Insectivorous birds have very diversified diet, but particular species usually show some specialisation, which leads to a varying level of dependence on special prey. Their reproductive cycles are dependent on the availability of appropriate arthropods; in the case of Blue Tits Cyanistes caeruleus and Great Tits Parus major reproduction is usually coordinated with the availability of caterpillars as the key food for nestlings. Therefore a picture of nestling diet, with some estimates of the actual frequency of caterpillars and alternative prey, is an important component of explanations of aspects of Tit life-histories. As in most cases a rough assessment of diet composition and relative proportions of prey items is satisfactory, we suggest that faecal analysis is a feasible method to get such a picture. Droppings may be collected to examine the diet of individual nestlings grouped in broods, at a particular age stage or at many stages reflecting development. The most time-consuming part of this method includes segregation and identification of prey remains in the laboratory. We draw attention to the procedures and the most diagnostically useful features of arthropod prey of Tits. Especially, we provide clues to identification of the remains of different arthropods. As an example, clypeus proved to be the most valuable structure to identify caterpillars, while chelicerae were the most diagnostically significant in Arachnids. Exemplary results on diet spectrum for the Blue Tit and Great Tit are also presented. Faecal analysis is fast and effortless at the sampling stage, with almost all effort being postponed to the stage of laboratory work.