11.4.1 Trends in Bird Feeding Guild Species Richness and Diversity
Our results show that total species richness and diversity of bird feeding guilds in the current landscape do not differ significantly among vegetation age classes. These results differ from previous findings that species richness and diversity increase with successional age (May 1982; Raman et al. 1998), and may be partly attributed to the low level of deforestation and fragmentation of our study landscape, which is dominated by >15-year-old forests (nearly 60 % of the total area). Our results suggest that the avian community perceives the landscape as fairly homogeneous and may be able to cross the successional matrix. Additionally, in our study we did not consider seasonal variation as we used 1-year cycle data and used no discrimination between the reproductive and migratory seasons; hence, seasonal variation may be masked. In contrast, Santamaria-Rivero (2011) found, for the same study area, significant differences in species richness among vegetation age classes during the reproductive season, and it has been suggested that species mobility could decrease as a response of individuals protecting their breeding territory, constructing nests, or providing parental care (Volpato et al. 2009). In contrast, he found no differences during the migratory season; this pattern could be influenced by the presence of migratory birds that may exert pressure as potential competitors on feeding resources, thus forcing species with enough plasticity to occupy other habitats (Greenberg 1990).
Interestingly, in our study we found significant differences in species richness of bark insectivores among forest classes, which suggests that diversity of habitat specialists increases with succession and the associated changes in forest structure, that is, increase in tree size and hence in bark surface. The habitat characteristics associated mainly with late succession, such as large trees (Thompson et al. 1995) or climatic buffering (Greenberg 1992), may influence the habitat selection of bark insectivores (Smith et al. 2001). Additional research is needed to account for other specialized groups of forest-dependent species that are less tolerant of habitat disturbance.