The study of foraging behaviour of 20 forest bird species was conducted during two different phenophases of a vegetation period in the West Carpathians oak-dominated natural forests. Using a standard and a modified a priori approaches, seven foraging guilds were distinguished, whereas only five significantly different guilds were clustered by a a posteriori approach. Four common guilds for all three approaches were quantitatively compared. The methods differed especially in the classification of foraging generalists. Differences in guild densities between a priori and a posteriori approaches emerged in both, the pre-foliage and the foliage periods, chiefly in the guilds of insectivores on ground surface (47.3–89.6%) and insectivores on buds/leaves and twigs (15.0–45.9%). The modified a priori method provided moderate densities of separate guilds. The modified a priori method with more detailed density calculation is recommended for the quantitative guild structure comparisons of different bird assemblages. The method takes into consideration proportional use of foraging substrates in combination with food type.