Functional traits can determine pairwise species interactions, such as those between plants and pollinators. However, the effects of biogeography and evolutionary history on trait-matching and trait-mediated resource specialization remain poorly understood.

We compiled a database of 93 mutualistic hummingbird-plant networks (including 181 hummingbird and 1,256 plant species), complemented by morphological measures of hummingbird bill and floral corolla length. We divided the hummingbirds into their principal clades and used knowledge on hummingbird biogeography to divide the networks into four biogeographical regions: Lowland South America, Andes, North & Central America, and the Caribbean islands. We then tested: (i) whether hummingbird clades and biogeographical regions differ in hummingbird bill length, corolla length of visited flowers and resource specialization, and (ii) whether hummingbirds’ bill length correlates with the corolla length of their food plants and with their level of resource specialization.

Hummingbird clades dominated by long-billed species generally visited longer flowers and were the most exclusive in their resource use. Bill and corolla length and the degree of resource specialization were similar across mainland regions, but the Caribbean islands had shorter flowers and hummingbirds with more generalized interaction niches. Bill and corolla length correlated in all regions and most clades, i.e. trait-matching was a recurrent phenomenon in hummingbird-plant associations. In contrast, bill length did not generally mediate resource specialization, as bill length was only weakly correlated with resource specialization within one hummingbird clade (Brilliants) and in the regions of Lowland South America and the Andes in which plants and hummingbirds have a long co-evolutionary history. Supplementary analyses including bill curvature confirmed that bill morphology (length and curvature) does not in general predict resource specialization.

These results demonstrate how biogeographical and evolutionary histories can modulate the effects of functional traits on species interactions, and that traits better predict functional groups of interaction partners (i.e. trait-matching) than resource specialization. These findings reveal that functional traits have great potential, but also key limitations, as a tool for developing more mechanistic approaches in community ecology.

We compiled a dataset of 93 quantitative hummingbird-plant interaction networks from localities distributed widely across the Americas. Each of the 93 networks describes interaction frequencies within assemblages of hummingbirds and their food plants in a specific location. We only included mutualistic interactions in which a given hummingbird was observed drinking nectar and touching the stigma / anthers of the given flower, thereby potentially acting as a pollinator. The networks were sampled to represent all hummingbird clades and hummingbird-visited plant families without any taxonomic bias. Species names of the hummingbirds follow the International Ornithological Committee World List (IOC version 9.2; www.worldbirdnames.org). Hummingbirds were divided into nine clades following McGuire et al. (2014). Species names and families of the plants follow ‘The Plant List’ (TPL version 1.1; www.theplantlist.org), with a few exceptions where species names of recorded plants were not found in TPL.