Plot Size as a Factor in Winter Bird-Population Studies

Abstract

The influence of plot size on the results of a Winter Bird-Population Study was explored by dividing a 58.3-ha (144-acre) area of apparently homogeneous mature pine habitat into nine 6.5-ha subplots. Bird density and species richness were estimated for the nine subplots. To simulate progressively larger sample areas, all possible combinations of the subplot survey results were made. Variation in estimates of the density and species richness in the 6.5-ha subplots was large and it decreased as plot size increased. Most of the variation in the distribution of birds among subplots was related to variation in the habitat. In addition to the effects of plot size on census results, the choice of an appropriate size involves consideration of factors such as grid size, sampling speed and time available for censusing. The optimal size in this open pine habitat was between 20 and 25 ha. Plots of this size can be surveyed easily in the early morning hours and would provide estimates of approximately 80% of the species seen on a plot three times as large. In the present study, plots of 6.5 ha provided estimates of only 40% (range 16% to 60%) of the species observed on 58.3 ha. Rarefaction, a statistical technique, is applied here as a method of comparing species richness among plots of different size. If one standardizes the number of individuals to the lowest number in a set of plots to be compared, rarefaction can be used to determine the number of species expected in a sample of that size.

Keywords

Affiliated Institutions

• Florida State University US

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