The annual breeding season for birds generally starts in winter and continues through spring and summer. In Australia, winter starts in June, and for this reason, Canberra Ornithologists Group (COG) usually aligns its reporting period with the breeding season. This means that COG survey reports are for the financial year, July to June. The Garden Bird Survey follows this pattern. Thus the first year of the survey was July 1981 to June 1982 (1981-82), the second year 1982-83 and so on, and the most recent year reported, the 27th, was 2007-08.
There are two types of graph in the book. Both depict the abundance (A) of a species. This is the average number of individuals of a species recorded over all sites each week over the full Garden Bird Survey year. For the Galah, a frequently observed bird, the A value is around six. That is, on average, around six Galahs were seen at every site each week when observations were made. If a bird is less common, such as the Grey Fantail, the A value will be lower, in this case 0.33. If the bird is only an occasional visitor, such as the Glossy Black-Cockatoo, the A value will be very small indeed, with A = 0.0037.
Annual Abundance Graphs
This graph shows the abundance (A value) of a species each month of the year, at all sites, averaged over the 27-year survey period.
The graph below shows that Silvereyes were most abundant in April-July, where the average number recorded at a site each week (A value) was nearly eight, and least abundant in November-December, where the number recorded was almost three.
For birds, such as the Grey Fantail, which occurred less frequently and at fewer sites, the A value was much lower, ranging from 0.1 to 0.7. Note the graphs look similar, but for each the scale of the Y-axis (the A value) is quite different. If a bird is recorded once during a year, the A value will be 0.000016.
27-year Abundance
The second type of graph shows the abundance of birds each year over the 27-year survey period. This graph shows an average abundance A of around 6 for the period from the 1981-82 Garden Bird Survey year until 2001-02. From there there has been a steady decline to an A value of about 3 in 2007-08.
Note that a blue trend line is included to give some indication of overall rise or fall in abundance. Trend lines have been included only when the R2 value (in this case 0.7479) is over 0.7, indicating a reasonable correlation between the trend and the year to year values of A.
The graph for a Black-shouldered Kite displays a different trend, and the A value is about 160 times less than that for the Silvereye. No trend line is given, but it is still possible to discern a long-term decline.
Rank and Breeding Rank
The birds recorded in the survey have been ranked in order both for number of records (rank), and for the number of breeding records (breeding rank). The Species List on page 101 shows the rank order of birds observed, while the Breeding Records List on pages 106 does the same for breeding records. Throughout the book the Rank (R) and the Breeding Rank (BR) are included in the description of the species.