On the importance of niche packing for local species richness in taxonomic bird groups of the Neotropical region

In passerines, high species richness is driven mainly by dense occupation of the trait space rather than by an increased volume of the trait space. This niche packing mechanism would be allowed by increased specialization. Here we studied the Tyranni to investigate how species from the parvorders Tyrannida and Furnariida share the trait space. Because Furnariida include more specialized species in diet and foraging, we expected niche packing to be a predominant mechanism driving species richness in this group and tested (1) whether differences exist in local functional structure and the spatial regularity of occupied functional space between the groups and, if so, (2) whether the differences in local functional structure are explained by species trait combinations, species abundances, or both. We calculated indexes of functional diversity (alpha diversity), variation in trait composition (beta diversity), and generalized linear mixed models (GLMMs). We recorded 32 and 30 species in each passerine group using point counts for 4 years in an undisturbed continuous forest of southern Brazil. Although both groups presented similar overall richness, our models indicated that Furnariida had more species per transect and higher diversity values than those of Tyrannida but lower dissimilarity and turnover of traits. Furthermore, linear models of species richness and functional richness had higher slopes for Tyrannida than for Furnariida. Our study suggests two different strategies in Tyranni: while species richness in Tyrannida is reached by higher dissimilarity among local assemblages, in Furnariida species packing is the dominant mechanism.