| Summary: | <p>The breeding, moult and migration strategies of a Portuguese population of the little-known Savi's Warbler, <em>Locustella luscinioides</em>, were studied and compared with other populations and species. The biometrics, the variation in body condition during the breeding season, the breeding phenology and success, the variation in egg and clutch size, and the nest-site selection were described. In both sexes, mating and breeding success benefited from early arrival and breeding; females produced enough fledglings to balance the typical mortality rates of passerines. Although this species depend on pure reeds to feed, the prefered nest-sites were located in high and dense <em>Juncus</em> and <em>Rubus</em> with thick litter; and predation was greater in sites with less litter.</p> <p>The phenology, sequence and extent of the post-juvenile and post-breeding moults were also described. Later-moulting birds retained more primaries and tended to start moult in an outer primary. This was interpreted as an adaptation to replace as many as possible of the most important flight feathers in the time available.</p> <p>The winter moult extent was determined by analysing the stable isotope signatures of the wing feather generations present in birds captured in spring. This is a novel application of this technique. Carbon, Nitrogen and Hydrogen isotope ratios of feathers grown in Europe differed from those grown in Africa and were used to classify the origin of unknown feathers. This species has a suspended post-breeding moult and a suspended or arrested pre-breeding moult, a strategy which was not been described previously.</p> <p>The data from birds captured in Portugal were analysed to assess the phenology of the post-breeding migration, fuel deposition rates and flight range. Many of the adults were able to reach the winter quarters without refuelling, which suggests that potential staging sites were bypassed by birds; as has been predicted for time-selected migrants. The possibility that juvenile migration might be energy-selected could not be excluded, as these birds left the breeding quarters with only enough fuel to reach the next staging site.</p>
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