Time of passage, habitat use and mass change ofAcrocephaluswarblers in a South Wales reedswamp

Studies on the mass change of Reed Acrocephalus scirpaceus and Sedge Warblers A. schoenobaenus were undertaken in a coastal area of Phragmites swamp and Willow carr at Llangennech, south west Wales, in 1986–88. Ringing was also used to assess time of passage, and habitat use throughout 1988. Both species occurred at the site from April onwards. Peak autumn passage occurred in the first week of August, though Reed Warblers occurred for a period after Sedge Warblers had left. First years occurred at the site for a longer period than adults, particularly in Reed Warblers. In both species, mean mass declined significantly during brood rearing and then increased again from late July onwards. As at other British sites, many Reed and Sedge Warblers probably began migration without large lipid reserves. Nevertheless, during early August over 30% of adult Sedge Warblers showed masses typical of fattening birds (> 13 g ‐20 g), and over 20% of first years showed a similar trend in late August. Such large proportions of heavy birds are more typical of sites in southern England and France, and it is concluded that Sedge Warblers at Llangennech fatten to a degree unexpected for Wales. Patterns of habitat selection showed that they switched increasingly to reeds at this time, which would be consistent with feeding on Plum Reed Aphids Hyalopterus pruni. Aphids occurred abundantly at Llangennech and are important elsewhere to Sedge Warblers during fattening.