The simplest approach: providing misleading results?
When we examine the time series that results from a simple average of bird counts (Figure 1), it would appear as though the mean number of tūī per garden has increased from 2007 to 2013. But is this really the case?
At its simplest, the average number of a particular bird species per garden can be calculated as the total number of birds of that species counted across all gardens divided by the total number of gardens surveyed. This is calculated separately each year and results plotted as a time series to give an indication as to whether the population is changing over time.
In 2010, for example, 4732 tūī were counted in 4124 garden bird surveys, resulting in 1.14 tūī per garden. The following year 3826 tūī were counted in 3092 gardens, resulting in 1.23 tūī per garden, etc.
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