Monday, 27 February 2017

The Dun Sentinel of the marshes

This an odd one. A short post on a snail.

Assiminea grayana
to be precise. Common name, the Dun Sentinel. A tiny salt marsh specialist extending to just above the high water mark, common on estuaries between the Humber and Pegwell. Found around the southern North Sea, also very recently (the 1990s) discovered in Ireland and the north-west of England.

Why mention on a birding blog? Studies have shown that where the Dun Sentinel is present, it can amount to up to 98% of the food resource for any estuary Rock Pipits. Dietary differences. The resident Rock Pipits on other parts of our coast get away with staying put in a maritime environment on a diet largely consisting of Melarhaphe neritoides- the Small Periwinkle. Driven out by the cold further north, our wintering visitors have found a different food resource to exploit

I admit to having never knowingly seen a Dun Sentinel, but I think I'll now include on my (bait-)bucket list and make an effort this spring once things have warmed up a bit and the winterers have departed. Rock Pipits are another favourite of mine after all.

The taxonomic fun over the years, separating Rock, Water and Buff-bellied Pipits (only given to full species status by the British Ornithologists Union some 19 years ago) is based only in part on visual differences. Habitat specialisation when breeding and wintering comes into the argument. The Dun Sentinel goes some way to explaining just why migrant Rock Pipits can get by on estuaries in winter- they've found a prey item out on the saltings they can do very well on. Some argue there's very little overlap between the species/subspecies in their wintering habitats but, some inland sites such as Stodmarsh/Grove in East Kent, are now throwing up headaches for Water/Rock aficionados.

Field i.d. is a blogpost for another day, but for now is always worth remembering the visual identification of each species is not as easy as some would have it. If you read the monographs rather than the sparsely worded field guides, you will find most distinctions use caveats of terms such 'generally' and avoid claiming many identification pointers to be as clear cut as we would like them to be. Small wonder it took us so many years to pigeonhole to species status...

The more we learn, the more there is to know.

I'm no conchologist, but I'll enjoy stringing a shell in a few weeks, all the time chuckling in the knowledge the snail taxonomists are still arguing about whether Dun Sentinel is actually a separate species, or if they need to lump it in with other Assiminea down around the Mediterranean.


Spot the Rock Pipit, hunting out the other two percent of his diet.
(Horrid Hill, March 2015)


No comments:

Post a Comment