The anti-predatory behaviour displayed by female Great Tit (Parus major) and Blue Tit (Cyanistes caereleus) in the nestbox was studied to observe possible differences in response between the species, and also between individuals of the same species, but with a different life history. In the study area the females could have a background as immigrants, born in the area and raised by their own parents, artificially introduced to foster parents of the other species as part of a cross-breeding experiment that has been taking place in the area for years, or they could be the offspring of cross-bred individuals.
Possible changes in the behaviour over time, in the different stages of nesting (incubation of eggs, young hatchlings, nestlings and fledglings) was also observed, to see if an individual would become more or less aggressive in the later stages.
The field tests were conducted by imitating a predator in different ways, in an attempt to provoke the female into giving an aggressive response, while she was in the nestbox, both during the incubation period and in later stages, until the fledglings left the nest.
While no general difference was observed between the two species in their frequency of aggressive behaviour, there were some observations that could suggest a difference in the preferred type of aggressive response, and there also seemed to be some differences in how the two species adjusted their behaviour during the different stages of nesting.