An on-going study of Great Tits at Wytham Woods in Oxfordshire was started in 1947 and it appears that they are coping well as changes in climate have altered the availability of their key food. This study is reputedly the longest running population study of wild animals anywhere in the World. The population is estimated at 400 breeding pairs producing 2000 offspring a year.
Researchers found that Great Tits are laying eggs two weeks earlier in the spring than 50 years ago. This is to keep step with the earlier emergence of the winter moth caterpillars, which are an important food source for Great Tit chicks, but are only available for about two weeks of the year. Winter moth caterpillars can make up 90% of all creatures found in oak trees during this crucial feeding time. Each of the Great Tit's eight or nine chicks will eat about 70 caterpillars a day, so you can see how important it is for the Great Tit to coincide its breeding season with the emergence of the winter moth larvae. Scientific studies have clearly shown that the emergence of the caterpillars is temperature linked.
Providing live mealworms in your garden at this time can be a great benefit for the birds, particularly if for some reason they are slightly out of synch with the caterpillars.