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The Eurasian three-toed woodpecker Picoides tridactylus is considered to be a specialist and obligatory insect-feeder, but the significance of other food sources for its survival and reproduction is unclear. We investigated the importance of tree sap as an alternative food source. We found that three-toed woodpeckers spent up to 33% of their foraging time and 38% of foraging events obtaining phloem sap from coniferous trees in spring before the start of their breeding. Sap use was most common in April and early May, and decreased significantly during nesting in late May and June. The woodpeckers made large numbers of new rows of sap holes in trees in the spring, the maximum being 399 fresh rows in 17 trees during one spring within a single territory. The sap trees were mostly at mature forest edges with rows oriented towards southern open areas indicating that woodpeckers utilized beneficial thermal conditions to obtain sap. These patterns suggest that sap is of high importance especially during the critical period of gaining energy for the start of nesting. We conclude that sap use is an important adaptation that may buffer variation in the availability of insect food in spring, and may increase the probability of three-toed woodpeckers commencing successful breeding. [ABSTRACT FROM AUTHOR]

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]