....and crimson.
Why are so many berries – including the species below - red?
Guelder rose
Hawthorn
Sorbus (intermedia?)
Bittersweet
The standard answer is that bird vision is particularly sensitive at the red end of the spectrum, which is why they’re attracted to red fruits. Bird pollinated flowers in the tropics tend to be red too. It’s undoubtedly true that there is a correlation between this colour and attractiveness to birds, but it’s also now known that birds are also able to see ultraviolet, at the opposite end of the spectrum – a wavelength that our eyes are not sensitive to. So we can’t make an absolute direct comparison between the colours that we see and the colours that birds see – when we see purple, for example, it’s a combination of blue and red light reflected from opposite ends of our spectrum of visual perception. Bird purple would be a combination of ultraviolet and red – whatever that might look like. Although we can’t have absolute knowledge of the colours in bird-world, what researchers can do is to present birds with colour choices to see what they prefer...... and that produces some interesting results. It turns out that it’s not just the colour of the berries in our visual spectrum that's important - it's also the contrast between the colour of the berries and the background, including UV light reflected or absorbed by leaf sufaces, that’s also important – the contrast between berries and the background affects the choices that birds make, not just the colour of the berries. Also the waxy ‘bloom’ on many black, blue and purple fruits (like bilberries) reflects UV light, which makes them conspicuous to birds – rub the bloom off and birds tend to be less attracted to them. Finally, experience counts too - juvenile and adult redwings show different preferences, so learning has a role in associating fruit colour with the best food sources. As in so many branches of science, research hasn't yielded clear-cut answers yet, but it has produced some very interesting questions. The story has as many twists and turns as a Shirlock Holmes mystery. What is certain is that simply trying to interpret what birds (or any other animals) see according to what we see is unlikely to give a true impression of the way the world looks to them.
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