But some are even weirder than we thought. Because researchers have found that certain small birds massively increase their temperature when feeding their young. So much so, in fact, that they almost overheat and die.
How much do they heat up, you ask? Well, small birds known as passerines normally have a body temperature of around 41°C (106°F). But when feeding their young, this can increase by more than 4°C (7.2°F) and exceed 45°C (113°F).
"A body temperature of over 45°C must be close to fatal even for small birds", said Jan-Åke Nilsson, a professor at Lund University in Sweden, in a statement. He’s the lead author on a paper describing these findings in Proceedings of the Royal Society B. The other author is Andreas Nord.
The cause of the temperature increase was that the birds were working hard to feed their young, constantly flying back and forth to the nest, and their breast muscle had to work overtime. The bigger their brood, the more work they had to do and the less chance they had to cool down between flights by panting, leading to a higher body temperature.
This study focused on marsh tits (Poecile palustris), found throughout Europe and northern Asia. The team made their broods larger and smaller, and then watched how hard the birds had to work, measuring the temperature of parents when they returned to the nest. High body temperatures like those seen here have previously only been found in birds that live in hot and arid regions.
“It was rather astonishing that our marsh tit parents operated at body temperatures comparable [to] birds in arid areas,” Nilsson told IFLScience.
One of the weirdest things was that their brains could continue functioning despite the increase in temperature. Us humans, by comparison, considerably struggle with fever or even die when our body temperature rises by a similar amount.
But this study suggests that a warming climate could affect small birds like these, which seem to live right at the limit of how much heat they can handle. About 46°C (115°F) is thought to be the predicted lethal level.
"If the climate becomes warmer, it could make small birds more vulnerable,” Nilsson said in the statement. “Warmer springs would force them to produce and raise fewer offspring because they cannot feed them as often without risking death."