University of Oxford study finds cold snaps and heavy rain impact survival of UK great tit nestlings
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New research from the University of Oxford shows that cold snaps and heavy rain can stunt growth and reduce survival prospects in UK great tit nestlings.
Relying on 60 years of data for more than 80,000 individual wild great tits in Oxford’s Wytham Woods, researchers discovered several startling facts about how weather can impact birds.
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To come to their conclusions, those involved in the study first 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.
When extreme cold hit during the first week after hatching, the effects were particularly detrimental. Intense rainfall was also recorded as being detrimental to nestlings as they grew older, with both events reducing mass at fledging by up to three per cent.
However, when extreme heat coincides with heavy rain, fledgling mass drops even further by up to 27 per cent.
‘In the Wytham population, great tits have adjusted to warmer springs by breeding earlier to track peak abundance of their main prey, caterpillars,’ said lead researcher Devi Satarkar.
‘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.’
As hatchlings do not possess feathers, cold snaps mean they divert energy to staying warm instead of growing. As well as this, 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 – reducing their mass.
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 16-17C.
In 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.
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.
