Abstract
Avian nectar-robbing is common in some floras but its impact on plant-pollinator mutualisms, flowering phenology, and the evolution of floral traits remains largely unexplored. Surprisingly, there have been no quantitative studies of the topography and extent of floral damage inflicted on any flowering species by nectar-robbing birds. I studied nectar-robbing of orchard oriole (Icteridae: Icterus spurius) on the large reddish-orange flowers of trumpet creeper (Bignoniaceae: Campsis radicans), an ornithophilous liana of eastern North America. Floral traits that inhibit nectar-robbery by hummingbirds and bees, such as the thickened calyx and sympetalous corolla, are ineffective in deterring orioles. Orioles target the zygomorphic trumpet-shaped corollas at the 11:00 h or 01:00 h positions with a closed-bill puncture and then enlarge the incision with bill-gaping to reach the nectary. More than 92% of flowers were robbed when orioles were present. Fruit set was nil until orioles departed on fall migration in late July-early August. The timing suggests oriole nectary-robbery may be a potent selection agent for an extended flowering season or delay in the onset of flowering. The biological and geographic attributes of the Campsis-Icterus association make it a promising model system for studying the consequences of avian nectar-robbery on pollination biology and floral trait evolution.