Results of ten years ringing Acrocephalus warblers in the estuary ofthe Loire River, western France

摘要

Since 1994 birds have been mist-netted and ringed each summer (15 July to 25 September yearly, occasionally up to mid October) in a 20 ha Phragmites australis reedbed at "Le Massereau" nature reserve, on the southern bank of the estuary of the river Loire. Nearly 58,000 birds have been caught from 1994 to 2003, including circa 40,000 Acrocephalus warblers. We present some preliminary results for these four warbler species, focusing on phenology of occurrence and the origin of ringed birds. Great Reed Warbler Acrocephalus arundinaceus accounted for less than 1% of captures. The species reaches the northwestern limit of its breeding range near the study site, and it is considered that most records refer to individuals from local populations. However, a minority of captures occurs rather late in the season (mid August to mid September) and may concern passage migrants from small, isolated, populations in northern Europe (mostly The Netherlands). Reed Warbler Acrocephalus scirpaceus (16,589 birds caught, i.e. 42% of all Acrocephalus) is a common breeder in the area, and the distribution of captures (frequency of adult birds rapidly declining, highest abundance of juveniles by mid August) suggests that a majority of birds come from local populations. Passage migrants occur from early August (occasionally mid July) and are mostly juveniles coming from northwestern Europe and, exceptionally, from Central Europe. With 22,642 individuals (59% of all Acrocephalus) caught over 10 years, Sedge Warbler Acrocephalus schoenobaenus is the most abundant bird species at the study site. Migrants from northwest Europe and particularly from Britain progressively replace local birds. It is suggested that the study site and other reed beds around the Loire estuary are important migration stopover sites for this species. Aquatic Warbler Acrocephalus paludicola occurs yearly and its capture rate has increased markedly (c. 150 individuals ringed in 2002) as a result of the systematic use tape luring in recent years.