An investigation of the effects of urbanization on corticosterone physiology of blackbird (Turdus merula) nestlings in Debrecen, Hungary. The relationships between baseline corticosterone and habitat type (urban vs forested) were evaluated. The influence of respiratory rate on stress hormones was investigated while the experiment was controlled for the sex and body mass of the nestlings. No correlation was found between corticosterone concentrations and habitat type, respiratory rate, and sex, while body mass predicted baseline corticosterone concentrations. The validity of cort-fitness and cort-activity hypotheses in urban environments was evaluated.