Woodland birds at greater risk from cold springs Great tits were particularly badly affected, with the average span from laying the first egg to hatching lasting 32 days. The figure for birds at urban sites was 17 days. A delay in hatching generally leads to a smaller brood, and fewer chicks leaving the nest. Writing in the PLOS ONE journal, the researchers said this was most likely because the weather had led to a scarcity of caterpillars, a key type of prey. Those in urban areas had a wider range of other food sources with which to compensate. Rainfall, which washes caterpillars off leaves and makes them harder for birds to find, and cold temperatures, which affect their reproduction and growth, were found to be the main factors. Dr Nancy Harrison, one of the researchers, said: "The birds breeding in the good woodland habitat really struggled during last year's cold, rainy spring. Blue tits and great tits in urban areas often forage for other prey and so are less reliant on one particular food source. These "urban scavengers" were better able to cope in 2012 when these caterpillars were in short supply." Over the whole of the ten-year study woodland birds produced larger and healthier broods, but this may not always be the case, she added. The breeding season is controlled by a hierarchy of factors including daylight and temperature, with temperature playing a key role in caterpillar reproduction and growth. If these extreme weather events become more commonplace due to the effects of climate change, then birds living in urban environments may have the advantage.