Composition of Marsh Tit Poecile palustris nestling food was studied in the Białowieża National Park (E Poland). The birds relied exclusively on natural food sources. Repeated visual observations of food brought to young in over 500 broods, during 14 seasons (1993–2007), showed that Marsh Tit used a highly specialised diet in rearing young. Soft-bodied, folivorous caterpillars typically composed > 80% of the nestling diet both in riverine and oak-hornbeam habitat. Spiders formed the second most important prey type (c. 12%). Diet composition fluctuated across years but the proportion of caterpillars in the diet remained high (70–90%) despite more than hundredfold variation in the caterpillars' supply across years. Within years, diet composition changed with nestling age and the broods' synchronisation with the seasonal peak of caterpillars availability. Young that were fed within 15 days before and 10 days after the peak received a high (> 80%) proportion of caterpillars, independent of their age. The share of caterpillars in the diet dropped only outside of that period. In most seasons, Marsh Tit young appeared in nests more than two weeks before the caterpillar peak; consequently, the share of caterpillars fed to small young in the earliest broods was lowest (c. 62% on average). Marsh Tits strove to collect caterpillars even when they were scarce, rather than switch to alternative food types. Spiders were brought in highest numbers (up to 20%) to the youngest (1–7 days old) nestlings. This suggests that spiders contained specific nutritional ingredients required by the small young.