Information on nestling diet composition is crucial for understanding the variation in birds breeding seasons at both the ultimate and proximate levels. We studied Nuthatch nestling food composition along with availability of its presumed main food resource — folivorous caterpillars — in a primeval forest, free from direct human impact (Białowieża National Park, Poland), in 1998–2004. Food brought to young during 4281 feeding visits (235 broods) was recorded. Nestlings were fed mostly caterpillars (53% of visits), but winged insects (14%), beetles (11%) and spiders (10%) were also regularly provided. The proportion of caterpillars increased with increasing caterpillar availability (measured by frassfall), with a maximum around the seasonal peaks and a later decline. The proportion of caterpillars in the diet was lower on days when frassfall was low (< 0.1 g/0.25 m²/day), irrespective of the temporal distance from the frass peak, while the proportion of winged insects increased on such days. Above the 0.1 g frassfall threshold, the proportion of caterpillars regularly exceeded 50%, reaching 90% on individual days. As in the majority of years the ‘caterpillars’ remained above the 0.1 g level over long periods, this could account for a comparably small interyear variation in the proportion of caterpillars in the diet, despite nearly tenfold differences in caterpillar abundance across years. ‘Caterpillars’ were brought significantly less often to the smallest, 5-day old, nestlings (52%), than to older ones (64%, 71% and 72% for 10-, 15- and 20-days old young, respectively). Spiders were most often provided to the smallest nestlings.