A 60-year study of over 80,000 great tits in Oxford's Wytham Woods shows that cold snaps and heavy rain reduce nestling body mass and survival chances. Birds breeding earlier in the season appear better shielded from these effects. Moderate warm spells, however, can boost chick growth by enhancing food availability.
Researchers at the University of Oxford analyzed 60 years of data on more than 80,000 wild great tits in Wytham Woods, pairing it with daily weather records. The study, published on March 11 in Global Change Biology, examined how extreme weather during critical chick development stages affects fledging mass, a key survival indicator.
Severe cold in the first week after hatching harms nestlings most, as they lack feathers and must expend energy on warmth rather than growth. Heavy rainfall poses a greater threat later, when chicks demand more food. Both conditions can reduce body mass at fledging by up to 3%. When intense heat coincides with heavy rain, the effect worsens, dropping mass by as much as 27%, especially in later-season broods.
Lead researcher Devi Satarkar noted: "In the Wytham population, great tits have adjusted to warmer springs by breeding earlier to track peak abundance of their main prey, caterpillars. This overall earlier laying is beneficial, buffering them against many impacts of extreme weather -- but it also exposes them to cold spells early in the season. Even small early-life deficits can have large implications for survival. It will only get tougher for birds to keep up as extreme weather increases in frequency and intensity with climate change."
Bad weather limits parental foraging and reduces caterpillar availability by knocking them from plants. In contrast, moderate warm extremes in Oxfordshire link to heavier fledging weights. Satarkar explained: "Extreme weather events are affecting wild bird populations in complex ways. The level of warmth we see in these heat extremes in Oxfordshire might boost growth because it can increase insect activity and visibility -- making caterpillars easier to find -- while letting parents forage more and reducing nestlings' thermoregulatory costs. The high water content in caterpillars also helps against dehydration. This contrasts sharply with hotter regions like the Mediterranean, where similar events can exceed 35°C and harm nestlings."
Early-hatching broods benefit from abundant caterpillars during safe warm spells, while later ones face lighter fledglings despite similar peak temperatures of 16-17°C. Over time, cold and rain slightly lower adult survival odds, but warm extremes offer minor positives. Earlier breeding within a season helps mitigate unpredictable weather.
As climate change heightens extremes, the study underscores the need to monitor microclimates and habitats for conservation, such as nestbox placement in woodlands. Researchers will continue tracking the Wytham population to assess shifting weather effects.
