Double brooding and polygyny in Sedge WarblersAcrocephalus schoenobaenusbreeding in north-west England

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Abstract
Sedge Warbler breeding was studied at a site in Greater Manchester, UK, during 1991 and 1993. Birds were individually colour-ringed and, in the 2 years of the study, 18 pairs and 38 nesting attempts were observed. In both years, the majority of pairs (> 90%) attempted a second brood after successfully raising their first. Overall nest survival was 44% and the median reproductive output of individual males was 6.5 fledglings in each year (range 0–10 and 0–15 in 1991 and 1993, respectively). After fledging, one or both parents continued to feed the young for up to 15 days (median 11, range 0–15 days). One male was apparently bigamous in 1991 and in 1993, raising four broods successfully in 1993 and 25 fledglings over the 2 years. These observations suggest that double brooding is the normal strategy for Sedge Warblers in this population and that polygyny may be common, although easily overlooked.
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