Breeding biology of the Blue Tit Parus caeruleus in a montane Mediterranean deciduous forest: The interaction of latitude and altitude

Laying date and clutch size in a population of the Blue Tit were evaluated during five years in a montane deciduous oak forest central Spain. By comparison with other Mediterranean and non-Mediterranean Blue Tit populations in the same habitat type of the Palearctic region, we found a similar timing of breeding than in central and North European populations. However, the low clutch size is in agreement with the latitudinal tendency in clutch size observed by LACK (1947, 1966). The relatively altitude constraining the distribution of this habitat type may affect the delayed breeding season and low average clutch size observed. The degree of isolation of this habitat type in the Spanish Central Range, and the gene flow of Blue Tits populations setting in bordering habitats of poorer quality, may also contribute to the smaller clutch size observed in our study population.