Putting out food for birds may help them through the winter. In spring, though, it can ruin their chances of mating

FEEDING birds sounds the sort of thing that no one in his right mind could oppose. In Britain, the world capital of amateur ornithology, roughly half of households put food out for their feathered friends, and it is estimated that around 30m of the country's birds are given nourishment this way every year. Other places are somewhat less generous, but the general principle holds. Encouraging birds is good, and what better way to encourage them than to feed them?

Science is nothing, however, if it is not iconoclastic, so one scientist, Valentin Amrhein of the University of Basel, in Switzerland, asked himself if feeding birds this way really is good for them. The answer, it turns out, is “no”—at least, not always.

Dr Amrhein's team conducted their study in the suburbs of Oslo, in the spring of 2007. The objects of their attention were 28 male great tits, each of which was observed at dawn three times, with 16-17 days between the observations.

The first observation took place in the absence of supplementary food. Immediately afterwards, a feeder was provided within the observed male's territory. Half of these feeders were kept filled with food, but half remained empty. Full or empty, each feeder was left in place for 16-17 days, at which time a second observation was made and the feeder was removed. The third and final observation was made 17 days after this.

The object of the exercise was to look at the effect of feeding on a male's performance in the dawn chorus. This chorus might sound lovely to human ears, but it is a gruelling challenge for birds. Males have to get up early and, having fasted all night, expose themselves to extreme cold while singing their little hearts out to show females that they are worth breeding with. In general, those males that start singing long before the sun comes up get the best mates and best defend their territories from rivals.

Dr Amrhein expected that males who were being given extra food would perform better during the dawn chorus than those that were not. To his surprise, he discovered exactly the opposite. Those who received food supplements got lazy. He and his colleagues report in Animal Behaviour that 36% of the males whose feeders were filled started singing only after the sun had already come up. Among the birds without this extra food, that happened only 10% of the time. Moreover, the effect was sustained after feeders were removed, for it was still apparent at the time of the third observation.

Why extra food has this effect is unclear. What is clear is that delaying dawn singing is a dangerous game for males to play. The worm is not the only thing caught by the early bird. Fat and lazy tits risk losing out on a mate, as well.