The BirdTrends report raises species alerts for population change to conservation bodies when the best available estimates of long-term decline are statistically significant and pass criteria set at -25% and -50%.
Species with declines close to these threshold values often change category between years. Discussion tables A1–A3, however, indicate just two changes to the long-term alerts since BirdTrends 2016, affecting two different species, plus one additional species listed in Table A3 that did not raise a formal alert.
- For Dipper the 40-year WBS/WBBS decline has fallen below the 25% threshold and is also no longer statistically significant, and therefore no longer raises an alert. This species is currently amber-listed.
- For Tawny Owl the 25-year CBC/BBS decrease now raises a lower level alert. The 48-year trend still does not raise any alerts.
- The green-listed Garden Warbler raised an alert for the first time in BirdTrends 2015, but the CBC/BBS decline dropped back below the 25% threshold in BirdTrends 2016. It reappears in Table A3 in the current report, as the decline has climbed back above 25%. However, it does not raise a formal alert due to the wide confidence intervals around the current estimate.
Amber and red listings use similar criteria and were reviewed in 2015. This report, using three further year's data not available to BoCC4, suggest potential updates to current conservation concern for House Martin, Little Grebe, Sedge Warbler and Greenfinch.
Alerts from WBS/WBBS (Table A4) are unchanged except for the change for Dipper which also occurred in Table A2 and hence is described above.
For CES (Table A5) the change for Greenfinch over a 25-year period now raises a high level alert rather than the low level alert raised in BirdTrends 2016. This species has suffered a severe decline over recent years, attributed to trichomonosis (Robinson et al. 2010b) and raises a low level alert for both the 25-year and 48-year CBC/BBS trends.