* Correspondence author Email: bgycbt@leeds.ac.uk © 2006 British Trust for Ornithology Ringing & Migration (2006) 23, 65-79 Survival rates of adult Reed Warblers Acrocephalus scirpaceus at a northern and southern site in England CHRIS B. THAXTER 1 *, CHRIS P.F. REDFERN 2 and RICHARD M. BEVAN 1 1 School of Biology, Ridley Building, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK 2 School of Clinical Medical Sciences, Paul O’Gorman Building, Newcastle University, Newcastle upon Tyne, NE2 4HH, UK Mark–recapture ringing data from Wicken Fen in Cambridgeshire and Gosforth Park in Newcastle upon Tyne were used to estimate adult survival rates in a southerly and a northerly population of Reed Warblers in England. The computer program MARK was used to estimate survival and recapture rates, adjusted for transient birds, with respect to sexes at each site. Expressed as percentages, survival rates between 1988 and 2004 at the northerly Gosforth Park site were 60.3 ± 6.0 (se) for males and 54.9 ± 10.3 for females. Survival rates at the southerly Wicken Fen site between 1995 and 2004 were 32.9 ± 16.0 for males and 52.0 ± 22.4 for females. Rainfall in the Sahel region of West Africa did not account for variation in survival rates over time at either site and did not correlate with variation in adult Reed Warbler abundance in the UK. At both Wicken Fen and Gosforth Park, Reed Warbler populations increased during the study periods. At Wicken Fen, new recruits due to immigration had a greater contribution to population growth than survival of adults returning to the site, whereas at Gosforth Park the reverse was found. In the UK, the Reed Warbler Acrocephalus scirpaceus has extended its range north and west over recent years (Gibbons et al 1993); however, recent analyses have suggested an overall decline in abundance (Clark et al 2003). Reedbeds of Common Reed Phragmites australis are most extensive in the south, particularly the Norfolk Broads and East Anglia, of which Wicken Fen in Cambridgeshire is one of the largest, with more than 40 ha of reeds (Cadbury 1997). Suitable habitat is less common in the north and small populations of Reed Warblers in these patchy habitats are likely to vary in relation to range expansion, habitat suitability, and reedbed management. In the northeast of England, Reed Warblers are scarce and the largest population is found at Gosforth Park in Newcastle upon Tyne (Redfern & Davidson 2003). Further north, they occur only as sporadic breeders. An important step towards understanding the reasons behind changes in bird abundance has been the use of bird ringing data to estimate survival rates and productivity (Baillie 1990, 1995). Survival rates are a vital measure of bird populations and monitoring how these vary over time can highlight where conservation effort is needed. For example, British Trust for Ornithology (BTO) ringing data show that decreasing juvenile/first- winter survival explains the decline in Song Thrush Turdus philomelos numbers from 1975 to 1993 (Thomson et al 1997). Survival rates in many species have also been linked to major climatic variables such as precipitation and temperature (eg Conroy et al 2002). Populations of trans-Saharan migrants such as the Whitethroat Sylvia communis (Winstanley et al 1974, Hjort & Lindholm 1978), Sand Martin Riparia riparia (Jones 1986), and Sedge Warbler Acrocephalus schoenobaenus (Peach et al 1991) fluctuate in relation to rainfall in the sub-Saharan Sahel region of West Africa. The Reed Warbler is also a trans-Saharan migrant and, although little is known of the wintering areas, recoveries from birds ringed in the UK have shown that they migrate southwest and may winter in West Africa from Senegal to Nigeria (Redfern & Alker 2002). Bird populations in the UK may also show regional variation in their survival rates and population changes. For example, data from the Constant Effort Sites (CES) project have been used to identify the reasons underlying changes in abundance of Willow Warblers Phylloscopus trochilus, a species that within the UK has shown marked declines in southerly populations in contrast to those in the north, with lower adult survival rates reported for declining areas (Peach et al 1995). Reed Warblers within the UK may also show regional variation in survival rates. The aim of this study was to compare adult survival rates and populations of Reed Warblers at two sites in England: a small northern site and a large southern site