Young great tits living in the forest grow less rapidly and die more often due to extremely hot days. Urban great tits do not appear to be bothered by this.

The earth is getting warmer – with more exceptionally hot days as one of the consequences – and our planet is urbanizing. What effect does the combination of these two developments have on animals? Which investigated behavioral ecologist Ivett Pipoly from the University of Pannonia in Hungary with colleagues by studying nests of young great tits. The result: great tits in the city seem to be more resistant to heat than tits in the countryside.

Birdhouses

When you look at the effect of heat on animal populations in cities, you can reason in two directions. Firstly, you can say: it is generally warmer in cities than outside (the so-called heat island effect† As a result, urban animals suffer more from hot days than animals in the wild. But you could also say: due to the same heat island effect, urban animals can deal with heat better than their counterparts elsewhere.

To see the rationale for great tits, Pipoly and her team spent six years, from March through July, tracking young great tits in birdhouses they installed. Some of those birdhouses hung in the Hungarian towns of Veszprém and Balatonfüred, some hung in the forest.

The result: Although higher maximum temperatures and hotter days were recorded in the cities than in the forest, only the young great tits in the forest suffered those days. The ‘forest birds’ died more often and also lost weight more often.

caterpillar scarcity

So why is it that the urban tits can cope better with the heat? Among other things, young great tits that live in cities have fewer feathers and more bald spots. They are also a bit smaller on average, making them a relatively large surface and can therefore lose heat more easily.

In addition, the urban tits have fewer young at a time. And, as Pipoly and colleagues write: “Fewer offspring produce less heat and have more space to keep their distance from each other.”

It may also be the case that extremely hot days mainly mean that great tits living in the forest have less food. This is because ‘bush tits’ mainly eat caterpillars – which are available for a shorter time and become smaller in size when it gets warmer. Urban tits feed their young all kinds of things, and are therefore not so dependent on the caterpillar position at that time.

Surprising

“It’s a pretty compelling study,” says Menno Schilthuizenevolutionary biologist (Naturalis and Leiden University) and author of the book Darwin in the city† “Great tits are known to often show strong local evolutionary adaptation to their habitat on a surprisingly small spatial scale. Indeed, in this case, it appears that urban populations have adapted to the heat island effect and are much more resilient to extremely hot days. This is surprising, because you might think that urban populations are already at the maximum they can tolerate and that extremely hot days are more likely to kill them. But the opposite seems to be the case.”