DEKHUIJZEN HM & SCHUIJL GPJ (1996) Changes in breeding success of Great Tit Parus major and Pied Flycatcher Ficedula hypoleuca on the Veluwe and the Gooi, 1973 - 1992. LIMOSA 69 (4): 165-174.

Great Tit Parus major European Pied Flycatcher Ficedula hypoleuca A long-term study (1973-1992) was conducted on reproductIve success of Great Tit and Pied Flycatcher breeding in nesting boxes in various study areas in two parts ofthe Netherlands: Veluwe and Gooi. Parameters investigated included number of broods (nests containing at least ?ne ~gg), clutch size, breeding success (% of eggs resultmg m fledghngs) and number of fledglings (per successful first brood). Only first broods of Great Tit were taken into account. The principal study area (Deelerwoud) is situated in the south of the Veluwe and is mainly covered with coni~ erous woods intersected with birches on poor sandy SOlIs. Most study areas elsewhere on the Veluwe are situated in comparable habitats. The study areas in Gooi are characterised by a wealthier vegetation and a larger component of deciduous trees. Numbers of Great Tit broods showed no overall trends in the study areas, but breeding performances were different. In Deelerwoud and in most areas elsewhere on the Veluwe, breeding success and numbers of fledghngs decreased. In Gooi, both either remained stable or increased. Numbers of Pied Flycatcher broods increased in most areas, but a recent decrease was recorded in Gooi. Clutch size and breeding success increased in Deelerwoud and Gooi, but at different levels. Pied Flycatchers in Gooi produced larger clutches (5.9 and 5.4, resp.), had better breeding success (84% and 74%) and fledged more young (4.8 and 3.8) compared with their conspecifics in Deelerwoud. The significant decline in breeding success of Great Tit is confined to areas on poor sandy soils. This can be explained, at least partly, by the dependence of adult females and nestlings on Ca-rich items for egg laying and skeletal growth, respectively. Recent studies proved that the decline in breeding success of Great Tits is due to a decline of snail shells on Ca-poor soils. Acidification has almost certainly contributed to this decline. On ric~ er soils, snail populations are less influenced by acid ram. Breeding performances in Pied Flycatcher are better than those in Great Tit. This is explained by the fact that these birds catch millipedes and wood lice, which are good sources of calcium. Causes of the recent decrease in numbers of Pied Flycatchers in several areas remain unclear; the trend is certainly not the result of declining breeding success.

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