The 121 species in this report can be accessed in any order, via the alphabetic and taxonomic 'Species links'. The taxonomic sequence is that maintained by the British Ornithologists' Union and updated in in its current British List.

The vernacular and scientific names we use are also drawn from that list. Given this report's limited geographical scope, we use British rather than the international English names. Depending on the availability of data, the following will be found beneath each species heading:

1. Conservation listings

Global, European and UK conservation categories are given, in that order.

Global listings

BirdLife International is responsible for maintaining the global red list for birds that is part of the cross-taxa listings being compiled by IUCN (International Union for Conservation of Nature). On the BirdLife International web site, there is a page of information for every species in which justification for its conservation listing is given (BirdLife International 2015a). We show the global conservation category for each species, with a link to its BirdLife species page.

The IUCN categories relevant to this report are:

• VULNERABLE (VU) - A species is Vulnerable when the best available evidence indicates that it meets any of the criteria A to E for Vulnerable (see IUCN Red List Criteria), and it is therefore considered to be facing a high risk of extinction in the wild.
• NEAR THREATENED (NT) - A species is Near Threatened when it has been evaluated against the criteria but does not qualify for Critically Endangered, Endangered or Vulnerable now, but is close to qualifying for or is likely to qualify for a threatened category in the near future.
• LEAST CONCERN (LC) - A species is Least Concern when it has been evaluated against the criteria and does not qualify for Critically Endangered, Endangered, Vulnerable or Near Threatened. Widespread and abundant species are included in this category.

European listings

Conservation listings for Europe that use the same categories as the global assessment have been recently provided by BirdLife International for the first time (BirdLife International 2015b). A broad geographical definition is used for Europe as well as a political one (EU27) that covers the very much smaller area represented by the countries of the European Union. We show the whole-European red list category, with a link to the relevant species page on the BirdLife International web site, along with the EU27 listing if it is different.

These listings supersede the 'species of European concern' (SPEC) categories formerly used (BirdLife International 2004).

UK conservation listing

The UK conservation listing is taken from The Population Status of Birds in the UK (Eaton et al. 2015 (BoCC4); see PSoB pages). These assessments supersede three earlier Birds of Conservation Concern listings (Gibbons et al. 1996, Gregory et al. 2002, Eaton et al. 2009). There are three categories, as follows:

• Red – high conservation concern
• Amber– medium conservation concern
• Green– all other species (except introduced species, which are not classified)

The main reason or reasons for listing as red or amber, which are tabulated in the full paper (Eaton et al. 2015) are summarised here.

Like its predecessor, BoCC4 also classifies races, for polytypic species, where two or more races occur regularly in the UK. On occasion the listing for a race may differ from that for the species as a whole. These race-level assessments are given alongside those for species level in our species pages.

A note appears in this section if the species is one for which the Rare Breeding Birds Panel currently requires all UK breeding records to be submitted, or on which it has reported in the past.

2. Long-term trend

This summarises the headline trend in population size since 1967 from CBC/BBS,1975 from WBS/WBBS data, or 1984 from CES data. If there are no data available from these schemes, any assessment of trends covers the period since about the mid 1960s, but may also take historical data into account. Increases and declines that are described as 'shallow', 'moderate' or 'rapid' are generally statistically significant (see the population trends table). The following terms are used:

• Rapid decline: >50% population decline according to CBC/BBS, WBS/WBBS or CES
• Moderate decline: 25–50% population decline according to CBC/BBS, WBS/WBBS or CES
• Shallow decline: 10–25% population decline according to CBC/BBS, WBS/WBBS or CES
• Decline/Increase: information has been derived from sources other than CBC/BBS, WBS/WBBS or CES
• Probable/Possible increase/decline: information has been derived from sources other than CBC/BBS, WBS/WBBS or CES, and the information is uncertain – see the status summary for details
• Stable/Fluctuating, with no long-term trend: no overall change, or change <10%
• Uncertain: the information from two monitoring schemes conflicts, or the data are unrepresentative of the species' total UK population – see the status summary for details
• Unknown: no information on the UK population trend is available
• Shallow increase: 10–50% population increase according to CBC/BBS, WBS/WBBS or CES
• Moderate increase: 50–100% population increase according to CBC/BBS, WBS/WBBS or CES
• Rapid increase: >100% population increase according to CBC/BBS, WBS/WBBS or CES

3. UK population size

Estimates of population sizes of birds in Britain and in the UK, for the breeding season and for winter, are agreed periodically by the Avian Population Estimates Panel (APEP), on which BTO, GWCT, JNCC, RSPB and WWT are represented (Stone et al. 1997, Baker et al. 2006, Musgrove et al. 2013, Woodward et al. 2020). UK population estimates from APEP's fourth report (Woodward et al. 2020) are given for each of our species, with a shortened reference (APEP4) and a summary of how each estimate was derived. Any new information potentially superseding APEP4 is also presented.

4. Key facts table

This table giving a summary of key facts for migration, habitat and diet.

5. Status summary

This section provides a brief summary of the trends detailed for the species.

European trends are also described in this section for species for which they are published (PECBMS 2020a). Note that the terms used to describe the European trends are as given in the PECBMS report. These terms are sometimes the same in the PECBMS report as those used to describe UK long-term trends ('moderate decline'/'moderate increase'), but are assessed in a different way, as described on the PECBMS website, and so do not have the same meaning as the equivalent UK long-term trend categories used in this report, listed above. Therefore, to avoid potential confusion, the European trends within the species accounts in this report is usually described simply as 'decline' or 'increase'.

6. Population trend graphs

The first, headline graph shows the most representative long-term trend in abundance for the species, and is followed under the 'Population changes in detail' header by further graphs from other schemes, including BBS graphs for separate UK countries, as available. Generally for these graphs there are annual estimates (dots), with a smoothed trend line and its 85% confidence interval.

The Methods section provides details about how the trend data are calculated for each scheme. Index values provide a relative measure of population size on an arithmetic scale relative to an arbitrary value of 100 in one of the years of the sequence. If an index value increases from 100 to 200, the population has doubled; if it declines from 100 to 50, it has halved.

A narrow confidence interval indicates that the index series is estimated precisely, and a wider one that it is less precise, though the scale of the y-axis varies throughout and must always be taken into account. The use of 85% confidence limits allows relatively straightforward comparison of points along the modelled line: non-overlap of the 85% confidence limits is equivalent to a statistically significant difference at approximately the 5% level (Anganuzzi 1993).

CBC/BBS joint trends are produced only where there was no significant difference between CBC and BBS trends during the period of overlap between the two schemes (1994–2000). Where a joint CBC/BBS UK trend cannot be justified it is sometimes possible to present a CBC/BBS England one, provided that CBC and BBS trends were not significantly different across the 'Fuller rectangle' during the overlap period (see CBC/BBS trends, Alert system). CBC/BBS England trends use all data from England and become the headline trend if no long-term UK index is available.

7. Population trends table

This table provides details of summarised percentage changes in population size, over the maximum period from each source, and from the past 25 years, 10 years and 5 years, where these figures are available. Further columns indicate the years included, the average number of census plots included in the analysis for each year, the percentage change (an increase if presented with no sign) and the upper and lower 90% confidence limits of that change.

Note that positive and negative percentage changes are not directly equivalent: for example, a decrease of 20% would require an increase of 25% to restore the population to its former level. Where the confidence interval does not include zero, population declines are regarded as statistically significant.

The 'Alert' column indicates where a statistically significant population decline is estimated to be of greater than 50% (>50) or between 25% and 50% (>25) (see the Alerts section for further details). The 'Comment' column lists any caveats that must be considered when interpreting the estimates. The caveats include:

• Small sample: For CBC/BBS, WBS/WBBS and CES data, a mean sample size of less than 20 (but more than 10) census plots was available; for BBS data from individual countries, a mean sample of less than 40 (but more than 30) plots was available.
• Unrepresentative?: Some trends may be marked as possibly unrepresentative of the stated region, owing to the original CBC plots being self-selected by observers and thus potentially a biased sample. This judgment was made either because the species' average abundance in 10-km squares containing CBC plots was less than that in other occupied 10-km squares, as measured by 1988–91 Breeding Atlas timed counts or frequency indices (Gibbons et al. 1993) or, where these figures could not be calculated, on expert opinion.

8. Population trends by habitat

This section appears for a subset of the most abundant and widespread species. It refers to BBS data for the 16-year period 1995–2011 and has not been updated to the current year. A chart shows the species' BBS trends for each of 12 broad, mutually exclusive habitat types.

The data presented vary by species according to their sample sizes. The vertical axis shows the estimated percentage change over the period, with its 95% confidence interval, in relation to the overall change, indicated by a dashed line.

Under 'More on habitat trends', the data for each habitat trend are presented as a table and as a graph.The graphs allow the patterns of change to be compared between habitat categories over time. There is more information on these trends here on the BBS pages.

9. Demography graphs

Graphs from Constant Effort Sites, Nest Record Scheme or Ringing Scheme data illustrate trends in productivity and survival. NRS graphs show annual means, with error bars to denote ±1 standard error; and quadratic or linear regression lines with their 95% confidence interval. For CES data, the smoothed trends are plotted with their 85% confidence limits (see CES section for details). CES survival graphs and ringing recoveries survival graphs show annual estimates with their 95% confidence interval, but trends for these data have not been assessed.

10. Demography table

This provides details of changes in demographic variables since 1968 (or a more recent year, depending on the availability of data). It lists the period of years concerned, the mean annual sample, the type of trend ('curvilinear' is for a significant quadratic trend, 'linear' is for a significant linear trend, 'none' is where the linear trend is not significantly different from horizontal), the modelled values (from the appropriate regression) for the first and last years and their difference (provided only where the trend is significant), and any caveats that must be considered when interpreting the data.

Changes are presented either in the units given or as percentages, and are increases unless a minus sign is shown. The caveat 'Small sample'; is given when the mean number of nest record cards contributing annually was in the range 10–30, or when the mean annual number of CES plots recording the species was less than 20 (but more than 10). Note that where the trend is curvilinear, although inclusion in the table indicates that a significant quadratic trend has occurred, the overall change between 1968 and the current year may be small. 

11. Causes of change

Further information on the causes of the population changes we have observed is given here. A brief summary is followed by more a detailed account which discusses any relevant references from the scientific literature and any relevant demographic information from this report which contribute towards our understanding of the drivers of population change.

The length of this section is very variable: scientific research is usually focused on declining species which are of conservation concern and hence much more information tends to be available for these species. The evidence presented in this section is sparse or even lacking for some species; in most cases these are species which are increasing and are therefore of lower conservation concern.

12. Information on Conservation Actions

Further information relating to research about potential conservation actions is discussed in this section. As for the Causes of Change, the length of this section is very variable and most research has focused on actions to help declining species.

For a small number of species which may be having a negative impact on other species, this section may also include information about conservation actions which aim to reduce these negative impacts.

13. Additional information

Links to atlas maps and tables from previous atlas surveys, and the relevant pages of BirdFacts, BirdTrack and Garden BirdWatch, as available from the BTO website, are provided on the side bar of each species page.