Morphological and behavioural adaptations to feed on nectar: how feeding ecology determines the diversity and composition of hummingbird assemblages

Abstract

Hummingbirds are the most specialised nectarivorous birds and show close ecological relationships to their food plants. Their small body size, bright colors, and unique behaviour have fascinated generations of naturalists. In this review, we investigate the morphological and behavioural adaptations of hummingbirds to feed on nectar and arthropods, and explore their diffuse co-evolution with their food plant species. Further, a list of plant genera including species mainly pollinated by hummingbirds is presented. Summarising the existing knowledge on hummingbird feeding ecology, we find that much of the variability in morphology and behaviour of hummingbirds is determined by their unique feeding mode and the constraints set by their food plants. Based on the existing literature, we developed a hierarchical system explaining how different environmental factors have shaped the current richness of hummingbirds, and their morphological and behavioural diversity. We propose that climatic stability within and between seasons and days determines the constancy of food availability, which in turn is the most important factor for species richness in hummingbird assemblages. However, the assemblage composition of hummingbirds is also influenced by phylogenetic factors, especially under harsh environmental conditions. Unsurprisingly, the highest morphological and behavioural diversity is observed in the most species-rich assemblages. This diversity may have at least partly evolved to reduce inter- and intraspecific competition. Independently of which morphological character we consider, the 360 different hummingbird species have evolved a large morphological variability to adapt to their individual feeding niches.