Exotic trees affect breeding success of UK birds

Published: 13 February 2013 at 11:46

Researchers find blue tits and great tits struggle in ‘man-made’ parks and gardens

Research by scientists at Anglia Ruskin University has shown that the presence of exotic, non-native trees and shrubs is having a negative effect on blue tit and great tit populations in British parks.

A collaboration between Anglia Ruskin and the Centre for Ecology and Hydrology has seen scientists compare the success of birds breeding in nest boxes in city parks to those breeding in nest boxes in traditional woodland and hedgerows.

Dr Nancy Harrison, Senior Lecturer in Life Sciences at Anglia Ruskin, led a team monitoring nest boxes in the Botanic Garden in Cambridge.  The team recorded clutch size, chick weights and fledging success, as well as the energy expended by breeding adults to establish the birds’ field metabolism.

Their research found that blue tits and great tits have more difficulty raising chicks in a man-made environment such as the Botanic Garden.  Dr Harrison’s colleague, Dr Julia Mackenzie, found that very few great tits raise chicks to fledging in the Cambridge University Botanic Garden, and these are often underweight, with an uncertain future.

Both the larger physical gaps between trees and also ‘functional’ gaps in the form of exotic trees and shrubs, which offer little reward for the birds, meant that the city dwellers had a much lower breeding success, despite working harder, than the birds living in natural woodland.

Exotic trees perhaps appear to represent a greater challenge for the birds than spaces with no trees at all.  One reason could be that birds are not making the right decisions in a mosaic of exotic trees and shrubs, spending time searching for food in the wrong places.

The average weight of an 11-day-old great tit in the Botanic Garden was 14.5g compared to 17.5g for nestlings of the same age in woodland habitats, while 11-day-old blue tits averaged 9.0g compared to 10.6g in a woodland environment.

Dr Harrison said:

“The birds in our cities live longer because we feed them, but they struggle to breed and raise fewer chicks.  As a result more of the birds breeding in Cambridge are older, mature individuals.
“Nest boxes with breeding blue tits and great tits bring people a lot of pleasure, and gardens and public green spaces are important for our well being in urban and suburban environments.
“Our research shows that even very small spaces can become hotspots for birds, but for an urban habitat to support wild birds we need to protect our mature, native trees, limit the number of exotic species that are planted and not insist on keeping gardens manicured.”