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Cavity-nesting birds can save time and energy by reusing old cavities. We studied cavity reuse and its connections to nesting success and timing in the three-toed woodpecker Picoides tridactylus in a 170-km2 area in southern Finland during 1987-2015. The data include 520 nest trees, 645 nest cavities and 833 nestings in 86 territory sites, including 211 cases of cavity reuse. Twenty-five percent of nestings was in previously used cavities. Twenty-eight percent of cavities and 25% of nest trees were used more than once. Reuse improved nesting success and facilitated early nesting in the year following first nesting. Reuse of nest trees with several cavities was observed in 15% of nest trees, and 62% of reused cavities were in those multi-cavity trees. Cavity reuse and multi-cavity trees were most abundant in long-term territories with stable habitats. In boreal forests, cavity and tree reuse can be an important adaptation allowing efficient nesting during a short breeding season. [ABSTRACT FROM AUTHOR]