Latitudinal trends of reproductive traits in the Blue Tit Parus c ruleus

Geographical variation in laying date, clutch size and proportion of second clutches was studied in the Blue Tit Parus c ruleus in order to test current predictions about latitudinal trends of these reproductive traits. The effects of latitude, altitude, habitat type (deciduous, evergreen and mixed) and year of study were taken into account. Location: Data were obtained from 87 populations of the western Pal arctic along a latitudinal range of 27 degrees (33 to 60 deg N). Methods: General Linear Models (GLM) were performed on mean laying date, clutch size and proportion of second clutches of Blue Tit populations. Habitat type was included as a factor and the rest of variables as covariates. Results: Laying date was later in evergreen than in deciduous habitats, was positively correlated with altitude and showed a quadratic relationship with latitude. Blue Tits laid smaller clutch sizes in evergreen habitats compared with mixed or deciduous habitats. Clutch size was negatively correlated with altitude. The variation of clutch size on latitude was curvilinear. The proportion of second clutches was higher in mixed and deciduous than in evergreen habitats and its variation along the latitudinal cline also was better fitted to a quadratic than to a linear model. Conclusions: The latitudinal variation found in all these life history traits can not be only explained by classical hypotheses about latitudinal reproductive variation. As recent studies have shown, factors other than daylength, seasonality of resources and predation appear to operate in the determination of the reproductive parameters of passerine birds in a geographic context. Mixed forests appear to be the habitat where Blue Tits are more productive (large clutch sizes and high proportion of second breeding attempts). Habitat quality associated with the position (peripheral and central) of the distribution of a bird species in combination with other factors, namely ambient temperature and energy requirements, probably contribute to explain the curvilinear pattern found in reproductive traits of this species.

Fargallo, J. A., Ardeola 51(1), 2004, 177-190