Friday, 20 June 2014

Stilts step forward

A lot of time has been spent over the last year or so thinking about birds that may colonise the UK.  The latest influx of Black-winged Stilts was totally predictable and resulted in clusters of birds being found both along the south coast and inland.  Breeding immediately looked likely at several sites.  After a failed attempt in Cambridgeshire and display elsewhere, birds settled onto nests at Cliffe Pools and Medmerry. Breeding populations have been increasing recently in France and in the Netherlands with generally more breeding attempts in The Netherlands per year since 1989 than during the previous two decades.

The breeding attempt at Medmerry is a notable success for this new site.   It is the largest open-coast managed-realignment scheme in Europe, and the RSPB's newest reserve. It was created between 2011 and 2013 by the Environment Agency, with RSPB input and advice, and consists of mudflats, tidal lagoons and saltmarsh.  The scheme was designed to protect people and property from flooding as well as creating coastal habitat.  The birds are breeding on a freshwater flood storage lagoon (photo above) that forms part of the site and looks a good choice by the stilts.  Cliffe Pools supports around 10% of the UK's saline lagoon habitat in old clay excavations adjacent to the river Thames. The stilts are breeding on islands within the lagoons, amongst Avocets and Black-headed Gulls.  This looks a poorer choice but we shall see.

So what do Stilts require?  They generally prefer temporary, shallow brackish and freshwater pools.  They feed on adult and larval insects, particularly flies, beetles, bugs and dragonflies, which they pick from the surface of the water or forage for in shallow mud.  In Western France, their favoured breeding pools usually dry out in late summer,  between June and late September, and re-flood before the end of the year. These pools typically have a salinity of 2-5 parts per thousand (ppt) in winter, rising to about 15-20 ppt in spring. They are usually fairly well vegetated, typically containing Brackish Water-crowfoot and Sea Club-rush.  Black-winged Stilts typically prefer slightly less saline, and more vegetated, water bodies than Avocets. It is probably important to flood pools early enough in autumn to enable pools to be colonised by aquatic invertebrates before these become inactive in winter.

Towards the northern limits of their range in Europe, most Black-winged Stilts breed at shallow, temporary, nutrient-rich fresh water bodies. The Reserve de Grand-Laviers, near Abbeyville in Northern France (photo below), consists of 81 ha of former sugar beet lagoons, most of which dry out each year in late summer and re-flood in early winter. In 2013 these lagoons supported 16 pairs of breeding Black-winged Stilts, as well as 42 pairs of Black-necked Grebes.  Now what else is predicted  as a potential colonist?  Ah yes, Short-toed Eagle.