Seasonal Changes in Flight Direction of Migrant Butterflies in the British Isles

Abstract

For many years naturalists were reluctant to admit that butterflies, moths and other frail insects were capable of undertaking unidirectional 'migration' flights of hundreds of miles or more. When the evidence of this became overwhelming, it was held that the flights were 'over-flow' movements from over-populated districts, and could not be compared with the 'real' migration of birds. True migration must consist of a to-and-fro movement of the population in one direction at one time of the year and in an opposite direction at a later season. In fact, in the north temperate areas, what was considered necessary to establish the existence of migration was evidence of a movement to the north in the spring and to the south in the autumn. Round about the year 1920 only one butterfly was generally admitted to be a migrant in this sense: the monarch (Danaus plexippus L.) of North America. In this insect there was considerable evidence of a southward movement in the autumn, and slight evidence of a return movement in the spring. In 1930, I published a book on the Migration of Butterflies (Williams, 1930), bringing forward evidence, of various degrees of incompleteness, for migration in over 200 species of butterflies from many parts of the world. There was also small evidence of a return flight in about half a dozen species. Even at that date, however, there were competent entomologists who would not accept the idea that such movements were anything but exceptional outflows, without any relation to regular migration of birds. Dr A. H. Clark, one of the best of the North American lepidopterists, did not even accept the reality of the migration of the monarch butterfly till about eight years later. Heape, in his book on Emigration, Migration and Nomadism, which was published posthumously in I 93 i, but actually written before my own book appeared in I 930, expressed fully the then prevailing view that 'to-and-fro' migration was fundamentally different in origin from overflow movements in one direction only, and that true migration was only known in one single species of butterfly-the monarch. In 1936, Grant brought forward evidence of a northward movement of the red admiral butterfly (Vanessa atalanta L.) in the British Isles which lasted during May, June and July, and of a southward movement in September and October. This will be referred to again later. In I942, I brought forward evidence for a return flight at a different time of the year in a dozen species of butterflies and two species of moths (see Williams, Cockbill, Gibbs & Downes, I942, pp. 240-8). The butterflies included, in addition to those mentioned above, Phoebis sennae L. in America, Catopsilia pyranthe L. in India; Colias croceus Fourc. in western Europe; Kricogonia lycide Godt. and Ascia monuste L. in southern North America and in Central America; Vanessa cardui L. in Europe; Dione vanillae L. in Florida; Libythea labdaca Westw. in West Africa; and Danaus berenice Cr. in Florida. In fact I had then come to the conclusion that evidence for a return flight is found in a species as soon as close observations are made and a sufficient number of observers are available. It appears that in many cases the flight in one direction is gregarious and in large numbers, while the flight in the opposite direction is in small numbers with the butterflies flying singly. So long as we remained dependent for information on the accidental reports of field naturalists, the evidence was biased in favour of the conspicuous flights in one direction; when more careful observations were made the second movement began to be noticed. Shortly after the publication of my book in 1930, an organization was formed in south-eastern England, largely by the energy and enthusiasm of Captain T. Dannreuther, to watch for evidence of directional movements of butterflies and other insects in the British Isles; and also to collect records of the occurrence of known immigrants in Britain even if there was no immediate evidence of directional movements. This organization is known as the 'Insect Immigration Committee of the SouthEastern Union of Scientific Societies'. Hundreds of