Arthropod assemblages living in tree canopies in two habitat types — a rich woodland and poor parkland were studied in 2003–2005 as a background for describing the dynamics of caterpillars; the study system was located in central Poland. Caterpillars, the optimal food of breeding tits, varied in abundance between both habitats and years. The peak abundance of caterpillars was at least 3 times higher in the woodland than in the parkland habitat. There were also pronounced differences in caterpillar production between years, with every consecutive year during 2003–2005 being trophically worse than the previous one. The Blue Tits' timing of breeding coincided roughly with the timing of caterpillar abundance in particular years. Mean clutch size corresponded strictly to the inter-habitat and inter-year pattern of variation in caterpillar availability, suggesting adaptive responses in Blue Tit reproduction to food resources.