The COG Gardenbird Survey
This section contains pages describing the Garden Bird Survey itself, and other aspects related to the survey, such as weather, species and breeding lists, references and a glossary of terms used in the book.
The Garden Bird Survey Year
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.
Weather
It is not clear how weather patterns affect the Canberra bird population. There has, as yet, been no systematic analysis of the Garden Bird Survey data to see if there are any relationships between rainfall or temperature and the bird abundance. of garden birds. However, readers might find useful some information about the variations in weather over the survey period. The rainfall and temperature information is based on data from the weather bureau at Canberra Airport.
Rainfall
Since the survey year, which coincides with the annual bird breeding cycle, runs from July to June, the annual average rainfall for each financial year is shown in the graph below. On the whole, the rainfall throughout the period showed expected variability around an annual average of about 595 mm, although the long-term average is slightly higher at 617 mm.
The average rainfall over the last ten years of the survey (98-99 to 07-08) was just over 550 mm, compared with an average of 650 mm for the previous ten years.
Eight of the 27 years of the survey were in drought, including 1981-82, 82-83, 90-91, 97-98, 02-03 (the driest year of the survey period), 03-04, 04-05, 06-07 and 07-08. There were only two particularly wet years, 1983-84 and 1988-89 (nearly 1000 mm). In 1983-84, most of the rain fell in May 1983 with late spring and summer being particularly wet, with almost 200 mm falling in January 1984.
The graph Monthly Average Rainfall shows that over the years, Canberra gets less rain in winter, and more in late spring and summer.
Temperature
An inland city, Canberra experiences a considerable range of temperature, although its altitude prevents temperatures from climbing too high in the summer. The average summer maximum temperatures are in the range 25?30 degrees Celsius, winter maximums being around 12?15 degrees, and with winter minimums hovering around 0 degrees Celsius.
To give some idea of how temperatures have varied over the 27-year survey period, three graphs are provided. The first is the annual average temperatures over the years. This shows which years were warmer or cooler on average.
2006-07 was the warmest year, with an average daily maximum temperature of 22 degrees Celsius. This was more than three degrees warmer than 1992, a comparatively cool year. The average daily maximum temperatures in January 1981 and 2007 were around 31.5 degrees Celsius where by comparison, January in 2000 was about 7 degrees cooler. The comparison of the maximum temperatures throughout 1983-84 and 2006-07 are in the graph below.
1983-84 was the coldest year, with an average daily minimum temperature of 5.9?degrees Celsius. This was two degrees cooler than 2006-07, a comparatively mild year. June 1984 was quite cold, with daily average minimum temperatures at 2.7?degrees Celsius.
