that Professor Poulton has, as regards the remarks of Mr. Banta, not followed the usual course of the modern theorist.
He has had the courage to take up the cudgels and reply to Mr. Banta in Nature. The reason of this unusual course appears to be that Mr. C. F. M. Swynnerton has made some observations in South Africa which Professor Poulton considers are in favour of his pet theory.
According to the Professor, Mr. Swynnerton, as the result of three and a half years’ investigation in South-East Rhodesia, “has obtained the records of nearly 800 attacks made by 35 species of birds belonging to 30 genera and 18 families, upon 79 species of butterflies belonging to 9 families or sub-families.”
Professor Poulton does not seem to see that the researches of Mr. Swynnerton are altogether subversive of the theory of protective mimicry. In order that natural selection may totally change the colouring of a butterfly (as it does according to the theory of protective mimicry), that butterfly must be habitually preyed upon by large numbers of birds, which must be so vigilantly and unceasingly on the look-out for it, that its only chance of escaping from their attacks must be for it to assume a disguise.