New research from the University of Oxford published today (11 March) shows that cold snaps and heavy rain can stunt growth and reduce survival prospects in UK great tit nestlings. However, breeding earlier within a season appears to buffer against many of these weather-related effects.
The study relies on 60 years of data for over 80,000 individual wild great tits in Oxford's Wytham Woods, combined with daily historical weather records. Researchers identified the coldest, wettest and hottest days of each breeding season, and counted how many of those occurred during specific developmental windows, to examine their impact on body mass when nestlings fledged (left their nest) – a key predictor of their survival.
The researchers discovered that extreme cold during the first week after hatching is particularly detrimental, while intense rainfall is worse for nestlings as they grow older, with both events reducing mass at fledging by up to 3%. However, when extreme heat coincides with heavy rain, fledging mass drops even further by up to 27%, particularly in broods laid later in the season.
Lead researcher Devi Satarkar (Department of Biology, University of Oxford) says: "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."
Hatchlings can't effectively regulate their body temperature as they lack feathers, so during cold snaps they divert energy to staying warm instead of growing. Both cold and rain extremes can deter parents from foraging, and heavy rain dislodges caterpillars from vegetation, lowering the food available for the growing nestlings that have very high energy needs.
Perhaps the most surprising result was that hot extremes increased fledging mass during the nestling stage. We might expect high temperatures to cause heat stress, but these results may be because these warmer spells in Oxfordshire remain mild compared to the searing heat extremes faced by southern European populations.
Devi explains: "Extreme weather events are affecting wild bird populations in complex ways. The level of warmth we see in these heat extremes in Oxfordshire might boosts 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-season broods seem to benefit from warm spells during spring, when caterpillars are plentiful and temperatures stay well within thermal tolerance ranges. Later broods don't fare as well: fledglings are around a third lighter, even though the 'hot days' they experience reach similar absolute temperatures of about 16-17ºC. Over the long-term, extreme cold and rain slightly reduce the chances of surviving to adulthood, whereas extreme heat can have small positive effects. However, breeding earlier within a season appears to buffer against many of these weather-related effects.
As species increasingly interact with the effects of climate change, there is a growing need to monitor microclimates, habitat variation, and finer-scale weather to predict population resilience. Research like this can help inform targeted conservation like nestbox placement or woodland management to protect vulnerable nestlings at critical developmental stages.
The researchers plan to continue tracking Wytham's great tits to see how these extreme weather effects evolve as the climate changes, particularly to see if current 'moderate' heatwaves start becoming detrimental with further warming.
