Urban birds cope better in extreme weather

Published: 23 September 2013 at 15:05

Anglia Ruskin study finds woodland blue tits and great tits struggled during cold, wet 2012

Birds breeding in native British woodland are more susceptible to the effects of extreme weather conditions than those in urban environments, according to a new study published by online science journal PLOS ONE.

The research, led by scientists at Anglia Ruskin University and the Centre for Ecology and Hydrology, examined breeding patterns of blue tits and great tits in Cambridgeshire over a 10-year period.

The scientists discovered that birds breeding in urban areas are better able to cope during unusually cold and wet weather because they are less reliant on feeding their chicks a single food source.  The study compared 2012 – a year with temperatures significantly lower than average, including a particularly cold and wet spring – to the previous nine years. 

Of the three sites monitored, the birds at Brampton Wood nature reserve (a typical mixed deciduous woodland of common ash, English oak and field maple) struggled most during 2012, the wettest year in England since records began in 1910. 

The other sites surveyed were the Cambridge University Botanical Gardens in Cambridge city centre and Cow Lane nature reserve, a mixed riparian zone of willows and reed beds close to the banks of the Great Ouse.

Both the number of chicks in the brood and their individual weights were lower than normal across all sites in 2012, but the most significant decrease was seen in the traditional woodland habitat of Brampton Wood.   

Usually, blue tits and great tits lay one egg per day until their clutch is complete and then begin to incubate them.  However, the Brampton Wood birds delayed their incubation in response to the onset of cold weather, which in turn delayed chick hatching. 

This prolonged stalling of the nesting cycle was unprecedented during the 10-year period of the study at any of the sites and was likely due to the negative effect of the cold on the woodland bird’s caterpillar prey.

For great tits, the period from laying the first egg to hatching was 32 days tits at Brampton Wood, a period almost twice as long as the urban site (17 days).  Typically, a long delay in hatching leads to a smaller brood size and fewer chicks successfully leaving the nest.

Dr Nancy Harrison, Senior Lecturer in Life Sciences at Anglia Ruskin, said:

“The birds breeding in the good woodland habitat really struggled during last year’s cold, rainy spring. 
“The breeding season is controlled by a hierarchy of factors including daylight and temperature, with temperature playing a key role in caterpillar reproduction and growth.
“Variability in the lifecycle of caterpillars will inevitably impact on birds that prey upon them.  Although we believe temperature was more significant than rainfall on this occasion, heavy rain can wash caterpillars off leaves and increase their scarcity for predators.
“Blue tits and great tits in urban areas often forage for other prey and so are less reliant on one particular food source.  These ‘urban scavengers’ were better able to cope in 2012 when these caterpillars were in short supply.
“Over the 10-year period of the study, birds living in the traditional woodland habitat fared significantly better and produced larger and healthier broods than their city cousins.  However, if these extreme weather events become more commonplace due to the effects of climate change, then birds living in urban environments may have the advantage.”

The full article can be read online at http://dx.plos.org/10.1371/journal.pone.0075536