It was found that cold days in the first week after hatching and heavy rains can lead to a reduction in chick mass by up to 3% by the time they fledge. In cases where heat coincides with heavy rains, weight loss can reach 27%—especially concerning late broods. Chicks that do not yet have feathers cannot adequately regulate their temperature, causing them to expend energy on heating, while rains wash caterpillars off plants, depriving the birds of a primary food source.
An interesting finding was that the moderate temperatures characteristic of Oxfordshire contribute to an increase in chick mass: warmth activates insects and makes them easier to find. At the same time, in hotter regions such as the Mediterranean, similar temperature conditions are already causing negative consequences.
Early nesting helps avoid most of the negative effects of weather: early broods coincide with the peak abundance of caterpillars and avoid summer temperature anomalies. Nevertheless, researchers warn that with the increasing frequency of extreme weather events caused by climate change, birds are finding it increasingly difficult to adapt to new conditions.
