Refining the use of point counts at the scale of individual points in studies of bird-habitat relationships.
Pierre Drapeau, Alain Leduc, Raymond McNeil.
Useful sampling guidelines have recently been provided to improve the precision of bird population parameter estimates from point count data. Less attention has been given, however, to the issue of accuracy or completeness of counts at each sampling point. While it may not be critical in studies that compare average counts among study plots, completeness of counts at the level of individual points may pose several problems in studies that assess relationships between response variables (species' occurrence or abundance) and explanatory variables (vegetation structure, size and isolation of the fragment, proportion of interior habitat) on a point by point basis. Reliability of models in such studies is greatly dependent on the accuracy of counts at each point. We examined how bias of incomplete counts could be minimised through refinements of the point-count method. Six forest types of the northeastern deciduous forest in Quebec (Canada) were intensively surveyed during an entire breeding season. Census performances of the point-count method for measuring species richness at individual points were assessed by resampling the original data set for a simulated census protocol of two visits at a point. Effects of daily variation, time of season, time of day, and count duration were tested. Census performances of counts conducted the same day were significantly poorer than counts from different days. Census performances were influenced by the time of season during which counts were conducted. When pooled across habitats, census performances for counts that combined visits at the beginning and at the end of the breeding season were significantly better than counts that were concentrated in either one of these periods. The 05:00 to 08:30 morning period yielded the best results for five of the six habitats. Census performances in open field were, however, significantly better during the first sampling period (04:30), suggesting that bird censuses in open habitats could be initiated before sunrise. Longer counts (25 and 20 min) greatly improved species richness estimates at a point. For studies that focus on comparisons at the scale of individual points, longer counts are preferable to short counts (15 and 10 min). We stress the importance of the scale (study plots or individual points) at which statistical analyses of point count data are conducted in the decision-making process of point count sampling procedures.