There are a number of ways of presenting the summary results of the GBS of which the two most commonly used are:
- A (for Abundance): the aggregate number of birds of a given species reported each active survey site-week; and
- F (for Frequency): the proportion of sites in a given period - typically a year - in which the species was reported at least once.
The Chart below shows these two summary statistics for each of the 30 years of the GBS. (Note that each year begins on 3 July and Year 1 was 1981-82.) As usual click on the image to get a larger version.
The broad picture offered by both series is similar: a reasonably constant level of reporting until about year 19 (1999-2000) followed by a strong increase since then.
Constant SitesAn issue of concern was that this pattern was in some way influenced by changes in the composition of the Survey panel in the later years. Since the sites are self-selecting (ie anyone who wishes to participate is welcome to do so) it might be possible that something had happened to change the nature of the set of sites.
A simple way of compensating for such effects is to use a subset of sites which have participated in the survey for a large number of years. 29 sites have been in the GBS for 15 of the 30 years: this seemed like a 'reasonable' way of ensuring that changes in the results should not be unduly affected by changes in the panel. 29 sites met the criterion of 15 or more years of contribution. typically they contributed between 25% and 30% of the sites active in a year, although this was higher (above 50%) when the overall survey participation went though a low patch in the late 1990s.
I calculated both numerators and denominators for A and F (as defined above) for the subset of sites active for 15 years or more. The next two charts compare the values of these two summary statistics for all sites (as shown above) with the equivalent statistics for the >15 year sites.
Changes in Flock SizeReviewing the first chart in this post suggest that the value of A is rising faster than the value of F. This implies that flock size s increasing as well as the frequency of observing the species. Within the GBS Summary statistics, G (Group size) shows the average flock size by year.
In broad term this shows a higher proportion of very small groups (1,2 or 3 birds) in the early period, approximately similar distributions for the mid-size flocks and a lower proportion for the larger flocks (8+ birds).
I have also examined the seasonality of reports over these two periods.