Introduction of Exotic Tree Species as a Threat to the Azores Bullfinch Population

Abstract

1. The Azores bullfinch (or Priolo), Pyrrhula murina, is largely confined to native forest in the east of the island of S. Miguel (Azores). This paper assesses the impact of a recent large-scale invasion of the native forest by exotic flora on the population and distribution of this species. It examines the population size and importance of vegetation characteristics to explain the seasonal distribution of this bird. The conservation of this population, including the planning of forest management practices is addressed. 2. Point-counts and capture-recapture indicate a population of 60-200 pairs. The bird occurred all year round in the largest fragment of native vegetation to the east of the range and was recorded in a smaller patch to the west only in autumn. 3. The Azores bullfinch's monthly density was much higher in laurel forest than in exotic forests (Cryptomeria japonica and Pittosporum undulatum). There were peaks in the density of birds in exotic forests in summer and autumn. The edge of the native forest was important from May to December and the interior from January to April. The degree and intensity of the selection of habitat structure varied seasonally. More habitats were selected over summer than over winter. Native forest and Clethra arborea were highly preferred at all seasons. Bare ground and short vegetation were selected in summer and avoided in winter. 4. The Azores bullfinch was sedentary but ranged widely locally. They appeared more mobile over summer than over winter. Larger-scale altitudinal movements were carried out in May. 5. Monthly habitat selection may be interpreted as preference for habitats where feeding resources are more abundant. A large home range is needed in summer because birds feed in openings that are separated by unsuitable tall vegetation. 6. Both demographic and environmental stochasticity are important factors in the conservation of the small population of Azores bulfinch. The first factor may not be the major one because the population may have increased recently due to the introduction of C. arborea and also because recruitment seems to compensate annual adult mortality. The invasion of the native forest by aggressive exotic flora seems to be the most important environmental stochasticity factor affecting this population. Present exotic forests are too dense but they could be valuable habitats in summer if they were more scattered. Improvement of the habitat quality of the existing native forest and expansion of its area are important nature conservation strategies for this population.