Earlier this year, we published a paper about the choices birds make at different coloured bird feeders. Last week, I was contacted by a TV researcher with a question about hummingbirds. He’d seen a video clip of a hummingbird trying to feed from a man’s orange baseball cap, and wanted an explanation as to why this might happen. The obvious answer is that hummingbirds like red (and reddish colours), but do we know why? Asking a behavioural ecologist “why?”? I had to know.
First a disclaimer: I am not claiming to be an expert in hummingbird behaviour, pollination ecology, or floral trait evolution. I don’t even live anywhere where there are hummingbirds. As part of the bird feeder project, I briefly looked into the literature on choices at supplementary feeders, which happens to be entirely (until we published out paper) about hummingbirds. The first thing that jumped out from that brief survey of the literature was that the assumption that hummingbirds like red is actually wrong.
Why, then, is the hummingbird attracted to the hat?
Generally speaking, hummingbirds feed from red flowers, and flowers that are pollinated by hummingbirds tend to be red. So, this leads naturally to the assumption that hummingbirds like red. This is probably bolstered by the fact that if you wanted to buy a hummingbird feeder, you’d probably have to look quite hard to find one that wasn’t red.
Instead of asking why hummingbirds like red, we can turn the question around and ask instead why hummingbird-pollinated flowers tend to be red (although they are not exclusively red). There are actually two competing hypotheses as to why this is the case[2,3,4,5]. The first is that hummingbirds have an innate preference for red, and find it easy to locate red flowers (birds have excellent colour vision). The second is that the red colour is less attractive to other potential pollinators, such as bees, who find it more difficult to locate red flowers (bees do not have red receptors, and so are sometimes considered ‘colour blind’ to red, although studies have shown they can discriminate). By either mechanism, flowers that are pollinated by hummingbirds have generally evolved to be red, even though they might be quite distantly related, in a case of convergent evolution.
For the first hypothesis, there’s a body of experimental work investigating preferences for flower colour in closely related species of plants suggesting a preference for red and orange flowers over yellow ones. But if we look at experimental work on hummingbird choices at artificial feeders, we don’t actually see this pattern – there is no particular preference for red over other colours – except when the hummingbirds are trained to feed from red feeders, or are captured from the wild on or close to red flowers[1,3]. Other than that, one study that seems to show that when artificial feeders are put in new locations, red ones seem to be investigated first by hummingbirds, but afterwards colour makes no difference[1,3]. What’s important are things like location and nectar quality. So, the support for an innate red preference in hummingbirds is actually quite weak, and the answer is not nearly as simple as “hummingbirds like red”.
The other hypothesis, that the flowers are red to deter bees, has a body of empirical support[6,7]. For example, bees find it harder to locate red flowers when they are against a green foliage background, especially when there is no UV component, prefer white over pink flowers when there is no difference in nectar reward, and avoid foraging on red flowers. Overall, there seems to be quite a lot of support for “bee avoidance” rather than “hummingbird preference” as an explanation as to why hummingbird flowers are red. But why might flowers evolve to avoid being pollinated by bees? Aren’t bees what spring to mind when you hear the word “pollinator”? Because bees are potentially less satisfactory as pollinators: they tend to eat some of the pollen, and are more likely to result in self-fertilisation for the plant. By avoiding pollination by bees, the plant can retain more of its nectar resources for its ‘preferred’ hummingbird pollinator, thereby attracting hummingbirds due to the greater nectar rewards that are associated with red flowers.
So back to the original question, if hummingbirds don’t really have a preference for red, why the attraction to the hat? Experience might play a role here here. If hummingbirds are attracted to resources that are of a familiar colour (and we know they can be trained to particular colours), associated with higher nectar rewards, and the flowers (and artificial feeders) they are feeding on tend to be red, and if red is also conspicuous in novel locations, then the hummingbird might well choose to explore the opportunities offered by a novel orange object in their environment. On arriving at the hat, the holes may bear enough of a similarity to the narrow corolla tubes (the central part) of hummingbird-pollinated flowers for the hummingbird to try feeding from them.
 Rothery, L, Scott GS & Morrell LJ. (2017) Colour preferences of UK garden birds at supplementary seed feeders. PLoS ONE 12(2): e0172422 doi: 10.1371/journal.pone.0172422
 Lunau K, Papiorek S, Eltz T, Sazima M. (2011) Avoidance of achromatic colours by bees provides a private niche for hummingbirds. J Exp Biol. 214: 1607–1612. doi: 10. 1242/jeb.052688
 Handelman, C & Kohn, JR (2014) Hummingbird colour preference within a natural hybrid population of Mimulus aurantiacus (Phrymaceae). Plant Species Biology, 29: 65-72. doi: 10.1111/j.1442-1984.2012.00393.x
 Rivest SA, Austen EJ & Forrest JRK (2017) Foliage affects colour preference in bumblebees (Bombus impatiens): a test in a three-dimensional artificial environment. Evolutionary Ecology 31: 435–446 doi: 10.1007/s10682-017-9893-4
 Bergamo PJ, Rech AR, Brito VLG & Sazima M (2015) Flower colour and visitation rates of Costus arabicus support the ‘bee avoidance’ hypothesis for red-reflecting hummingbird-pollinated flowers. Functional Ecology 30: 710-720 doi: 10.1111/1365-2435.12537
 Schemeske DW & Bradshaw HD (1999) Pollinator preference and the evolution of floral traits in monkeyflowers (Mimulus). PNAS 96: 11910–11915 doi: 10.1073/pnas.96.21.11910
 Gegear, RJ, Burns, R & Swoboda-Bhattarai, KA (2017) “Hummingbird” floral traits interact synergistically to discourage visitation by bumble bee foragers. Ecology 98: 489-499 doi: 10.1002/ecy.1661