A standardised system for setting 'alerts' in this report has been agreed between the providers and users of population monitoring information in the UK. Alerts are raised by population declines of 25–50% and of >50% over short, medium and longer terms (five years, ten years and 25+ years respectively) and noted in the 'Alert' column in the population change and demography tables. These help to highlight the scale and timing of declines, and act as an aid to interpreting the trend graphs presented.

These alerts are important for conservation practitioners who need to set priorities for conservation action, but we hope that they will also interest readers of the report more generally. Similar Alerts for wetland birds are provided by the Wetland Bird Survey (Woodward et al. 2019).

Our main emphasis in this section is on long-term declines measured over the longest period available (usually 50 years) and over 25 years, which is one of the periods used to determine 'Birds of Conservation Concern' red and amber listing for the UK (Eaton et al. 2015).

Alerts triggered over the short term should be considered as early warnings, indicating that conservation issues may be developing for the species concerned. Some short-term declines might stem, however, from normal fluctuations in abundance, from which the population is able to recover without assistance. The steep decline of a suite of species of similar ecology should be considered as a stronger indication that potential problems may be developing. Details of the methodology used to raise alerts are given in the Methods section.

Where this section discusses red-listed or amber-listed species, it uses the current version of these lists, introduced in December 2015 and abbreviated as BoCC4. The full paper (Eaton et al. 2015) details the criteria by which each listed species qualifies for its red or amber status and these criteria are also summarised on our species pages under 'Conservation listings' (see Key to species texts). Our tables here of red and amber species include only those that met the criteria (red or amber, respectively) for UK breeding population decline.

Long-term trends of 'Birds of Conservation Concern' red-listed species

The species considered in this section are red listed under BoCC4 wholly or partly because of severe UK population declines revealed by annual census data, amounting to more than 50% over the 25-year period 1987–2012, the 45-year period 1967–2012, or both. The latest long-term population changes and alerts for these severely declining species are shown in Table A1, over the maximum period available (usually the 50 years 1967–2017) and over 25 years (1992–2017). This table thus updates the figures that were used to produce the new BoCC4 red list, by five years.

The 24 species in Table A1 are listed in descending order of their longest-term percentage change. Turtle Dove remains the species with the strongest long-term UK decline (-98%). Tree Sparrow, which headed this table recently, has shown significant increases in numbers since 1995 and is now in second place, albeit still with a decline of 96% since 1967. The figures for Lesser Spotted Woodpecker are likely to be a very large underestimate of the current population change, because the species had by 1999 become too rare for further annual monitoring. Were recent data available, this species might easily surpass Turtle Dove and Tree Sparrow in the strength of its decline. Similarly, there is strong evidence that the decline for Woodcock has continued since it was last included in CBC/BBS monitoring.

Two other species, which are also red listed under BoCC4 because of severe UK population declines, are not included in Table A1 as long-term monitoring data are not available: Wood Warbler and Whinchat. Shorter monitoring histories from BBS show that both species have declined by more than 50% over 22 years (1995–2017).

For Song Thrush and Grey Wagtail, the populations have increased over the last five years, so the long-term decline is now less than 50%, prompting a lower level alert; and the 25-year decline is now less than 25% for both species so no longer triggers an alert. These species were on the red list under BoCC4. Based on current figures they could potentially be changed to amber when the list is next reviewed. 

For ten other species – Yellow Wagtail,  Tree Sparrow, Corn Bunting, House Sparrow, Linnet, Skylark, Yellowhammer, Mistle Thrush and Lapwing – the 25-year change is now less than 50%, indicating that, while these species meet red-list criteria for long-term change, their rate of decline in more recent years has been slower than for most other red-listed birds, although their populations are still at a much lower level than in the 1960s. For Grey Wagtail and Tree Sparrow, the 25-year trend is effectively stable, and Song Thrush numbers have increased slightly. Though Curlew is red listed for its UK breeding population decline, its long-term CBC/BBS trends do not currently meet the >50% criterion; the key information for red listing comes from other surveys.

Long-term trends of declining amber-listed species

There are 25 amber-listed species under BoCC4 that are included in this report, of which about half (13 species) are listed because of UK population declines over the periods 1987–2012 or 1967–2012. Long-term trends are available from annual census data for 12 of these species (all except Swift); their trends are listed in Table A2 in descending order of longest-term percentage change (normally over the 50 years 1967–2017). A 25-year change (1992–2017) is also shown.

Three amber-listed species raise high alerts, having shown significant declines of greater than 50%, and so potentially are red-list candidates:

• Redshank has declined steeply in lowland Britain, according to waterways surveys, raising high alerts; a major decline is also documented for its breeding sites on saltmarsh, and BBS data show that declines have occurred recently across a wide range of habitats. BBS declines do not yet meet the red-list criterion, however.
• The English House Martin population meets the red-list criterion for long-term population decline measured by CBC/BBS trends (over 50-years), although BBS shows that numbers in Scotland and Northern Ireland have increased over 22 years (the longest trend available for this species in those countries), and consequently there has been little change in the UK as a whole since 1995.
• English Willow Warblers also meet the red-list criterion for long-term population decline (over 50-years), but there has been little change in Wales and the overall change in Scotland and Northern Ireland since 1995 has been upward.

Although it is not included in Table A2 as no long-term trend is available, the shorter length trend from BBS (1995–2017) shows that a significant decline of greater than 50% has also occurred for Swift over 22 years.

​Five other species raise only the lower level of alert. Common Sandpiper and Meadow Pipit meet the 25% criterion (equivalent to amber listing) in both the 25-year and the 50-year periods. Populations of Bullfinch and Dunnock have been recovering and both show increasing trends over the shorter, 25-year period. Tawny Owl raises an alert over the 25-year period but does not do so over the longer 50-year period. Though amber listed for population decline, Dipper, Reed Bunting, Kestrel and Shelduck do not formally raise alerts on the present data (note that in the case of Shelduck long-term CBC/BBS trends cannot be produced and the data presented here are for the 31-year period to 1999). 

Long-term declines of species that are not currently red or amber listed (for declines)

This section of the report draws attention to declines which currently surpass red or amber criteria but which were not recognised in the BoCC4 listings (Table A3). These species may be candidates for conservation listing (for declines) at the next review.

The WBS/WBBS trend for Snipe is based now on a very small sample of plots, the species having deserted so many of its former riverside haunts. It is currently amber-listed solely because its UK breeding range has contracted sharply, especially in lowland England, and not for UK population decline. BBS data indeed do not show any decline at the UK scale over the longest period covered by this survey (22 years).

Little Owl meets red-list criteria for population decline but, as a species introduced to the UK, is not eligible for any conservation listing.  Whitethroat also raises a high alert over the long term, but the species is currently in recovery from its sudden losses in the late 1960s and therefore does not warrant a conservation listing. WBS/WBBS also indicates a possible strong decline for Little Grebe over both the 42-year and 25-year timescales, with the 42-year trend raising a formal alert in this report. However, small waterbodies are not well-covered by WBBS and relative stability on BBS squares casts doubt upon the true nature of this species' population trend.

Greenfinch, Garden Warbler and Sedge Warbler are currently green listed, yet have all decreased by more than 25% over 50-years, raising alerts. In the case of Greenfinch, this decline is over 50% and so raises a higher alert, which also applies to the 25-year period, and is the result of a substantial population decline since 2006. A recent assessment of UK species which applied IUCN criteria and categories to UK populations rated Greenfinch as 'Endangered' (Stanbury et al. 2017). Another green listed species, Sparrowhawk, raises an alert having declined by more than 25% over 25-years.  

Potential declines of >25% have also occurred for Tufted Duck over the 25-year period, but this estimate has wide confidence intervals and is not statistically significant, so does not formally raise an alert. The apparent decline of Red-legged Partridge is also not statistically significant, and is of no conservation concern because the species is not native to the UK.

Declines along linear waterways

The Waterways Bird Survey and Waterways Breeding Bird Survey supplement the results from CBC and BBS, which include all habitat types, by measuring trends in bird populations alongside rivers and canals. Joint WBS/WBBS trends allow trend assessments to be continuous since 1974 for up to 25 species that were covered by WBS. WBBS, ongoing since 1998, includes all bird species but trends are presented here only for waterway-specialist species, for which joint WBS/WBBS trends are available.

For 13 species that are abundant in waterway habitats, WBS/WBBS provides the headline population trend for this report, generally because sample sizes exceed those from CBC/BBS. These species include one that is red-listed (Grey Wagtail), seven amber-listed species (Greylag Goose, Oystercatcher, Common Sandpiper, Redshank, Snipe, Kingfisher and Dipper) and four green-listed species Tufted Duck, Goosander, Little Grebe and Sand Martin), along with Canada Goose, which, as a non-native species in the UK, is excluded from the BoCC4 listings.

For four of the WBS/WBBS headline species that are in decline (Common Sandpiper, Redshank, Snipe and Grey Wagtail), latest trends appear also in Tables A1, A2 or A3, as appropriate. Dipper also appears in Table A2 as it is amber-listed as a result of declines, but does not currently raise an alert. Two other species (Oystercatcher and Tufted Duck) appear in Table A3 as a result of potential declines (of >25%, but not statistically significant). Even where WBS/WBBS is not the headline trend for a species, however, the waterways data provide valuable supplementary information from this sensitive habitat.

Table A4 lists all statistically significant declines of greater than 25% recorded from the full period of waterway monitoring (nominally 42 years, 1975–2017).

Six species are included here for which the WBS/WBBS trend is not the headline one and so is not listed in Tables A1–A3. These species are discussed briefly below. The trends for Yellow Wagtail and  Sedge Warbler are consistent in direction with the 50-year trends reported from CBC/BBS, but the declines on waterways have been more severe. The CBC/BBS trend for Reed Bunting is not statistically significant, but shows a substantial increase in the first eight years until the mid-1970s followed by a substantial decline in the late 1970s and early 1980s, and therefore would be consistent with WBS/WBBS if both trends had started in 1975. The Pied Wagtail declines along waterways are particularly intriguing because they contrast markedly with the fluctuating but generally upward trend, in more terrestrial habitats, as measured by CBC/BBS.

In the early 1980s, population increases for Lapwing reported by WBS/WBBS contrasted sharply with decline on CBC/BBS sites but long-term trends from both schemes show there has been a steep decline. It is possible that the initial WBS/WBBS increases may have been caused by redistribution of breeding birds into wetland areas during the early stages of the decline.  Moorhen numbers have dipped sharply by all measures over the last ten years, perhaps through extra mortality in cold winters, and its long-term WBS/WBBS change has tipped over the alert threshold.

Alerts raised by WBS/WBBS, and long-term increases detected by that index, are tabulated in WBS/WBBS alerts and population increases. A full set of this year's WBS/WBBS trends can be obtained from the Table generator.

Declines on CES plots

The Constant Effort Sites Scheme provides trends from standardised ringing in scrub and wetland habitats. It is possibly our best scheme for monitoring some bird populations inhabiting reed beds, but its main objective is to collect integrated data on relative abundance, productivity and survival for a suite of species. The longest trends currently available from the CES cover a period of 33 years (Table A5).

Most of the species that are declining on CES sites show broadly similar trends to those from CBC/BBS or WBS/WBBS data. Willow Tit is red listed on the strength of its long-term CBC/BBS declines (Table A1). Willow Warbler and Reed Bunting are similarly amber listed (Table A2). Greenfinch and Sedge Warbler are currently green listed but the long-term population trends now show a decline of >50% and >25% respectively (Table A3).

For reasons unknown, CES trends for Whitethroat, Reed Bunting and especially Lesser Whitethroat are considerably more negative than those from census data over similar periods.

Chaffinch also raises a CES alert following several years of population decline. Recent BBS data also show a sharp decline but as this followed longer-term increases it has not yet triggered any BBS alerts.

A full set of alerts raised by CES and long-term increases are tabulated in CES alerts and population increases.