Lack of caterpillars key to survival of urban blue tits

Scots scientists researching the poor breeding record of one of the country’s most popular birds have attributed part of the problem to a shortage of caterpillars.

Blue Tit

Many animal species suffer reproduction problems in urban habitats, despite many additional breeding and feeding opportunities.

Biologists have now shown conclusively that among urban blue tits, part of the problem is linked to poor nestling diet, particularly to a scarcity of their preferred food, caterpillars.

The research adds to growing concerns that urban environments can become ecological traps for urban-dwelling species.

The increasingly rapid process of urbanisation has now placed more than 50 per cent of the human population in cities. Birds and other species can be attracted to these habitats by human food and shelter, but these benefits can be offset by major ecological deficits.

Although blue tits are widespread these songbirds often suffer in cities. In 2015, blue tit parents managed an average of less than one chick per nest in city parks in Glasgow, compared to more than five chicks per nest in the Loch Lomond National Park.

Researchers from the University of Glasgow, the National Environment Research Council’s Life Sciences Mass Spectrometry Facility and the Scottish Universities Environmental Research Centre think they have found the reasons.

Researchers counted caterpillars, other insects and spiders on the trees where blue tits forage. They found that caterpillars were greatly reduced in the city and did not show the typical seasonal peak on which chicks can thrive.

Filming the parents as they entered the nest boxes, the researchers discovered that the more caterpillars the parents delivered, the higher the fledging success of their young.

Although urban parents worked harder for each of their offspring, they found far fewer caterpillars than forest birds. Instead, brought alternative foods, including peanut granules that the chicks were unable to digest. Dietary differences were also confirmed by analyses of stable isotopes in blood and egg samples of blue tits, which give clues to the food the birds had consumed.

Dr Barbara Helm, of the University of Glasgow’s Institute of Biodiversity, Animal Health and Comparative Medicine, said: “These losses of birds’ breeding success are avoidable if we take better care of urban biodiversity.

“Not all breeding seasons are as bad as 2015, but if conditions become tough, urban birds are hit much harder than birds in natural forests. Increasing caterpillar numbers will directly benefit birds in our cities”.

The research results make the strongest case to date that urban birds are not receiving a suitable diet. However, more research is needed to confirm similar patterns for other cities in the world.

One way people might be able to help would be to encourage plants in their gardens that caterpillars like.

Dr Davide Dominoni, now at the Netherlands Institute of Ecology, added: “We should increase the birds’ provisioning of caterpillars from moths and butterflies, for example by prioritising native plant species that the caterpillars like, including oaks, and by reducing use of insecticides.”