Point Counts of Birds in Bottomland Hardwood Fotests of the Mississippi Alluvial Valley: Duration, Minimum Sample Size, and Points Versus Visits
Vydavatel US Fish and Wildlife Service, Department of the Interior
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Provides guidelines concerning sampling effort to achieve appropriate level of precision regarding avian point count sampling in the MAV. To compare efficacy of point count sampling in bottomland hardwood forests, duration of point count, number of point counts, number of visits to each point during a breeding season, and minimum sample size were examined. Minimum sample sizes were computed from the variation recorded during 82 point counts from 3 selected localities containing 3 habitat types wet, mesic, and dry across 3 regions of the Mississippi Alluvial Valley northern, central, and southern. For each of these point counts, all birds seen or heard during the initial 3 minutes and during each minute thereafter up to 10 minutes within three concentric distance categories 26 m, 25 to 50 m, and 50 m were recorded. In a second study, the effect of increasing the number of points or visits was determined by comparing the results of 150 4minute point counts obtained from each of four stands on Delta Experimental Forest. Within each stand, bootstrap estimates of the mean cumulative number of species each year were obtained from all possible combinations of six points and six visits. Similar analyses of 384 counts obtained from 132 points distributed among 56 sites in west Tennessee bottomland forests were undertaken. Mean number of species recorded during 5 and 10minute counts were 10.3 and 12.9 and 11.3 and 14.7 for the lower MAV and west Tennessee, respectively. There was significant variation in numbers of birds and species between regions and localities nested within regions; neither habitat nor the interaction between region and habitat was significant. Sample size sufficient to detect actual differences of some species e.g., wood thrush Hylocichla mustelina was 500; for other species e.g., prothonotary warbler Protonotaria citrean, this same level of precision could be achieved with 10 counts. Significant differences in mean cumulative species were detected among the number of points visited and among the number of visits to a point. Although no interaction was detected between number of points and number of visits, when paired reciprocals were compared, more points invariably yielded significantly greater cumulative number of species than more visits to a point in the lower MAV. In west Tennessee, more points yielded either similar or significantly more cumulative species than more visits.