This paper describes the seasonal variation in Great Bustard Otis tarda numbers in two adjacent lekking areas within Castro Verde, the most important site for this species in Portugal. Furthermore, the first data on the breeding biology of the species in the region are presented. The pattern of flock size variation was similar to the one observed in other areas of the Iberian Peninsula, with larger flocks occurring during winter. The use of the two lekking areas was distinct, with one area having more birds during the breeding season (more important as display ground) and another during winter (more important as feeding ground). Cereal fields were the most important nesting habitat for Great Bustards, followed by first year fallow fields. Mean clutch size was 2.6 eggs/nest.
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