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Abstract

The naturalist John James Audubon famously gave an account of a migration of the now-vanished passenger pigeon (Ectopistes migratorius). After attempting to count the passing flocks that together made up the vast procession, he abandoned this task as impractical and continued on his journey, noting that at the end of a full day’s travelling, the birds still continued to pass by and did so still for several more days thereafter (Audubon 1870). Breeding colonies consisting of hundreds of millions of pairs of these birds were reported during the 1800s, and it is estimated that the largest migrations contained billions of individuals (Schorger 1955). During the latter decades of the nineteenth century, as Americans in the eastern states bore witness to these huge flocks, those living further west were contending with periodic outbreaks of another multitudinous animal, the Rocky Mountain locust (Melanoplus spretus). One infamous swarm of 1875 was estimated to have covered half a million square kilometres and to have contained several trillion locusts (Piper 2007). Like the passenger pigeon, the Rocky Mountain locust was to be extinct shortly after the turn of the century. Today, juveniles of the extant – though declining (Atkinson et al. 2004) – Antarctic krill (Euphausia superba) in the Scotia Sea of the Southern Ocean form super swarms trillions strong that can be 30 m deep and extend over several km, sometimes containing hundreds of individuals per cubic metre. So large are these swarms that the majority of the total population can be contained within just a few such aggregations (Tarling et al. 2009).