The migration of desert locust swarms: Facts and possible implications for bird migration

PROCEEDINGS OF THE ASSOCIATION FOR THE STUDY OF ANIMAL BEHAVIOUR

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THE MIGRATION OF DESERT LOCUST SWARMS : FACTS AND POSSIBLE IMPLICATIONS FOR BIRD MIGRATION . By R . C . RAINEY, Anti-Locust Research Centre, Kensington, London . Desert locusts make regular long-range mi- subtropics, where the modern locust data show grations, over distances of thousands of kilo- that even randomly-orientated flight . making metres, to a seasonal pattern which enables the possible the temporary colonization of the species to exploit effectively the scanty and ephemeral habitats provided by the erratic erratic rainfall characteristic of its typical rains of the semi-arid and arid zones, is of subhabitats, where these migrations accordingly stantial survival value ; the proverty of the fossil record of birds is consistent with the possibility have a very important survival value . It is, however, now clear that the part played by of much of the evolution of the group having the orientation (heading) of the individual fly- occurred under arid conditions . Might it perhaps ing locusts, in determining the direction of these have been the great planetary wind-fields, parimpressive migrations, is certainly very small and ticularly the Trade-winds and the inter-Tropical probably negligible . The considerable data now Convergence Zone (which are likely, on meteoravailable on the directions both of large-scale ological grounds, to have existed far back into seasonal migrations, and of the local movements geological time), which produced the firstof individual swarms, are in fact consistent with seasonal displacements of initially randomlyan effectively random orientation of the flying orientated fliers, acting upon these primitivelylocusts . This inevitably results in a down-wind equipped pioneer birds in a manner essentially direction of displacement ; and the seasonal pat- similar to the way in which these wind-fields still tern of locust migration, together with its appear to determine the migration of locusts? biologically significant association with rainfall, And was it perhaps population displacements, necessarily emerges from down-wind displace- initially produced in this manner, which proment and the meteorological fact that convergent vided the `familiarization flights' needed for the low-level wind-flow is essential for the pro- development of bird navigation to the impressive duction of signicant rainfall . standard which it has now reached? The InterMigrating locusts thus appear very much as Tropical Convergence Zones show not only creatures of circumstance, with not only the seasonal but also short-term fluctuations in directions but even the destinations of their position, which result in to-and-fro movements of locust swarms, thereby suggesting possibilities spectacular flights largely if not wholly deterof elementary training of primitive birds in mined by the corresponding synoptic meteorvisual navigation by local familiarization flights ology. This is in such complete contrast with the standard of navigation exhibited by birds that repeated at intervals of days or weeks . it has seemed, until recently, that the migrations One might thus come to regard ability to of birds and of locusts must be two processes so navigate in flight as, geologically, a relatively fundamentally different that even the use of the recent refinement, rather than as a necessary same term was to be avoided . first step in the evolution of bird migration . The locust story may, however, suggest a Even if the first birds were already capable of possible mechanism by which birds might rudimentary terrestrial or marine navigation, as originally have begun to learn to migrate and to some modern reptiles may be, a mechanism navigate, initially with sense-organs and central- comparable with that of locust migration might nervous system of perhaps no more than reptil- still have operated to give them sufficient opporian standard . The first birds presumably may tunity to evolve solutions to the different and well have been poikilothermal, and perhaps probably more difficult problems of aerial accordingly mainly restricted to the tropics and navigation .

APHID DISPERSAL AND ITS BEARING ON THE GENERAL CHARACTER OF INSECT MIGRATION BY FLIGHT. By C . G . JOHNSON, Rothamsted Experimental Station, Harpenden . The nature of the migration process in insects has been seen in terms of mass migration in butterflies, and as a flight, whose direction is controlled by the insects concerned . Passive drifters such as aphids were not regarded as true

migrants ; and dispersal and migration were seen as different processes . But strong-flying locusts, apparently in control of their direction of displacement, are displaced along wind trajectories ; this blurred the