The effects of census date on the results of intensive Common Birds Census surveys

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Abstract
An experimental investigation of how the timing of intensive short-term Common Birds Census-type surveys influences their results was conducted in 1977 and 1978 on a census plot on Aston Rowant NNR in the Chilterns in Buckinghamshire [Britian, UK]. The community spectra obtained from census work concentrated into periods between late April and early June were highly correlated with the results of 4 normal CBC censuses conducted on the plot over the whole period, most so in the case of the mid-May censuses. Individual species showed some seasonal differences in the population levels estimated for them in the intensive work, and the estimates of changes in these populations between the 2 yr were correlated rather poorly (though significantly) with the changes estimated from the extended normal CBC results. Intensive surveys are thus comparable with normal CBC results in assessing community structures and are adequate for monitoring gross population changes on census plots not otherwise censused.