Bee Exclusion in Bird-Pollinated <i>Salvia</i> Flowers: The Role of Flower Color versus Flower Construction

Abstract

Premise of research. In plants, evolutionary shifts from one pollinator guild to another are normally associated with coordinated changes of several floral characters. These might be floral morphology, color, and scent. On the basis of the animals’ different sizes and shapes as well as their visual and olfactory sensitivities or preferences, floral visitors exert different selective pressures on flowers. Floral traits might act as floral filters, attracting pollinators or excluding unwanted flower visitors. We use the genus Salvia, in which several shifts from bee to bird pollination are known, to examine the potential role of floral color versus floral construction as floral filters.Methodology. We studied 26 bee-pollinated, 30 bird-pollinated, and 3 intermediate Salvia species. Besides documenting floral morphology and floral guides, we measured floral spectral reflectance and characterized the flowers’ perception by bees.Pivotal results. The color loci of half of the bird-pollinated species (red to humans) are in the achromatic center and thus are less conspicuous to bees. All other species (mostly bluish) have significantly greater color contrast and spectral purity and can be recognized by bees and birds. Nectar guides occurred in almost all bee-pollinated species included in the study but in no bird-pollinated species. The results suggest that color might play a role as a floral filter against bees in only half of the bird-pollinated species, while mechanical bee exclusion appears to be important in all bird-pollinated species.Conclusions. Our data indicate that bees are excluded from bird-pollinated flowers mainly by floral construction. The red color and the absence of nectar guides in bird-pollinated flowers can be interpreted as adaptations for reducing nectar and pollen theft by bees via visual exclusion without losing attractiveness for birds.

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