Composição e dinâmica de bandos mistos de aves da Mata Atlântica do Baixo Sul da Bahia, Brasil

Abstract

Mixed flocks of birds are clusters of two or more species whose formation and cohesion depend on behavioral interactions among members. This study investigated the specific composition of mixed flocks in the Atlantic Forest area of Bahia, Brazil, considering the seasonal dynamics of flocks, the vertical distribution of their species, and the determination of species-nucleus, which are responsible for attracting and maintaining the cohesionof other species flocks. From August 2012 to August 2013, monthly expeditions of five days were conducted at the region of the Ecopolo I,in Pratigi Environmental Protection Area (Ibirapitanga, Bahia).The following data were recorded for each mixed-flock contacted: the specific composition, the number of individuals of each species and, when it was possible, the height of foraging, the occurrence of agonistic interactions, estimated intensity of movement and vocalization for each species. Furthermore, the contacts of mixed-flocks component species were recorded when they were seen out of the flocks, observing the foraging height and evidences of reproductive behavior.Was registered 113 contacts with mixed flocks, with a total of 51 species of birds, representing about 30% of the species recorded in the study area.In flocks, the most representative families were Thraupidae and Thamnophilidae - counting this one with two endangered species. Mixed flocks occurred throughout the whole year, being more frequent in August and less frequent from September to April.Due to the constantly rainfall throughoutyear in the study area, there was no correlation between frequency, richness and size of flocks with the monthly pluviosity. There was a negative correlation with the frequency of flocks and temperature. The richness and the size of flocks did not correlate with the temperature.The average richness in flocks was 5,1 ± 2,6 species, and the average size of 5 ± 5,2, individuals, these two variables were correlated positively (r = 0.81, p = <0.0001). The richness was different between the vegetation strato, being the understory flocks richer than the ones in canopy. However, in relation to size of individual numbers, the understory and canopy flocks didn’t present significant difference, because the canopy flocks were composed by a great number of Thraupidaespecies, that happened in large intraspecific group. Inside or outside of mixed flocks, all species foraged in the same height range of vegetation.Among the five most frequent flocks species, three were considered to be species-nucleus(Herpsilochmus rufimarginatus, Lanio cristatusandMyrmotherula urosticta),presenting a higher degree of associations between all species, and capable of recruiting more species for flocks when they took place; flocks where these species-nucleus occurred were richer, indicating that, when put together, these species increased the ability to recruit other species.