NOTE: Species accounts are no longer included within the BirdTrends report and all links to individual species in this BirdTrends report now point to the relevant species page in BirdFacts. Species trends can now be viewed in the "Trends Explorer" which allows you to view a range of temporal trends for each species.

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 53 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 (Stanbury et al. 2021).

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 (see the Species Groups section of this report). 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 2021 and abbreviated as BoCC5. The full paper (Stanbury et al. 2021) details the criteria by which each listed species qualifies for its red or amber status. 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 BoCC5 wholly or partly because of severe UK population declines revealed by annual census data, amounting to more than 50% over the 25-year period 1994–2019, the 52-year period 1967–2019, 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 53 years 1967–2020) and over 25 years (1995–2020). This table thus updates the figures that were used to produce the new BoCC5 red list, by one year.

The 23 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 (-99%). 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. 

Note that Woodcock was included in this table in previous BirdTrends reports as it declined by 74% between 1968 and 1999, but it has been dropped from this report as the species cannot be monitored by BBS and we have no up to date trend data; however, there is strong evidence from dedicated surveys of this species that the decline for Woodcock has continued since it was last included in CBC monitoring and hence that the long-term trend for this species will also be greater than 50%.

Four other species, which are also red listed under BoCC5 because of severe UK population declines, are not included in Table A1 as long-term monitoring data are not available: Swift, Wood Warbler, Pied Flycatcher and Whinchat.

Shorter monitoring histories from BBS show that Swift, Wood Warbler and Whinchat have all declined by more than 50% over 25 years (1995–2020), whilst Pied Flycatcher has declined by more than 25% but less than 50% over the same period.

For 13 other species – Lapwing, Marsh Tit, Skylark, Mistle Thrush, Nightingale, House Sparrow, Tree Sparrow, Yellow Wagtail,  Linnet, Lesser Redpoll, Corn Bunting and Yellowhammer (listed in taxonomic order) – 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, the 25-year trend is effectively stable, and Song Thrush and Tree Sparrow numbers have increased slightly.

Although 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 BoCC5 that are included in this report, of which over half (18 species) are listed because of UK population declines over the periods 1994–2019 or 1967–2019. Long-term trends are available from annual census data for 17 of these species (all except Swift); their trends are listed in Table A2 in descending order of longest-term percentage change (normally over the 53 years 1967–2020). A 25-year change (1995–2020) is also shown.

Five amber-listed species raise high alerts, having shown significant declines of greater than 50% over the long-term, and so potentially are red-list candidates. However, four of these species (Redshank, Whitethroat, Willow Warbler and Meadow Pipit) also raised high alerts in the 2020 BirdTrends report and hence would have already been considered as potential red-list species as part of the BoCC5 review. In the case of Whitethroat, the BirdTrends trend includes the severe population crash between 1967 and 1969 but the period considered by BoCC starts in 1969. For the other three species, the headline trends presented in BirdTrends are not necessarily representative of the UK as a whole and other information available to the reviewers suggested that the UK-wide decline did not meet red-list criteria. The early years of the long-term trends for both Willow Warbler and Meadow Pipit are based largely on English CBC plots and hence the long-term trend presented here is for England only and may not reflect trends for the UK as a whole, particularly for Willow Warbler which is increasing in Scotland. The long-term trends for Redshank are based on WBS/WBBS data are not necessarily representative of trends away from linear waterways.

One amber-listed species raising high alerts in this report did not raise a high alert in BirdTrends 2020, and hence is a potential new candidate for red-listing:

• Common Sandpiper has been in decline in lowland Britain since the mid-1980s, and the long-term decline measured by WBS/WBBS over 45 years now exceeds 50%.

Eight other species raise only the lower level of alert, with all eight already being amber-listed for population decline. Six of the eight species raise an alert only over the long-term period: Populations of Song Thrush and Dunnock have been recovering and show increasing trends over the shorter, 25-year period. 

Grey Wagtail and Bullfinch are effectively stable, but populations of Dipper and Sedge Warbler have both declined over 25-years but do not raise an alert as the declines are less than 25%. Two species, Sparrowhawk and Tawny Owl, raise an alert over the 25-year period but does not do so over the longer period. For both species, the recent declines follow earlier increases.

Though amber listed for population decline, Shelduck, Moorhen, Kestrel and Reed Bunting 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 in BirdFacts are BBS trends from 1994). 

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 BoCC5 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 because its UK breeding range has contracted sharply, especially in lowland England, and not for UK population decline. BBS data do not show any decline at the UK scale over the longest period covered by this survey (25 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.  WBS/WBBS also indicates a possible strong decline along linear waterways for Little Grebe over both the 45-year and 25-year timescales, although only the longer-term decline raises 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. 

The green-listed Swallow raises a new alert in this report having declines by more than 25% over both the 25-year and 53-year periods. The decline of this species is concerning as it has occurred particularly rapidly over just ten years and follows increases from the 1990s to around 2010.

A similar rapid short-term decline has also occurred for Chaffinch, which raises a lower level alert over the 25-year period. Formal lower-level alerts are also raised over the 25-year period for Oystercatcher and Garden Warbler. Oystercatcher is already amber-listed for other reasons, whereas Garden Warbler shows large short-term fluctuations in abundance and hence the trend is unclear. 

Potential declines of >25% have also occurred for Tufted Duck over a 25-year period and for Garden Warbler over a 53-year period, but these estimates have wide confidence intervals and are not statistically significant, so do not formally raise an alert. 

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 (which are not well represented in the main survey).

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 seven amber-listed species (Greylag Goose, Oystercatcher, Common Sandpiper, Redshank, Snipe, Dipper and Grey Wagtail) and five green-listed species Tufted Duck, Goosander, Little Grebe, Kingfisher and Sand Martin), along with Canada Goose, which, as a non-native species in the UK, is excluded from the BoCC5 listings.

For six of the WBS/WBBS headline species that are in decline (Oystercatcher, Common Sandpiper, Redshank, Snipe, Dipper and Grey Wagtail), latest trends appear also in Tables A1, A2 or A3, as appropriate. One other species (Tufted Duck) appears 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 these sensitive habitats.

Table A4 lists all statistically significant declines of greater than 25% recorded from the full period of waterway monitoring (normally 45 years, 1975–2020).

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 53-year trends reported from CBC/BBS, but the declines on waterways have been more severe.

The CBC/BBS trend for Reed Bunting 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 34 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 and Greenfinch are red listed on the strength of their long-term CBC/BBS declines (Table A1). Willow Warbler, Sedge Warbler and Reed Bunting are similarly amber listed (Table A2). 

CES trends for Reed Warbler, Whitethroat, Reed Bunting and especially Lesser Whitethroat are considerably more negative than those from census data over similar periods, which may indicate habitat-specific differences in population status.

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 only triggered a BBS alert for the first time in this report; hence the decline did not meet amber-list criteria at the time of the BoCC5 review and the species remains green-listed.

The CES alert raised for Garden Warbler over 25-years is also consistent with the BBS alert raised over the same period, although the BBS data also show fluctuations in abundance for this species.

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