I’m okay. I know the Green Cross Code

NATURAL selection is a harsh interrogator at the best of times. But if you are a bird, it has an extra question, not asked so forcefully of animals that cannot fly: “is that extra gram of weight really necessary?” Contrary to the insult “bird-brained”, birds are not notably more stupid than mammals, but the pressure to keep organs light applies to the cerebrum as much as it does to anything else.

For the past century, though, birds have faced a new enemy that might require them to get smarter: the motor car. These days, cars and other motorised vehicles kill around 250m birds a year. That sounds like a significant selective pressure, so Anders Moller, an evolutionary biologist at the University of Paris-Sud, in France, decided to find out whether it really was.

Dr Moller’s hypothesis was that avoiding vehicles needs intelligence, and intelligence needs a big brain. The conclusion of this syllogism is that small-brained birds are more likely to be road-kill than large-brained birds are. To test this idea, though, he needed data on a lot of dead birds.

That serendipity plays a part in science is undeniable. Fleming’s chance observation of Penicillium mould on bacterial plates led to antibiotics. Kekulé’s dream of carbon atoms dancing in rings led to his model of the structure of benzene. Dr Moller’s serendipity was to meet, 30 years ago, a taxidermist called Johannes Erritzoe. Mr Erritzoe has, during his career, exhaustively recorded details of the specimens that have passed through his hands. These details include the weights of the internal organs, and likely cause of death, of 3,521 bird specimens of 251 species.

Since they met, Dr Moller and Mr Erritzoe have collaborated on many papers. This time, they asked whether there was a difference between the weights of the organs of birds killed by traffic and of those that had died of other causes. They found, as they report in Royal Society Open Science, that there was not—with a single exception. The smaller a bird’s brain, when controlled for its body size, the more likely it was to have been road-kill. Some 60% of the smallest-brained birds Mr Erritzoe handled had died this way. Among the largest-brained, death by traffic was unheard of.

All this suggests a selective pressure on birds in parts of the world with lots of traffic to acquire bigger brains, even at the cost of the extra energy required to keep those brains airborne. It also leads to a prediction, in a field of science—evolutionary biology—that is rarely in a position to make them. This is that the average weight of bird brains may rise over coming decades. Whether anyone with Mr Erritzoe’s enthusiasm for data collection will provide the means to test that prediction is, though, a different question.