Dear Readers, my science experiment proceeds apace, with my two magpies popping down to feed on my red and yellow doughballs every time no one is watching. They’ve given up calling, and instead it’s like being visited by ghost birds – I turn my back for five minutes and half the food is gone.
I’m now in the actual trial stage of the experiment, offering first 45 red dough balls and 5 yellow ones ten times over a few days, and then reversing the colours so there are 45 yellow balls and 5 red ones. As I mentioned last time, I have to record the details once there are between 15 and 35 balls left. As I write this, I am 6 trials through the 45 yellow/5 red stage, so should hopefully have all the data that I need to analyse in the next day or so. But what is the point of the whole thing?
What we’re looking at is the preferences of our predators, in this case the magpies, though the experiment isn’t particularly concerned with species (alas). Predators have a variety of strategies, which break down roughly into (bear with me) frequency-dependent selection, and frequency-independent selection.
In frequency-dependent selection, the predator chooses its prey by what is commonest, without any concern about physical features. In this scenario, when there are more red balls, you would expect the magpies to choose them, and when the yellow ones are in the majority, they’d go for them. When a predator chooses the commonest ‘morph’ (as different colourways are called), they will gradually change the make up of the population. Let’s take our balls as an example, and imagine that they are little animals who can reproduce and make lots of little doughballs. If, when the red balls are in the majority the magpie eats lots of them, fewer will survive to breed, and gradually the yellow ones, who are largely left alone, will become commoner, until they are in the majority. Then, if all the magpie cares about is easy pickings, it will start eating the yellow ones, and the red ones will get a chance to multiply. In other words, there’s a constant trend towards equilibrium, and although the doughballs might change in ratio of red to yellow, there will always be doughballs.
In frequency-independent selection, though, the magpie is more of a gourmand, and has particular things that s/he likes, even if they are harder to find. Also, it might be that the bird is more attuned to particular colours – red signifies ‘tasty’, while yellow means ‘bitter’, for example. So, maybe the magpie will choose red for preference, even when it’s rarer. In this case, you would expect the poor red doughballs to become rarer and rarer, until they’re so hard to find that the magpie has to turn to the yellow ones to get something to eat, giving the red ones a chance to recover. The red doughballs will never be as common as the yellow ones, because as soon as the population recovers, the magpies will shift to eating them again.
Real life is much more complex, of course – it’s not just magpies and doughball-insects, there’s a whole range of other plants and animals in a complicated ecosystem. But I am really looking forward to seeing what my results actually show. Plus the magpies are now my friends, and as I still have about a kilogram of dough (currently in the freezer) to get through, I can imagine that they’ll be my friends for quite a long time yet.