Oxford Great Tits Show How Extreme Weather Affects Chicks

A fresh analysis of Oxford’s well-known great tit population has found that extreme weather does not affect chicks equally. Its timing is crucial.

Cold spells immediately after hatching are particularly severe. Intense rain is more harmful later on, when chicks are larger and require more food.

When heat and heavy rainfall occur at the same time, conditions can deteriorate rapidly, leaving nestlings substantially lighter at fledging, particularly in broods laid later in the season.

Led by the University of Oxford, the study uses an exceptional dataset: six decades of records for more than 80,000 individual wild great tits in Wytham Woods, matched with daily historical weather information.

Researchers examined the coldest, wettest and hottest days of every breeding season, recording how many occurred within defined developmental “windows” for chicks.

They then assessed how these weather extremes influenced body mass at fledging, an important measure because fledging mass strongly predicts a young bird’s chances of survival.

Cold first, rain afterwards

The researchers found that severe cold in the first week after hatching is especially damaging. This is understandable, as newly hatched chicks have poor control over their body temperature and have not yet developed feathers.

In cold conditions, they must use energy to remain warm rather than converting it into growth.

Heavy rainfall had its greatest impact at later developmental stages, once chicks were older. Cold and rain each lowered fledging mass by up to roughly 3%. Although that may seem modest, small disadvantages early in life can bring meaningful survival costs in the wild.

The most striking decline occurred when extreme heat overlapped with heavy rain. Under these conditions, fledging mass decreased by up to 27%, most notably in broods laid later in the season. This is far from a minor effect: it could alter the prospects of an entire brood.

Great tits are adapting, but the strategy carries risks

Great tits at Wytham have already altered their breeding schedules. During warmer springs, they have begun breeding earlier to match the seasonal peak in caterpillars, their principal food source for nestlings.

Devi Satarkar, the University of Oxford’s lead researcher, says this shift is broadly beneficial, but has a drawback: breeding earlier can expose chicks to cold spells that still occur at the start of the season.

“In the Wytham population, great tits have adjusted to warmer springs by breeding earlier to track peak abundance of their main prey, caterpillars,” she said.

“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.”

In other words, breeding earlier is not simply better: it prevents some difficulties while introducing others.

Why cold and rain are so damaging for chicks

Several processes are involved. Firstly, hatchlings exposed to cold must expend energy on keeping warm, leaving less available for growth.

Secondly, cold and wet conditions can reduce how efficiently parents forage. During poor weather, adults may fly and hunt less, returning to the nest with fewer feeds.

Rain creates an additional issue, as it can physically dislodge caterpillars from vegetation. If caterpillars fall or are more difficult to locate, the food supply declines precisely when chicks need a continuous intake of calories.

Chicks also have very high energy needs. They must rapidly develop muscle and feathers before leaving the nest.

Hot days benefited Oxford great tit chicks, for now

Among the more unexpected findings was that heat extremes increased fledging mass during the nestling period.

Higher temperatures might be expected to cause heat stress. Yet in this system, the “extreme heat” appears mild enough to provide benefits rather than cause harm.

“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,” Satarkar said.

“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.”

What qualifies as a “heat extreme” in Oxfordshire may therefore remain comfortably within the birds’ thermal limits, while also helping parents find prey.

Early broods fare better than later broods

Position within the breeding season was highly important. Broods produced early in the season often benefit from warm periods, when caterpillars are plentiful and temperatures remain tolerable.

Later broods, however, do not appear to receive the same advantage, even when “hot days” reach comparable absolute temperatures of around 16–17°C.

Late fledglings were around a third lighter overall. This indicates that later broods experience a different ecological setting, probably including fewer caterpillars, different combinations of weather and potentially already overstretched parents.

Across the long term, extreme cold and rain slightly lowered the likelihood of survival to adulthood, whereas extreme heat produced small positive effects in this population.

Significantly, breeding earlier in a season seemed to cushion many of the effects associated with weather.

Implications for conservation

The central message of the study is that climate change concerns more than rising average temperatures. It also concerns the timing and combination of extreme events, and whether they coincide with vulnerable stages of life.

The researchers say that forecasting resilience will require closer attention to microclimates, habitat differences and weather at a fine scale.

This research could inform practical conservation action, including decisions over nest-box placement and woodland management intended to reduce exposure during crucial stages of chick development.

The team intends to continue monitoring the Wytham birds to establish how these patterns change as warming continues, particularly whether heat extremes that are beneficial today become harmful as temperatures rise further.

The research appears in the journal Global Change Biology.