Well, Readers, by the time you read this my latest assignment will be with my tutor, so I can reveal that woodpigeons and magpies prefer orange doughballs to yellow ones. So what, I hear you say. Well, the birds had previously voted on whether they preferred red dough balls to yellow ones, and the result there was a resounding ‘give me the red ones’. We know that birds have colour vision, and so there are really two questions here. Firstly, why do they prefer orange/red over yellow, and secondly, what impact would this have if the dough balls were actually prey species, either berries or insects?

What complicated the results a little this time was a) the weather, and b) that the squirrels discovered that they also like doughballs.

Firstly, who knew that birds hated windy weather so much? They seemed unfazed by torrential rain (though this did cause problems by making the doughballs disintegrate). In the end, I was only able to run 18 trials instead of my planned 20, but the results were pretty resounding anyway.

And secondly, what took the squirrels so  long to discover that they liked dough balls? They will eat meat if it’s on offer, so it’s  not really a surprise, and as they have dichromatic vision (i.e. they’re practically colour-blind), if they’d eaten too many this could have caused a problem. Fortunately, they would pick up one doughball in their tiny paws and nibble at it, rather than hoovering the lot up like the birds. I think we can discount their contribution.

So, back to the main questions. Why do birds prefer orange and red over yellow? The obvious answer (to me anyway) was that in plants, this colouration is usually an indication of ripeness. I’m not sure whether birds can taste sweetness, but at any rate the berries are at their maximum nutritional value when they’re full of  energy-giving sugar. So it came as no surprise that woodpigeons would go for the red or orange berries before the yellow ones. I was a little more surprised that the magpies did so, as they are much more omnivorous, but they too showed a clear preference.

The issue is complicated by research that was done in Mediterranean forests. There, a large number of unpalatable insects are coloured red, and when fake prey (yet more doughballs) was put out, with some coloured red and some coloured grey or green, it was found that the adult birds overwhelmingly chose the grey or green prey, while the juveniles chose red, green and grey equally. The adults appeared to have learned, by encounters with red live prey, that it didn’t taste good, and so avoided the red fake prey, while the juveniles were yet to learn. In the UK, some red insects, such as ladybirds, are unpalatable, but clearly the magpies either didn’t eat them, or decided to give it a go anyway. Or, of course, they could have looked at the doughballs and thought ‘that’s not an insect’. I’d put nothing past them.

Secondly, what would happen if the doughballs were real live prey, rather than something that I’d knocked up in my kitchen. It depends on whether the prey are berries or insects.

If the prey were to be berries, it would be good news for the orange or red ones, provided that the seeds inside the berries could survive the trip through the birds digestive tract. The seeds would be carried away from the parent plant and some, at least, would fall on fertile ground, maybe without the risk of competition or overshadowing. You might expect the red and orange plants to become more common, while the yellow ones languished because their seeds feel at the feet of their parent plant. Of course, even berries of  an unpopular colour will be eaten when birds are desperate, so this is all a numbers game, and the yellow berries may even have other survival advantages.

If the prey were insects, however, it’s a different story. At the rates of preference for my orange and red prey, these insects would become scarcer and scarcer, as they were gobbled up, while their yellow relatives would go about their business unmolested. The red and orange insects might even be driven to extinction, though what often happens is that the energy budget of the birds will make them switch when the red and orange insects become rarer and harder to find – if it costs more energy to find something than the nutritional value that it contains, it makes sense to switch to something easier to find, i.e. the yellow insects.

It’s important to realise that this is an important mechanism for evolution – after all, we have the example of the good old peppered moth as evidence. It was shown that, before the Clean Air acts, the dark morph of this moth was better camouflaged than the mottled one against the tree trunks where it rested, and so the mottled one became rare in the population, because it was preferentially picked off by birds. When the air became cleaner, and lichens began to recolonise the tree trunks, it was the mottled morph that was better camouflaged, and so the black morph became much rarer as it was suddenly more apparent to predators.

And so, a little back garden experiment with doughballs opens up a whole world of questions. Would the birds prefer red to orange berries? How quickly do they pick out the red/orange berries, and could this show strength of preference? If I spent hours making the red doughballs look like teeny tiny ladybirds would the magpies still eat them (I imagine that they’d take one look at my efforts and laugh their feathered heads off).