Monday, 7 October 2013

Small whites on the move

It wasn’t the sort of sight that leapt out at you, a series of small white shapes fluttering across the salt-marsh, but with a bit of studied thought it became clear that we were witnessing an ‘event’. The white shapes were small white butterflies and the unending procession of individuals, moving from west to east across the marsh, implied that these were not simple random wanderings; these flights were a population on the move, an autumn migration on a grand scale.

The small white is well known for its powers of dispersal. It is a wanderer, whose loose and open populations are to be found across virtually the whole of Britain and Ireland. The second brood, which tends to emerge in late June or early July, is much larger than the first, which may suggest that its numbers are boosted by the arrival of immigrants from the Continent. Later in the year comes the suggestion of a return movement, the butterflies moving south and east as autumn begins her tenancy of our countryside.

There has been some debate around the extent of migratory movements made by this species. That large numbers may arrive here from the Continent has been well documented. Writing in 1846, the Reverend Morris (a renowned lepidopterist) reported a case where a cloud of small white butterflies passed over a continental steamer and obscured the sun. Upon making landfall in England the great cloud of butterflies broke up and dispersed inland. Evidence for autumn movements is less striking but many lepidopterists will tell you that it does indeed occur. Work elsewhere in Europe, looking at the movements made by other species of butterfly, has revealed that complex migratory movements do take place, the different stages of the movements occurring within the different generations of the butterflies.

Whatever the underlying reason, it was clear to us that a great many dozens of these insects crossed out path that morning, all heading in the same direction and all flying into the prevailing wind. The total number of individuals involved must have been staggering, so great the area of coastal salt and grazing marsh over which they were passing. Some could even be seen passing offshore as we scanned for seabirds.

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