During mapping of breeding birds in 2300 ha of woodland, special attention was paid to Common Buzzards. In March-July, the study area was mapped six times (on average 9 min/ha spent, combined for the full period), Territories were defined according to observations of (1) calling and territorial conflicts, (2) contact calling below tree level, (3) alarm calls, indicative of an occupied nest, and (4) occupied nests. A distinction was made whether or not observed pairs had laid eggs. Observations in March turned out to be important in delineating territories, registration efficiency dropping considerably after mid-April, especially for pairs that were incubating (Table 1). Surprisingly, egg-laying pairs were more difficult to detect than non-laying pairs. Apparently, pairs with eggs or nestlings are less vocal than non-laying pairs, unless disturbed near the nest (in which case alarm calls are easy to hear). Whatever method is used during the breeding season to map territorial Common Buzzards, systematic searching for nests, in combination with notetaking of diagnostic plumage characteristics (necessary to be able to differentiate between closenesting pairs) is absolutely obligatory if it is the intention to obtain true population figures . Mapping in its typical sense, i.e. by using fixed routes and concentrating on behaviour indicative of territory-holders (and not searching for nests), is useless in Common Buzzards, because of its rather wide-ranging behaviour, large local differences in density, low profile during much of the breeding season and the presence of non-territorial surplus-birds.