In Gosney 2018a I listed a number of drawbacks of a traditional 'tetrad atlas survey' (ie. one which lists the species in every tetrad and shows the distribution of each species at that time). I argued instead that the use of 1-hour bird counts in Timed Tetrad Visits (TTVs) would give details of numbers, not just distribution, controls for the amount of effort involved and can be completed and published more quickly even if fewer observers are involved. However, this method has its drawbacks too: thenumber counted in any hour could vary according to the date, the time, the weather, the route and, most especially, the skills of the observer (see Gosney, 2020) and, in any hour, even the best observer is likely to miss more than 40% of theSeparate evening visits are needed to monitor nocturnal birds (see Gosney, 2018b, 2018c) and observer bias can be overcome by using only the most competent birders or using the same observers in successive surveys.
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