Wednesday, 20 April 2011

Boom boom boom boom

Night to day: moonlight, sunrise, feeding Great White Egret, Hairy Dragonfly.

Not John Lee Hooker but the Avalon Marshes in the middle of the night. The colonisation in recent years by Bitterns of this wet patch of Somerset has been a remarkable conservation success. As I stumbled around in the darkness as part of a co-ordinated survey to listen and watch for Bitterns, my personal tally was 12 ‘boomers’ although the area total may exceed 20 by the time all the results have been analysed.

When it comes to surveying Bitterns, the key census ‘unit’ is the booming male. There has been a complete count of booming Bitterns in the UK since 1990, from the low point of 11 boomers to the current high of 87 last year. Territories can overlap and males can be mobile so difficulties can arise sorting the individuals out. Survey results are drawn from careful mapping of boom locations combined with details on the characteristics of each individual bird’s boom. Booming reaches its peak around dusk and in the two hours before daybreak, but blundering around a moon-lit wetland has its advantages other than the repeated humpff humpff humpff coming out of the reeds. Cetti’s, Sedge and Grasshopper Warblers belt it out alongside the Marsh Frogs, a rattling Garganey flies over, and there’s always the chance of a whipping Spotted Crake. As day breaks, Little, Cattle and Great White Egrets all enter the notebook. As we settle in for a long watch over the reeds, my first Hairy Dragonfly and Red-eyed Damselfly of the year zip along the nearby ditch, and the water seems to ‘bubble’ with fish fry.

By contrast, the other, and arguably more important, census ‘unit’, the nesting female, is a much more difficult thing to quantify. When the eggs hatch, females are likely to make regular flights from the nest to feeding areas. A watch of around 6 hours is required to assess if such feeding flights are occurring. These may then need to be repeated every week or so to try and assess if the nest is successful. Watches suggested the first nests are now hatching in the Avalon Marshes. The likelihood of a successful nest is a relationship between habitat quality, food resource and predation. At the moment, the Avalon Marshes seem to have everything going for them.

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