Introduction of exotic tree species as a threat to the Azores bullfinch population.
Abstract
The Azores bullfinch (or priolo), Pyrrhula murina [Pyrrhula pyrrhula murina], is known only in the east of the island of S. Miguel (Azores), where it is largely confined to native laurel [Laurus azorica] forest. This paper assesses the impact on the population and distribution of the bullfinch of a recent large-scale invasion of the native forest by exotic flora (the trees Pittosporum undulatum and Clethra arborea, and the herbaceous plant Hedychium gardnerianum), which followed the original forest clearing for pasture and afforestation by (the exotic) Cryptomeria japonica. The paper examines the population size and importance of vegetation characteristics to explain the seasonal distribution of the bullfinch. The conservation of the bullfinch population is addressed, including the planning of forest management practices. Point-counts and capture-recapture data indicate a population of 60-200 pairs. The bullfinch 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. The monthly density of the bullfinch population 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 forest were highly preferred at all seasons. Bare ground and short vegetation were selected in summer and avoided in winter. The bullfinch was sedentary but ranged widely locally, and appeared more mobile over summer than over winter. Larger-scale altitudinal movements were carried out in May. Monthly habitat selection may be interpreted as a 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. Both demographic and environmental stochasticity seem to be important factors in the conservation of the small population of the bullfinch. 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 the bullfinch population. The existing 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 the bullfinch.