Reed and Sedge Warblers have recently been the subjects of much cooperative investigation. Based on the B.T.O. nest record cards, this paper contributes to the enquiry by contrasting the two species and pointing to areas requiring further study.THE REED AND SEDGE WARBLERS Acrocephalus scirpaceus and A. schoenobaenus are a pair of superficially similar birds which breed in marshes. Comparison of the two poses several questions on their ecological isolation (Lack 1971). Detailed studies have revealed differences in their habitat preferences and feeding behaviour (Catchpole 1972(Catchpole , 1973Green and Davies 1972). Reed Warblers tend to nest in reed Phragmites australis, though similarly structured tall vegetation is commonly used as a substitute. They feed in reeds but also in trees, and employ active techniques to catch mobile prey, especially Diptera (Davies and Green 1976). Sedge Warblers more commonly nest in lower, denser vegetation; they feed in the field layer especially near water, picking small relatively immobile prey items.The Reed Warbler's breeding biology was studied with intensive field effort by Brown and Davies (1949); and, more recently, Catchpole (1974) studied this species with emphasis on differences between habitats. By using British Trust for Ornithology nest record cards, this analysis aims to present a statement on the existing state of knowledge and draw attention to areas that would profit from further investigation. Of particular interest is the question of how the differences of habitat and foods influence the breeding biology of the two species.
METHODSThe major source material was the BTO collection of nest record cards up to the end of 1976, which amounted to about 4,000 and 2,000 for Reed and Sedge Warblers respectively.The nature of the data recorded on nest record cards and the biases inherent in their analysis have been described previously (Newton 1964). This paper relies heavily on conclusions based on comparison between species, a technique successfully used by ,others (eg Snow 1955, Mason 1976. Readers are cautioned against comparing the results presented here with those from other analyses without checking whether they were derived in the same manner.The analysis most susceptible to bias is of the failure rates of nests, mainly because those found have succeeded until that time. This study uses a new method of investigating the problem, which has advantage in using more of the available information and permitting study of the seasonal variation of failure rates.Additional information was collected by a request published in Ringing and Migration. This provided data on the brood size and timing of birth of nestlings which were recaught in subsequent seasons.
RESULTS
Breeding seasonsBreeding seasons were defined on the basis of laying dates of first eggs from nests where there was sufficient information. Eggs were assumed to have been laid at daily intervals, and incubation to have started on the morning of laying of the last egg and to have lasted twelve days. This assum...