Comparative Structure and Organization of Canopy Bird Assemblages in Honduras and Brazil
Difficulty of access into the forest canopy has hindered the study of canopy bird assemblages as well as attempts to unify concepts on their structure and organization. Our study takes several steps to make such an attempt possible. First, we describe for the first time a canopy bird assemblage from northern Middle America, thereby broadening our perspective of canopy birds in lowland neotropical rainforests. Second, we compare canopy bird assemblages of distant neotropical forests, on the basis of similar canopy-based censuses in Honduras and Brazil. These sites are particularly useful for such a comparison because they share a similar climate and forest structure yet offer relatively independent biogeographic histories, being separated by over 2000 km. Most importantly, we took three steps designed to clarify previously unresolved arguments on the composition of canopy bird assemblages: (1) by categorizing bird observations in Honduras into distinct strata we achieved a quantitative definition of the core canopy birds at that site; (2) we assessed the relative importance of dietary guilds and of edge species and migrants in Honduras and Brazil by comparing the observed composition of these groups with expectations based on species drawn randomly from each regional pool; and (3) by reclassifying data sets from Costa Rica and Panama by similar criteria, we are able to broadly characterize bird assemblages of lowland neotropical rainforest canopies.
Species Richness and Abundance
The species diversity of forest birds is notably higher in Amazonia than in Middle America at levels of both local (alpha) and regional (gamma) diversity (Karr et al. 1990, Terborgh et al. 1990, Blake 2007). Although we observed 27 more species in the canopy in Brazil than in Honduras, the overall richness of the canopy assemblages of the two sites did not differ significantly. Furthermore, if we include six species extirpated from the study site in Honduras (Ara macao, Amazona farinosa, A. autumnalis) or reduced below detectable levels by persecution (Spizaetus ornatus, S. tyrannus, S. melanoleucus), the difference between the sites in observed richness is 14%. Given that overall richness of forest birds is approximately 35% higher in Brazil, the similarity in species richness in the forest canopy is notable. The higher richness of migrants in Honduras, roughly three times that in Brazil, partially explains this finding. Especially notable was the greater richness of the family Parulidae, represented by 11 more species in Honduras than in Brazil, and the families Tyrannidae, Vireonidae, and Cardinalidae, which together contributed 15 species of migrants. Additionally, the number of hummingbird species observed in the canopy in Honduras was twice that observed in Brazil (14 vs. 7), despite equal species richness at the regional level. We suspect that in Honduras the high number of individual flowering trees in the canopy (principally Vochysia guatemalensis and Symphonia globulifera) may have attracted a greater diversity of hummingbird species into the canopy. During peak flowering, as many as eight individuals of seven species were observed in the canopy during a single 3-hr census, whereas in Brazil the median number of both species and individuals observed per census was one. This difference may be due to factors intrinsic to the Honduras site, because a similarly high richness of hummingbirds was not observed in Costa Rica (five species) or Panama (seven species).
Nonetheless, the additional migrants and hummingbirds in Honduras do not completely account for the similarity to Brazil in species richness. Clearly, in Brazil a smaller proportion of the regional forest species occurs in the canopy stratum. One plausible explanation is that differences in stratification result from the difference in taxonomic composition of the local avifaunas. At the Brazil site, the families Tinamidae, Cracidae, Furnariidae, Thamnophilidae, Formicariidae, Pipridae, and Troglodytidae, are all dominated by species of lower and middle forest strata, and richness of these families is more than double their richness in Honduras. In contrast, the few families with similar (Trochilidae, Vireonidae) or greater (Parulidae, Cardinalidae) species richness in Honduras are weighted with species of the upper forest strata, or with migrants, which seem to be more important locally in the canopy than at lower levels. Therefore, we suggest that taxonomic differences in the regional avifaunas explain in large part the similarity of richness in the canopy in Honduras and Brazil.
Despite this similarity in species richness, patterns of species abundance differed markedly. Our findings that the Middle American canopy was dominated by a few superabundant species and that species’ abundances were distributed more evenly in Amazonia are consistent with the findings reported by Robinson et al. (2000) for sites in Panama and Amazonian Peru. Robinson et al. described an “oligarchy” of eight common species, six from the understory and two from the canopy (Hylophilus decurtatus and Zimmerius vilissimus), that accounted for a disproportionate number (36%) of individuals at the Panama site. Likewise, in Honduras the seven most abundant canopy species each accounted for ≥4% of all detections and a combined 46% of all detections in the canopy, whereas in Brazil only a single species reached comparable abundance. Finally, the overall pattern of more rare species than common ones observed in both the Honduras and Brazil canopies mirrors results from other lowland sites in neotropical forest (Pearson 1977, Karr et al. 1990, Terborgh et al. 1990, Thiollay 1994b, Robinson et al. 2000).