A new study has found that blue tits, great tits and other native birds have learnt to peck away the tips of the galls formed by invading oak gall wasps to eat the juicy larvae inside, which are rich in protein. This helps them survive the crucial early spring period, when other food is scarce.
The new food source could help counteract the effects of climate change, which is causing some birds to lay their eggs too early in the year. The young then hatch before their main food – caterpillars that feed on oak leaves – becomes available. ‘What is exciting is that we’ve shown that the Andricus gall wasps are a really significant food source, and not just an occasional snack,’ says Professor Graham Stone of the University of Edinburgh one of the authors of the study. The tiny alien gall wasps, just 1-2mm long, lay their eggs in the buds of the, also non-native, Turkey oak Quercus cerris, which was introduced to Britain about 300 years ago, in early summer. The larvae produce chemicals that make protective galls grow around them. The oak galls grow over winter and early spring, with each tree carrying many thousands of galls.
This pattern of predation by tits and other birds was strongest at two sites in southern England, where the invasive wasps first arrived. The researchers think that this is because the birds take time to learn to find the larvae. They already eat the larvae of native gall wasps, in galls such as the familiar brown ‘oak apples’ on native oaks. In contrast, the alien galls look quite different and are just 2-3mm long and are hidden inside the buds of the Turkey oak. Birds may also take time to learn which trees have more galls, as some seem to be much more susceptible to gall formation than others.
Turkey oak is widespread and fairly common in Jersey where, as an exotic and potentially invasive tree, it is not necessarily very popular with local conservationists. Several species of Andricus wasps have been recorded in Jersey including A. quercuscalicis the main the species discussed in the study. A review of all gall wasps in Jersey was recently (Autumn 2012) published in Cecidology 27 (2): 81-84.
Read more on the study and download the results free here