The evolution of the traplining pollinator role in hummingbirds, specialization is not an evolutionary dead end

Trapliners are pollinators that visit widely dispersed flowers along circuitous foraging routes. Through coevolution with the flowers they pollinate, trapliners are hypothesised to become more morphologically and ecologically specialised than their non-traplining counterparts. We hypothesised that independent transitions to traplining in hummingbirds should entail convergent morphological specialisation, and we tested whether such transitions are irreversible and lead to lower rates of diversification as predicted by the hypothesis that specialisation is an ‘evolutionary dead end’. We find that there have been multiple independent transitions to traplining across the hummingbird phylogeny, but reversals have been rare or incomplete at best. Multiple independent lineages of trapliners have become morphologically specialised, convergently evolving relatively large bills for their body size. Traplining is not an evolutionary dead end however, since trapliners continue to give rise to new traplining species at a rate comparable to non-trapliners.