Natural forests have been progressively replaced by other forest types, mainly monoculture plantations. We compared the abundance of Great Tits and its main food competitors, the Chaffinch Fringilla coelebs and other tits species (Blue Tit Cyanistes caeruleus, Coal Tit Periparus ater and Crested Tit Lophophanes cristatus), and breeding parameters of Great Tits in four forest types: a mixed forest, a native broadleaf forest, a Maritime Pine Pinus pinaster plantation and a Blue Gum Eucalyptus globulus plantation in Central Portugal. The breeding parameters (laying date, clutch size, egg volume, hatching success, fledging success and chick body mass) were collected from nest boxes placed in the four forest types. The censuses indicated a higher abundance index of Great Tit and its food competitors in the mixed and broadleaf forest, when compared to the monoculture plantations. Occupancy rate of nest boxes was not directly related with tit abundance due to differences in the availability of natural nest cavities among forest types, and differences in preference for nesting cavities among tit species. However, a Great Tit abundance index gave similar results to the densities described in the literature, except for the Great Tit in the Eucalyptus plantations where the placement of the nest boxes should explain our value of 2.1 birds ha1, which is four times higher than that reported for other Eucalyptus plantations. Egg volume differed significantly among forest types; it was higher in the pine plantations and lower in the Eucalyptus plantations. Surprisingly, all other breeding parameters (laying date, clutch size, chick body mass, hatching and fledgling success) were similar among forest types and between years. On the other hand the number of eggs, hatchlings and fledglings were lower than those obtained in other studies in the Iberian Peninsula. Our results suggest that Great Tits have a high plasticity that enables them to adjust their breeding strategies even to forest plantations, if nest sites are available.
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