Sunday, 28 May 2017

St Mark's Fly

They start to emerge in late April just in time for the influx of warblers. St Mark’s flies are small, hairy, jet-black capsules of energy snapped up eagerly by hungry migrants after long flights out of Africa. So called because they traditionally appear first on the 25th April, the feast of St Mark, they’re about for very much longer, and a month after the saint’s day are abundant again today. Slow flying and easy to catch, they make ideal prey for reed and sedge warblers at Oxwich Marsh. Some days they seem to be everywhere, landing on jackets, hair and just about any available surface. The birds don’t really hunt them, needing merely to pick them up as they pass through the reeds and willows.

Just about all the migrants are in now, reed and sedge warbler song is at its peak, and whitethroats song-flight from scrub on the edge of the marsh. I’m always struck by the ebullient and complicated song of sedge warblers, they're great mimics, and just from the mouth of one tiny bird, and from the one nearby I can hear blue tit, swallow, starling, linnet and more. How anyone can confuse them with the monotonous reed warbler is a mystery.

I read that purple herons are at last breeding in the south of England, another sure sign of climate change. They were here at Oxwich during its heyday, but as far as I know never bred. In the early 1970s the marsh was wet, rich in flora and fauna, and its natural succession from old flooded fields to rich eutrophic marsh, provided maximum biodiversity and biomass at that time. For several summers the marsh attracted other ‘exotics’ too, marsh harriers, little bitterns and bearded tits were all regular visitors. A lack of management has allowed this once magnificent wetland to dry out significantly, which is now a shadow of its former self.

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