Preferential nectar robbing of flowers with long corollas: experimental studies of two hummingbird species visiting three plant species

Long flower tubes have been traditionally viewed as the result of coevoluti on between plants and specialized, legitimate, long billed-pollinators. How ever, nectar robbers may have played a role in selection acting on corolla length. This study evaluated whether hummingbirds are more likely to rob fl owers with longer corollas from which they cannot efficiently extract necta r with legitimate visits. We compared two hummingbird species with similar bill lengths (Lampornis amethystinus and Colibri thalassinus) visiting flor al arrays of artificial flowers with exaggerated corolla lengths, and also evaluated how the birds extract nectar rewards from medium to long corollas of three hummingbird-pollinated plants (Salvia mexicana, S. iodantha and I pomoea hederifolia). The consequences of foraging for plant fitness were ev aluated in terms of seed production per flower. Variation in seed productio n after legitimate visits of hummingbird-pollinated plants was mostly expla ined by differences in pollinator effectiveness. Seed production did not in crease with the number of legitimate visits to a flower, except in I. heder ifolia. We found that birds were more likely to rob both artificial and nat ural flowers with long corolla tubes. Nectar robbing was not observed on sh ort-corolla flowers of Salvia spp., but robbing negatively affected seed pr oduction of long-tubed flowers of L hederifolia. Significant differences be tween hummingbird species in the use of this behavior were observed, but ma les and females behaved alike. We suggest that short-billed hummingbirds wi th enlarged bill serrations (the edge of both tomia finely toothed) may hav e an advantage in illegitimately feeding at long-corolla flowers. This rais es the possibility of counter-selection on increasing corolla length by nec tar robbers.