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 indicate nine changes to the long-term alerts since BirdTrends 2017, affecting seven different species, plus three additional changes to species listed in Table A3 that did not raise a formal alert.
- For the red-listed Song Thrush, the 49-year CBC/BBS decline has fallen below the 50% threshold and raises a lower level alert. The 25-year trend for this species shows a shallow increase and does not raise either alert.
- For Corn Bunting (also red-listed) the 25-year CBC/BBS decline has fallen below the 50% threshold and now raises a lower level alert. The 49-year trend continues to raise a high level alert.
- For a third red-listed species, Marsh Tit, the 25-year CBC/BBS decrease now matches the alert level for the 49-year trend, with both raising a high alert.
- For the amber-listed House Martin, the 49-year CBC/BBS decline for England remains above 50%, but no longer raises a formal alert due to the wide confidence intervals around the current estimate.
- For Willow Warbler, also amber-listed, the 25-year CBC/BBS decline for England has dropped below 50% and now raises a lower level alert. The 49-year decline remains above 50%.
- The decline for Greenfinch, which is currently green-listed, now surpasses 50% and raises higher level alerts for the first time for both the 25-year and 49-year periods. A substantial population decrease has occurred in just the last ten years (-59%).
- The 41-year and 25-year WBS/WBBS decreases for Little Grebe no longer raise formal alerts due to the wide confidence intervals around both estimates.
- The green-listed Tufted Duck is listed in Table A3 in the current report, as the estimated WBS/WBBS 25-year decline is above 25%. However, it does not raise a formal alert due to the wide confidence intervals around the estimate and so is not statistically significant. Oystercatcher and Sand Martin were similarly listed in BirdTrends 2017 due to non-significant 25-year declines but do not appear in Table A3 in this report as the estimated declines are now below 25%.
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 Sedge Warbler and Greenfinch. In addition, although a long-term CBC/BBS trend is not available for Swift, the 21-year BBS trend suggests a potential update to the current conservation concern level for this species (from amber to red).
Alerts from WBS/WBBS (Table A4) are unchanged except for the removal of Little Grebe from this table as a result of the change to its alert status which is described above.
The alerts for CES (Table A5) are unchanged from BirdTrends 2017.