Wednesday, September 16, 2009

A Study in Scarlet.....

....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.

5 comments:

  1. Fascinating Phil.
    I’ve often wondered how birds ‘see’ things like colours. I do know they prefer the red berries on my Pyracantha to the orange ones. They eat them first :)

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  2. Hi Keith, insect vision is even harder to fathom - many of them see UV too and see patterns in flowers that are invisible to us. We seem to have a good berry crop up here in the North East this winter, so I'm hoping it will attract plenty of waxwings.

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  3. Lovely post, Phil. I've often wondered why Wild Ivy never evolved to red berries. They just stay in the hedgerows too long before being eatenOr do they multiple via other ways than fruit/seed?

    The continuous rain means that the Hawthorn has produced very little berries. Also Honeysuckle berries were very hard to find.

    I have nominated this blog for a Kreativ Blogger Award. Just go to my blog;
    http://wildlifeonwheels.blogspot.com/2009/10/kreativ-blogger-award.html

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  4. Thank you Yoke, that's very kind - I'm honoured! I've often seen pheasants eating ivy berries in spring, and I think migratory birds like redwing and fieldfare fatten up on them before they leave our shores. Ivy is a fascinating plant, not least because it flowers and fruits completely out of synchrony with most of our flora.

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  5. Ivy is also a killer. first if it gets a good hold on ony tree even the oak tree it will strangle it. mainly by inserting its root into the bark then taking the sap, then it will even demolish walls especially walls built with lime mortar the ivy's roots again take all the moisture and whatever to weaken the wall. and eventually the wall will collapse. your garden shed is also prone to this killer and now you cannot buy creosote you can use Jeyes fluid winter wash ( i used to use it to kill the bugs in my fruit trees in the spring and does not harm the trees)its the same just spray the shed , any Ivy that grows near your house and garden cut the stem close to the root. is sufficient to kill it. there is enough Ivy in the wild for our birds, if a bird eats the berries and comes in your garden and inevitable it will shit. nature will take over and you will have another plant to care for as you will.

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