THE BREEDING BIOLOGY OF THE CHIFFCHAFF PHYLLOSCOPUS-COLLYBITA IN BRITAIN - A COMPARISON OF AN INTENSIVE STUDY WITH RECORDS OF THE BTO NEST RECORD SCHEME

Most known aspects of the breeding biology of the Chiffchaff come from studies carried out in Central Europe. This study documents aspects o f its breeding biology in Britain by comparing data gathered during an intensive study in Wytham Woods (Oxford) from 1992 to 1994, and recor ds from 1933 to 1993 held by the BTO's Nest Record Scheme. Comparison of these two data sets showed close similarities in parameters such as : (1) laying dates; (2) length of the breeding season; (3) nest site u sage; (4) clutch size; (5) length of the nestling period; and (6) the relative importance of causes of nest failure. First clutches are laid in the second half of April, and early May, with second clutches in J une. Nests are built close to the ground, usually in Bramble bushes (R ubus spp.). Average clutch size decreases front 6 to 4 eggs through th e season. Incubation and nesting periods last 13-14 days. Nest losses are mainly due to predation, which accounted for approximately 75% of losses in both data sets, and the Weasel (Mustela nivalis) appears to be the main predator in Wytham Woods.