Caterpillars are important as food for many insectivorous birds. Their availability is highly seasonal in temperate regions. Global warming is known to advance this seasonality and can change the selection pressure on the seasonal timing of avian reproduction. Time series on caterpillar abundance and timing are often based on quantifying caterpillar faeces (called frass). Accurate estimates of caterpillar abundance and timing within and between years require temperature correction, the topic of this study. To estimate caterpillar abundance, we measured the rate of caterpillar frass production under tree crowns. To calculate the frass to caterpillar biomass conversion factor we kept wild caught caterpillars of a number of abundant species in cages outside and fed them daily with fresh leaves (Quercus robur or Betula sp.) in two studies, Dwingelderveld (Dv) and Hoge Veluwe (HV), in different time periods. We monitored caterpillar body mass and frass production on a 24-h time base to calculate the conversion factor as a function of temperature, study site, tree species and caterpillar species. One study (HV) concerned a reanalysis of earlier published data. The conversion factor strongly depended on ambient temperature, almost doubling from around 8 to 15 when daily temperatures reduced from 15 to 10°C, and differed somewhat between study areas and caterpillar species. Yet in 20 of 34 published studies that estimated seasonality in caterpillar abundance on the basis of frass fall, temperature was not corrected for. We illustrate the importance of the temperature correction of frass to mass conversion factors by comparing variation in raw frass data and temperature corrected caterpillar biomass estimates during the spring for 12 nest boxes in 16 years in the Dwingelderveld. We advise temperature correction of frass fall based estimates of caterpillar abundance in woodland, preferably based on local measurements in the study area and period.