Effects of weather, time of day, and survey effort on estimates of species richness in temperate woodlands

We used clearly defined measures of weather and time of day to determine their impacts on the number of bird species recorded in 5439 area-search surveys of 142 one-hectare sites in open temperate woodlands. Surveys recording the highest number of species spanned the full range of times of day and weather conditions. For our full dataset, the number of species recorded dropped by 16% at higher wind speeds (>20 km/h), 0.8%/h after sunrise (or 7.4% in the afternoon compared to morning), and 0.5% per degree temperature increase, but was unrelated to cloud cover. Resampling our full dataset, we separated impacts of effort from weather and analysed scenarios which might arise with time or budget constraints. Dawn surveys were less affected by time and weather, and recorded more species than other scenarios per unit of effort, especially at species-rich sites. We demonstrate that setting thresholds for these variables is not warranted if this reduces sample size in a study. Conversely, if researchers are restricted to a fixed number of surveys, then our data highlight that thresholds may be used to optimise results by avoiding periods when less productive conditions, particularly winds >20 km/h, prevail.