Assessing the reduction of caterpillar numbers by Great Tits Parus major breeding in apple orchards

摘要

Great Tits Parus major can reduce fruit damage in apple orchards by predating on caterpillars. A model by Mols (2003) predicted that damage reduction is mainly influenced by the hatching date of the Great Tit nestlings and the number of breeding pairs in the orchard. We tested these predictions by investigating predation pressure on caterpillars by Great Tits in various orchards. First we calculated the total number of feeding visits by Great Tits using two different methods: event counters and the daily weight gain of nestlings. For the latter method we needed to make assumptions on prey size selectivity of Great Tits to prevent overestimation of the number of visits. With the event counters we showed that the number of feeding visits increased with date and with the number of nestlings but there was no effect of hatching date on the total number of feeds over the whole nesting period. Next, we used these data on feeding visits to calculate the maximum proportion of caterpillars that Great Tits removed in organic apple orchards. The average number of three breeding pairs of Great Tits per ha removed on average 23% of the caterpillars. This percentage can increase up to 49% if all foraging takes place within the orchard.