In order to evaluate how a multi-brooded bird species adapts its breeding strategy during the breeding season, we investigated variations in clutch size, egg traits and sex of the final eggs in a population of the tree sparrow Passer montanus in northwestern Croatia. Of the three clutches of the season, the second was the largest and it contained the largest eggs. Furthermore, the females followed the brood survivor strategy for the second clutch, while they chose the brood reduction strategy for the first and the third clutches. These results indicate that female tree sparrows adjusted their reproductive strategy over the course of the breeding season and invested the most resources in the second clutch. We hypothesise that their decisions may serve to maximise the number of offspring that they can successfully rear in a single breeding season.