The importance of colour and other features of flowers in attracting pollinating animals is a widely-discussed topic in plant science. It also probably influences which pollinators you may, or may not, be able to find in your garden or local green space. Red has been suggested to be probably the most important colour for flowers to attract pollinating birds such as hummingbirds. There are two main theories as to how red may promote pollination of certain flowers by birds.
The first of these is that the red colour actively attracts birds. The second of these is that the red colour does not attract bee pollinators, therefore ensuring that the proportion of visits by birds is greater. Some evidence supports one or the other of these hypotheses, and it has also been suggested that a combination of both may actually be true. In order to gain more insight into the role of red colour on pollinator attraction to flowers, Zhe Chen, Yang Niu and colleagues from the Chinese Academy of Sciences collect spectral data from red flowers at a variety of locations, and show what this can tell us about how red flowers influence pollinators in a recent paper in Annals of Botany.
The authors use their measurements to address three questions. The first of these is whether red colour in the flowers they measure is more visible to birds than bees? The simple answer to this is yes, red is a probably more visible flower colour to birds than to bees. However, the authors find that the real situation may be a little more nuanced than this. For example, it turns out that the visibility of red to birds, whilst more than that of bees, is similar to some other colours. In bees by contrast, red was a clear low visibility colour in terms of contrast (how much is stands out compared to a background). In other words, whilst red may not be an especially ‘stand-out’ colour to birds compared to other colours, it is likely to be poorly-visible to bees.
The second question the authors address is whether there is any difference in colouration between red flowers that are predominantly pollinated by birds compared to bees? Again the answer to this appears to be yes, flower shades of red differ between bird-pollinated compared to bee-pollinated flowers. Interestingly, the authors find that this may have a lot to do with variation in the reflectance of different flowers, producing a ‘secondary peak’. In flowers pollinated by birds, this secondary peak is lower than that in flowers pollinated by bees. The authors speculate that this is likely because a high reflectance secondary peak is a known attractive feature to bees, but in birds decreases their ability to perceive contrast. This phenomenon is explainable through differences in the way bee vs. bird vision works.
The final question Chen, Niu and colleagues ask is whether there are differences in bird-pollinated flower colour in different locations? In bird-pollinated species, flowers from the ‘new world’ (I.e. the Americas) had generally lower secondary peaks than flowers from the ‘old world’ (Europe, Africa and Asia). In other words, flowers from the new world appear better adapted to avoid bees than flowers in the old world. The reasons for this may be multi-faceted, but the authors do point out that high secondary peak flowers do exist in the new world, suggested a possibly gradual evolutionary transition.
So seeing red is not the same everywhere, and varies according to both where you are and what pollinates you. The findings of Chen, Niu and colleagues are also consistent with red flower colour in cases of bird pollination reducing attractiveness to bees, rather than making the flower stand out especially to birds. So plants can sometimes get the results they want by simply avoiding everything they don’t want. A rather unusual strategy, but it seems to work for them!
Cover image: Martin Heade, Passion Flowers and Hummingbirds. Wikimedia Commons.