The impact of climate change is firmly on the global agenda, and we are at a make or break point.

The window to prevent the worst impacts of change closing quickly, according to the World Meteorological Organisation.

Songbirds are the “Canary in the Coalmine” for the health of our natural environment.  They show some of the first effects when something is wrong (or right).  Delicate little bodies and strict, season-linked breeding activities mean they are the visible sign that we need to make changes for the better.

Much has already been done to understand how changes in weather cause help or hindrance to some species. – but there is so much more we need to understand to try and fit together these puzzle pieces, alongside all the other factors affecting our wildlife, to understand the full picture.

“Although there is a substantial body of evidence for changes in the phenology of birds, particularly of the timing of migration and of nesting, the consequences of these responses for a species’ population dynamics is still an area requiring in-depth research. Overall, there is a range of intrinsic and extrinsic factors that could potentially inhibit adaptation to climate change and these are a high priority for research.” - Humphrey Crick. The impact of climate change on birds. Ibis 2004

Studies at the end of the 20th century into temporal migration, showed that birds had already found traditional summer breeding sites too warm and moved to the slightly cooler north by an average of 12 miles. This has continued and you may well have noticed this with the change in species nesting and breeding in your local area over the years.

At the same time, it was also observed that eggs were being laid earlier, raising questions around the cause– is this in response to temperature and rainfall changes or by the availability of insect-based food sources being available earlier?  The calculations made 20 years ago showed 75% of UK bird species will be starting their breeding season earlier by 2080.

Studies on specific species such as the blue tit, found early breeding activity starts in response to temperature and not availability of food. Warm springs are turning on the “breed” signs, but dry temperatures mean food abundance is low. Many young cannot get enough food and may perish. However, those who persist, have a longer summer season, improving their fitness for winter survival. So, in this species there is the possibility that natural selection allows populations to cope with global warming on a long term scale.

Dry springs followed by hot, dry summers, mean reduced insect life but also dry ground, impacting species including the thrush family, that rely on soil invertebrates for food and mud for nests.

A 2004 study on Song thrushes found the summer weight of both adults and chicks were negatively related with dry soils; adults would have to go further to find food causing increased risk of predation and reduced condition. The additional loss of favourable habitat such as damp ditches and hedgerows reduce food sources and increased chances of predation. Poorly fed youngsters then fall foul to cold days in winter and first-year survival rates are lowest during years with harsh frosty winters.

​Similar issues beset the reed bunting where winter survival rates are probably the main cause of their decline. The widespread use of herbicides has reduced availability of the small weed seeds these buntings feed on, leaving them under nourished and no longer able to unable to survive severe cold snaps and the long dry winters which are becoming a feature of the UK as climate change progresses.

Alongside long term temperature rises, we are also experiencing more extreme weather events. The small bodies of songbirds are less well adapted to cope with extremes of temperature than larger bodied birds – not only harsh winters but heatwaves can kill the fittest birds whilst baking eggs and nestlings in their nests. A 2006 study showed species with small thermal ranges (smaller bodies) showed the sharpest decreases in population growth rate in locations with the highest temperature anomalies during the 2003 summer heatwave.

What of migratory birds?

Poor weather during migration can cause an early or late arrival, but it is interesting to see that some species have shown a continuing shift to migrating earlier and earlier; e.g. the House Martin arrived progressively earlier (5.9 days) between 1981 and 2008 linked to spring air temperatures.

Studies have shown that birds that arrive early may also stay longer. A recent 2020 study on reed warblers found that breeding periods of individual females in the current century have extended by 2 weeks and produce 75% more fledglings compared to the 1980s.

The proportion of females raising second broods increased from 2.7% to 23.6% between the first and the second study period while the share of females that did not produce any young annually decreased from 48.1% to 15.5%.

The higher offspring production in recent years was related to more successfully fledged broods and an earlier start of breeding, which secured more time to re-nest.

Adversely, Swifts also arrive early and could benefit from a longer breeding season but have been found to suffer terribly due to extreme spring weather events. This is probably due to their morphology, being less able than other species to bounce back.  A 2006 study found that inclement weather during breeding season also caused a marked reduction in breeding success in the following year. Annual extreme weather events have a knock on effect for some delicate species.

It is not only the weather during summer breeding which can cause problems. Autumn and winter are becoming drier and colder in many over-wintering sites.  This negatively effects the abundance of food and suitable habitat available.  A 2017 study found reed warbler adult survival was negatively impacted by these conditions and a 2016 study found Meadow pipits and Chiffchaffs were being squeezed into smaller and smaller winter sites and may have to adapt and evolve and travel further to find suitable habitat in new locations.

Interestingly, a common theme in the studies points to winter survival of adults as one of the main indicators of species population changes.  A 2007 study on 10 UK resident species concluded “strong effects of weather on the survival rates of national bird populations” while a 2021 study found that wintering bird communities are tracking climate change faster than breeding communities – birds are not just biodiversity indicators, but also climate indicators.

A positive story

While most of the studies describe a negative impact there was one glimmer of hope from a 2014 study on long tailed tits.  We have seen marked increases in these sweet little birds with populations doubling in the last 40 years. Research found that despite wet springs reducing breeding survival, winter survival rates were higher and combinations of dry springs and autumns were actually favourable. “Furthermore, survival rates in this species are predicted to further increase under a wide range of future climate scenarios.”

This selection of studies provides a useful indicator of the impact of climate change on songbirds, but we are left with more questions than answers – as the esteemed Humphrey Crick said – “This is still an area requiring in depth research”.