Birds Perceive More Intraspecific Color Variation in Bird-Pollinated Than Bee-Pollinated Flowers

Abstract

Pollinator-mediated selection is expected to constrain floral color variation within plant populations. Here, we test for patterns of constraint on floral color variation in 38 bee- and/or hummingbird-pollinated plant species from Colorado, United States. We collected reflectance spectra for at least 15 individuals in each of 1–3 populations of each species (total 78 populations) and modeled perceived color variation in both bee and bird visual spaces. We hypothesized that bees would perceive less intraspecific color variation in bee-pollinated species (vs. bird-pollinated species), and reciprocally, birds would perceive less color variation in bird-pollinated species (vs. bee-pollinated species). In keeping with the higher dimensionality of the bird visual system, birds typically perceived much more color variation than bees, regardless of plant pollination system. Contrary to our hypothesis, bees perceived equal color variation within plant species from the two pollination systems, and birds perceived more color variation in species that they pollinate than in bee-pollinated species. We propose hypotheses to account for the results, including reduced long-wavelength sensitivity in bees (vs. birds), and the ideas that potential categorical color vision in birds and larger cognitive capacities of birds (vs. bees) reduces their potential discrimination against floral color variants in species that they pollinate, resulting in less stabilizing selection on color within bird-pollinated vs. bee-pollinated species.

Major Flower Pigments Originate Different Colour Signals to Pollinators

“…The role of pelargonidins in bird-pollinated flowers was previously reported in species of the Solanaceae and Gesnerioideae Smith, 2016, 2018;Ogutcen et al, 2020). Although birds do not show preferences for a particular colour, they have an excellent colour discrimination (Lunau et al, 2011;Stoddard et al, 2020;Whitney et al, 2020). Our results suggest that both carotenoids and pelargonidins pigments generate a higher conspicuousness to birds than other pigment groups, which would favour their accumulation in flowers of bird-pollinated species.…”

“…As the previous ones, this model also allows to measure perceived chromatic differences among stimuli (Endler and Mielke, 2005;Stoddard and Prum, 2008;Camargo et al, 2019). A considerable number of studies show that functional groups of pollinators can perceive differently the same flower colour (e.g., Ohashi et al, 2015;Bergamo et al, 2018;Whitney et al, 2020). In fact, it has been demonstrated that flower colours have evolved in different regions of the world to match the visual capabilities and preferences of local pollinator fauna (Burd et al, 2014;Shrestha et al, 2016;Camargo et al, 2019;Coimbra et al, 2020).…”

Colour Discrimination From Perceived Differences by Birds

“…Predictions from RN depend on the precise value of its various parameters, and in particular, on the magnitude of noise coefficients (e i ) assigned to the different photoreceptors (Lind and Kelber, 2009). In spite of the wide use of the RN model for answering questions regarding avian visual ecology such as: discrimination of parasitic eggs by hosts (Hanley et al, 2019), camouflage by female birds (Cain et al, 2019), and perceived flower colour variation by pollinating species at population level (Whitney et al, 2020); to cite just a few recent examples, no noise measurement data currently exist for any avian species. In most cases, the unknown but essential noise values are derived from applying theoretical assumptions of signal detection theory to photoreceptor density data (Vorobyev and Osorio, 1998), although it still remains unclear how neural processing by the brain may affect these assumptions.…”

How Much Pigment Should Flowers Have? Flowers With Moderate Pigmentation Have Highest Color Contrast

“…Of these types of pigments, anthocyanins are most commonly associated with flower color polymorphisms (Sapir et al, 2021). Indeed, many species for which flower color variation was reported (Frey, 2004;Rakosy et al, 2012;Renoult et al, 2013;Sletvold et al, 2016;Dormont et al, 2019;Paine et al, 2019;Streinzer et al, 2019;Gómez et al, 2020;Whitney et al, 2020) have blue, pink or purple colors that are commonly due to anthocyanin pigments. It would be interesting to know, for these species, how standing variation in pigmentation relates to the flower's conspicuousness to their pollinators.…”

“…Something to consider is that the perceived flower color variation probably differs across pollinators. Whitney et al (2020), for example, recently showed that for the same set of flowers, birds perceive more intraspecific variation than bees. It is known that different pollinators select for different flower colors (hues) owing to variation in spectral sensitivity and/or color preferences (Lunau et al, 2011;Shrestha et al, 2013;, but pollinators may also impose different selective pressures on the degree of variation around a certain hue.…”