noisy miners & GBS Breeding trends generally

["Philip Veerman" <>]

All good comments. (Coral may be paraphrasing the 18 year GBS Report that had changed a bit for the 21 year editions). There is a bit of a survey bias operating that we need to be aware of when comparing breeding data across broad periods of the GBS. This is that for the first 12 years of the GBS during which version 1 & 2 of the GBS charts were used, "officially" the only breeding recorded was two codes: the stage of eggs or young in the nest (a N on the chart) or dependent young (a F on the chart). In year 13 (version 3 and later of the GBS charts), many more breeding categories were added and have stayed. Whilst that was a good idea to do so, it means that the breeding data is not strictly uniform for both parts of the history of the GBS. As a result of the improvement, both the period during which breeding may be recorded and the likelihood of recording it have increased for years since year 13 (by unknown amount - which probably differs between species, e.g. no difference for King Parrots). It is very difficult to decide how to measure or reduce this bias. It is now just a fixture of the data, a simple consequence of the history and we can't go backward or guess what would have been recorded if the current regime existed in years 1 to 12. It should be mentioned in this context and I have been waiting for the time that it becomes necessary to do so. The history explaining how, when and why, is fully described in the GBS Report, (the section "Bird count and breeding data changes possibly due to changes in chart design" (over a page). So anyone reading it, will easily realise the issue, even if the species accounts don't devote a lot of attention to it.

 

Philip