Grasshopper Warbler Locustella naevia autumn migration-findings from a study in southeast Britain

This study examines the migratory behaviour of Grasshopper Warblers Locustella naevia on the south coast of Britain with respect to phenology, fuel loads, stopover behaviour and potential flight ranges. Data were derived from 5,455 mist-net captures gathered over 10 years between July and October at the Pannel Valley Reserve, East Sussex. For the majority of captures and recaptures, fat score, wing length and body mass were recorded. Data from recaptures were used to calculate fuel deposition rates and stopover durations, whilst fat scores and body masses were used to calculate fuel loads and potential flight ranges. The majority of birds were carrying moderate fuel reserves and very few birds refuelled at the reedbed site; indeed, many appeared to remain at the site for just one day. Estimated flight ranges suggest that most birds were capable of flying to the southern half of France. Therefore it is suggested that British birds typically accumulate fuel north of the south coast, possibly at or close to their breeding grounds, and do not refuel until reaching southern France. This is supported by the pattern of birds recovered abroad. That birds stop at the south coast at all is attributed to the English Channel acting as a barrier to onward flights.

Nicholas J. Bayly and Stephen J.R. Rumsey, Ringing & Migration (2007) 23, 147-155