| The annual breeding season for birds 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 last year reported, the 17 th, was 1997-98. |
There are two types of graph in this 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 Silvereye, a frequently observed bird, the A value is be around six. That is, on average, around six Silvereyes were seen at every site each week when observations were made. If a bird is a less common, such as the Grey Fantail, the A value will be lower, in this case 0.35. If the bird is only an occasional visitor, such as the Glossy Black Cockatoo, the A value will be very small indeed, with A = 0.00037. |
Annual Abundance Graphs
This graph shows the abundance (A value) of a species each month of the year,
at all sites, averaged over the 17-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 about eight, and least abundant in November-December, where the number recorded was about three. |
For birds, such as the Grey Fantail, which occurred less frequently 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 the year, the A value will be 0.000016. |
17-year Abundance
| The second type of graph shows the abundance of birds each year over the 17-year survey period. The number shown may be either for a calendar year (1982, 1983, and so on) or for the Garden Bird Survey Year (Year 1 = 1981-82). A calendar year is used for birds most abundant in the winter, such as the Silvereye. |
This format is used so as not to split the abundance of these birds across two years. The calendar year graph has 18 bars because 1991 and 1998 comprise six months only. |
The July-June year format is used for birds most abundant in the summer.
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