Gaining ecological information about Bayesian foragers through their behaviour. II. A field test with woodpeckers

The lesser spotted woodpecker Dendrocopos minor is a territorial species with its food resources distributed in patches. i.e. insect larvae in dead branches of trees. The species is threatened in Northern Europe, and therefore it is of interest to identify factors responsible for its decline. We hypothesise that (a) foraging behaviour is maximising fitness, (b) the woodpeckers, through their foraging behaviour, reveal their own opinion about their territory or their individual quality, and (c) that an inter-territorial comparison of behaviour and reproduction shows which factor is responsible for the breeding success of the woodpeckers. We collected data on giving-up density of prey (GUD) and patch residence time (PRT) during branch visits about one month before egg laying. The data was collected from a natural breeding population in southern Sweden, 1993-1996, A positive correlation between GUD and PRT within individuals is diagnostic for a Bayesian forager exploiting a clumped distribution of prey, This correlation was found in 22 of 28 investigated individuals. Average GUD of the breeding individuals was positively related to breeding success, i.e. there was a significant negative correlation between average GUD and commencement of egg laying, and a significant positive correlation between average GUD and number of fledglings. For the number of eggs laid there was a weak (not significant) positive relation with average GUD. Average GUD was not influenced by the age or sex of the individuals. Neither did these individual quality measures influence the relations between average GUD and reproductive success. Average GUD and average PRT of individuals was positively correlated. In concert, these results suggest that differences in average prey density across territories prior to breeding is the factor explaining most of the variation in breeding success of the lesser spotted woodpeckers in the studied population.