Response of passerine birds to an irruption of a pine processionary moth Thaumetopoea pityocampa population with a shifted phenology
Published: Volume 56(2), December 2009. Pages 189-203.
The main purpose of the present work was to investigate if the passerine community was able to respond to a localized irruption of a temporally shifted population of the pine processionary moth Thaumetopoea pityocampa, the larval development of which takes place during the summer, called summer population (SP). The work was conducted in the National Pine Forest of Leiria, a highly organised production forest, essentially consisting of maritime pine, Pinus pinaster, and located in the central west coast of Portugal (39º 50` N, 8º 57` W, 30 - 50 m a.s.l.). The SP irrupted in a homogeneous area consisting of young pines. Bird abundances and richness were estimated by point counts in the infested area and in two similar non-infested areas. The counts were made during all four seasons, covering all stages of the moth annual cycle and were repeated during two years. The overall abundance of birds was similar in both infested and non-infested areas. However, the richness of canopy gleaners was higher in the SP area than in a nearby non-infested area. Only two bird species responded numerically to the increased insect abundance: the blackbird Turdus merula, and the great tit Parus major. Thus the passerine community responded only to a limited degree to the irruption of the SP. Three factors might have accounted for the results: (i) the bird community consists mostly of territorial residents or short distance migrants and their low range of dispersal may decrease the probability of a response to a localized insect outbreak. (ii) The larvae of the pine processionary moth has urticating hairs which are considered to be an effective repellent defence against vertebrate predators, thus reducing the number of species that could react to the high caterpillar densities. (iii) The simple vegetation structure of the forest, homogeneous stands of small young trees, precludes the establishment of cavity nesters that constitute a large part of the canopy gleaners and most of the species which have been reported as potential T. pityocampa predators. However, results indicate that this important Mediterranean defoliator may have a positive effect on the canopy gleaners and on some species that are able to act as its predators.