Abstract
In mutualistic interactions, all parties are usually considered to benefit; yet there may be asymmetries in mutualisms where some partners/individuals benefit more than others. Such is thought to be the case in mixed-species flocks of birds, where following species are thought to benefit more than leading species, and leading species may not be able to escape the association if they are subordinate to other species. We measured dominance and affiliation patterns of a mixed-species flock system of southern China in an aviary where these variables could be measured in a standardized way. In eight wild-caught flocks, the leading species, David's Fulvetta (Alcippe davidi), was usually among the more subordinate flock members (ranking 9 of 13 species, with the the most dominant species ranked number one, and dominance measured by normalized David's scores). Dominance was strongly influenced by body mass, and not by bill length or the number of individuals. Female fulvettas in particular tended to be among the most subordinate individuals in the flock. There was evidence of a negative relationship between a species pair's affiliation, measured as the percentage of all observations in which these two species were found perching together, and their difference in ranking in the dominance hierarchy, particularly when some ground species, which are not likely to remain in flocks long term, were removed from the analysis. Species pairs that had more pronounced differences in their dominance rankings were less likely to be affiliated, which is consistent with the idea that subordinate species may avoid dominants. David's Fulvetta is a strong information provider, so other species are likely benefitted in this relationship. Our data suggest one reason that fulvettas stay in flocks is that they may be subordinate and therefore have little control over who associates with them.
