Discussion

The continued long-term decline in the status of UK bird populations

All previous BoCC assessments have highlighted a continuing decline in the status of UK bird populations, with an ever-growing Red list. Unfortunately, this trend continues with the results from BoCC5. We have now placed 70 species (28.6% of those assessed) on the Red list, and the length of the Red list has grown by three species since BoCC4, with 11 species moving on and six moving off to Amber. Two other species were not assessed for the Red, Amber and Green lists in BoCC5: Golden Oriole, a species that never had more than a toehold in the UK, has now been lost as a regular breeding species and joins the list of former breeders, while Aquatic Warbler (Red-listed in BoCC1 to BoCC4) was removed in BoCC5 owing to change in species selection criteria. Newly Red-listed species include some of our more familiar birds, such as Common Swift, House Martin and Greenfinch. Two rare breeders, Montagu’s Harrier and Purple Sandpiper, are close to being lost from the UK, with just two and one breeding pair, respectively, reported in 2019 (Eaton et al. 2021). The length of the Amber list has also grown by seven species. It is important to note that there were no changes in assessment criteria between BoCC4 and BoCC5, and any movements between lists were as a direct result of changes in species status.

BoCC4 saw the addition of 20 species added to the Red list with only three species moving off to Amber. The magnitude of the changes reported here in BoCC5 is less, but overall more species still qualified for the Red list than ever before (fig. 3). Since BoCC1 (25 years ago), and notwithstanding some minor changes to taxonomy and the details of some of our criteria, the number of former breeders has increased by four and the number of Red-listed species has nearly doubled (36 to 70), while the number of species on the Green list has decreased by almost a third, from 102 to 72. 

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Box 2

The declining status of our swifts and hirundines

Many of our aerial insectivores are in trouble. Both Common Swift and House Martin move from the Amber to Red list in BoCC5 owing to severe population declines of 58% (1995 to 2018) and 57% (1969 to 2018) respectively. The former is IUCN Endangered, the latter Near Threatened. The causes of Common Swift decline are unclear and more robust evidence is needed, but the loss of traditional nest sites is likely to a contributory factor. Conservation action has so far focused on protecting existing sites and providing artificial nest sites, through nestboxes and promoting the installation of Swift bricks within new housing developments. Similarly, House Martins prefer to use existing nests remaining from the previous year, and house owners are encouraged to leave them in place. The causes of the House Martin decline also remain unclear, but several factors on their breeding grounds and in Africa have been suggested, including climate and land-use changes (Woodward et al. 2020b; Kettel et al. 2021). Another aerial insectivore to show recent declines is the Barn Swallow, with a 31% decrease between 2008 and 2018 (Harris et al. 2020). Although it does not currently exceed a BoCC threshold over either assessment period, the recent decline led to Barn Swallow being classified as Vulnerable to extinction within the IUCN2 assessment. All three species spend the non-breeding season in the sub-Saharan humid and southern zones (Vickery et al. 2014). This is in contrast to Sand Martin Riparia riparia, which is currently Green-listed, winters farther north in the arid zone, and shows a 28% increase over the last 25 years (Harris et al. 2020).

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Criteria affecting the length of the lists

Much of the increase in the length of the Red and Amber lists arises because more species qualify by virtue of more severe declines in their breeding population. In BoCC5, 77 species showed breeding-abundance declines that exceeded at least the Amber-list threshold (54 for 25-year decline and 57 for longer-term decline), up from 29 in BoCC1 (25-year decline only). 

No species qualified for the Red or Amber list through non-breeding population declines in BoCC1 but 23 species, primarily wildfowl and waders, exceeded at least the Amber list threshold in this review. The publication of the most recent atlas (Balmer et al. 2013) saw the number of species qualifying for the breeding range criteria (BDr^1/2/BDMr^1/2) more than double from 21 in BoCC1 to 46 in BoCC4 (these data remained unchanged in BoCC5). The number of species qualifying for the Historical Decline (HD), localisation (BL/WL) and international importance (BI/WI) criteria shows a small reduction over time, with eight, 38 and 34 species qualifying in BoCC5, respectively. 

There has been a worrying trend towards more of the UK’s regularly occurring species being classed as threatened with global extinction by the IUCN (www.iucnredlist.org); with the addition of Leach’s Storm-petrel and Kittiwake Rissa tridactyla, there are now nine.

Existing and emerging themes

Previous BoCC reviews have highlighted the plight of bird communities breeding in farmland, the uplands and woodland. In BoCC5, we found no improvement in the overall status of the first two of these groupings (as defined by Gibbons et al. 1993 and Eaton et al. 2015); indeed, more species have been Red-listed. Since BoCC4, the number of Red-listed farmland species increased from 12 to 14, with the addition of Montagu’s Harrier and Greenfinch, while Common Whitethroat moved from Green to Amber. It is a similar story among species associated with uplands, where the number of Red-listed species increased from 12 to 14, with the addition of Ptarmigan (see also box 1), Purple Sandpiper and Dunlin, while Grey Wagtail showed a positive move back to Amber. Furthermore, Northern Wheatear and Red-breasted Merganser moved from Green to Amber. Note, however, that both Dunlin and Red-breasted Merganser moved lists owing to declines in non-breeding population; we do not have robust breeding trends for either species.

It is a mixed picture in woodlands. Three species – Pied Flycatcher, Song Thrush and Redwing –moved off the Red list to Amber; but we have lost Golden Oriole as a regular breeder and Eurasian Sparrowhawk moved from Green to Amber. It is also worth noting that some of the movements between lists documented in this review have been due to small variations in trend, rather than species showing dramatic changes in fortune. For example, Song Thrush and Pied Flycatcher, which moved from Red to Amber owing to trends of -49.9% and -43.4% respectively, remain (very) close to the -50% Red-list threshold. 

The plight of Afro-Palearctic migrants was first highlighted in BoCC3, with further declines noted in BoCC4 (see also box 3; fig. 4). The BoCC5 review has identified continued declines in the status of this group, with two migrant aerial insectivores joining the Red list: Common Swift and House Martin, and others moving up to the Amber list. BoCC4 highlighted declines in several of our wintering wildfowl and wader species and BoCC5gives us little cause for new optimism (see also box 4; fig. 5).

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Box 3

Afro-Palearctic long-distance migrants

The populations of many species of long-distance, European-breeding Afro-Palearctic migrants are in sharp decline. Potential causes are diverse and occur across widely separated breeding, migration and wintering sites. Degradation of breeding habitat as well the interactions between habitat degradation and climatic factors in their non-breeding areas, such as Sahel zone droughts, are believed to be key factors (Vickery et al. 2014). 

The worrying plight of our Afro-Palearctic migrants in the UK was raised in BoCC3 when Common Cuckoo, Whimbrel Numenius phaeopus, Golden Oriole, Wood Warbler Phylloscopus sibilatrix and Tree Pipit Anthus trivialis were all added to the Red list. These were joined by Common Nightingale Luscinia megarhynchos, Pied Flycatcher and Whinchat Saxicola rubetra in BoCC4. The status of these long-distance migrants continues to decline, with Common Swift and House Martin moving to the Red list, and Sedge Warbler, Common Whitethroat and Northern Wheatear going from the Green to Amber list in this review. Hewson & Noble (2009) were the first to note differing trends between those that spend the non-breeding season in the arid northern zone and the humid southern areas. Although the status of the arid-zone species continues to decline (fig. 4a), it is the species that winter farther south that are of higher conservation concern (fig. 4b). Only Pied Flycatcher shows a positive move from the Red list back to Amber, but the trend for this species remains close to the Red-list threshold. Wryneck Jynx torquilla and Golden Oriole joined the list of former breeders in BoCC4 and BoCC5 respectively.

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Box 4

Wintering waterbirds in decline

In this BoCC review, we highlight the declining status of our non-breeding waterbird populations. The UK holds internationally important assemblages of wintering wildfowl and waders, as demonstrated by 20 species meeting our Amber-list non-breeding international importance criterion. No wintering trends exceeded Red-list thresholds in BoCC1 and BoCC2 (fig. 5), but this has changed. BoCC3 and BoCC4 highlighted the plight of White-fronted Goose Anser albifrons, Greater Scaup Aythya marila, Common Pochard A. ferina and Ringed Plover Charadrius hiaticula. A further four species have now joined the Red list owing to declines in non-breeding populations: Bewick’s Swan, Common Goldeneye, Dunlin and Smew, while Red-breasted Merganser moved from Green to Amber. Pressures are wide-ranging, from threats on migration, illegal hunting, ingestion of lead ammunition to the impacts of climate change. Assessing drivers is further complicated by the fact that many wintering waterbird populations in the UK have been affected by ‘short-stopping’, whereby species have shifted their wintering grounds north-eastwards in response to increased temperatures.

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We need to wait for results of the current census to robustly assess the status of our seabird populations; however, Leach’s Storm-petrel moved from Amber to Red based on the change in its global status – now globally Vulnerable – and the dramatic declines noted by recent surveys on St Kilda (Deakin et al. in prep.). 

It is important to recognise and celebrate the few positive stories to come out of BoCC5. Following extensive conservation action, White-tailed Eagle has now moved from Red to Amber, as the species no longer qualifies for the Historical Decline criterion (Box 5). In addition, the ongoing increase in the Marsh Harrier Circus aeruginosuspopulation means that this species is now considered to have fully recovered from its historical decline, although it still qualifies for the Amber list through being a localised breeder. Other species that have increased sufficiently to move off the Red list are two rare breeders, Black Redstart and Redwing. These changes are as a direct consequence of recent population increases. 

The UK has seen continued colonisation, particularly by waterbirds, and we added five new species to this review: breeding Great White Egret, Cattle Egret, Little Bittern and Black-winged Stilt, and non-breeding Yellow-browed Warbler. While we welcome these additions to our avifauna, we should simultaneously recognise that the arrival of new species here owes much to man-induced climate change, which may have an adverse effect elsewhere in their ranges.

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Box 5

White-tailed Eagle reintroduction

White-tailed Eagle moves from the Red to Amber list in BoCC5, as the species no longer qualifies for the Historical Decline criterion and is now classed as HD recovering. Its population remains small, however, and it is considered to be IUCN Endangered. The species’ reintroduction into Scotland is a conservation success story. After the failure of two early attempts in 1959 and 1968, the release of 82 birds on Rum between 1975 and 1985, 58 birds in northwest Scotland between 1993 and 1998 and 86 birds in east Scotland from 2007 to 2012 brought a change in fortune, with successful breeding in Scotland recommencing in 1985. By 2002, a total of 25 territories were known, increasing to 52 in 2010, 100 in 2015, and an estimated 123 in 2019. The White-tailed Eagle’s range has also expanded to the extent that birds are now breeding as far north as Orkney, south to southern Argyll and eastwards into much of eastern Scotland north of the Central Belt. The population has now reached a level at which monitoring is becoming increasingly challenging, and it is estimated that about ten new territories are being established annually, though this may be an under-estimate. A reintroduction project has now started in England in the Isle of Wight.

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The updated assessment of extinction risk in Great Britain

The benefit of undertaking the internationally recognised standardised IUCN Regional Red List process, in addition to BoCC, is that we can compare the status of our bird populations with other geographical areas as well as with other taxonomic groups in Great Britain, plus it allows birds to be incorporated into multi-taxa assessments and higher-level biodiversity indicators. IUCN2 found that 46% of regularly occurring species and 43% of populations were assessed as being threatened with extinction from Great Britain. This is high compared with the equivalent figure for birds in Europe (13%, BirdLife International 2021) and most other taxonomic groups in Great Britain. The State of Nature report (Hayhow et al. 2019) showed that 15% of 8,431 species across 11 taxonomic groups in Great Britain were classified as threatened. Other groups showing relatively high proportions of threatened species are terrestrial mammals (26%, Mathews & Harrower 2020), vascular plants (22%, Cheffings & Farrell 2005) and butterflies (33%, Fox et al. 2010). 

As noted by Stanbury et al. (2017), we believe there are good reasons why the proportion of threatened populations in birds in Great Britain is high. First, the risk of extinction tends to increase as the geographical area considered becomes smaller. Secondly, there are fundamental biological differences between taxonomic groups which have relevance to the IUCN criteria. Even scarce invertebrates or plants can still have populations in the thousands of individuals, and thus do not approach the IUCN’s qualifying thresholds for small population size. Thirdly, data availability for birds far exceeds that of many other taxonomic groups thanks to the effort of volunteers, so species can be tested robustly against more criteria. We believe that, if similar data were available for other, similar-sized areas and for other taxonomic groups, levels of assessed extinction risk would be considerably higher than is estimated currently. Therefore, although the IUCN Red-listing process is scalable between regions and taxa, it is not perfect, and, on this basis, it is fair to assume there are limits to our ability to make comparisons.

There is much commonality between the results of IUCN2 and BoCC5 (table 3). Of the 108 species to have a population qualifying as threatened (CR, EN, VU) through the IUCN process, all but seven are on the BoCCRed or Amber lists (50 on the Red list, 51 on the Amber list). Since the IUCN Regional Red List process focuses on extinction risk, assessments tend to identify species with extremely low numbers, restricted ranges and/or rapid recent declines; justifiably so, since these are the species most likely to become extinct. Similar criteria exist in the BoCC process, albeit with different thresholds and over different time periods. Of the seven exceptions (IUCN threatened species on the Green list), one qualified for criterion D (very small population size), one for criterion C (small population and decline) and five by virtue of criterion A4 (a projected future decline). The last highlights an important difference between the two processes: BoCC focuses on the current and past status, while IUCN focuses on current and likely future trends (for species where reliable data indicate that a trend is likely to continue), providing an early warning signal of trouble ahead. Both Common Chaffinch and Barn Swallow qualified under criterion A4, show recent changes in fortune (Woodward et al.2020b) and may be worthy of additional conservation attention.

Unlike BoCC, the IUCN process does not consider the international significance of our populations or species showing severe declines prior to the three-generation-length assessment window; therefore, there is less commonality between IUCN2 and BoCC5 results than the other way around. Indeed, 49 Red- and Amber-listed species are assessed as of Least Concern. Examples of BoCC species whose past decline is of concern, but that are not IUCN threatened (because they are not currently declining to the extent they are threatened with extinction) include Common Cuckoo Cuculus canorus, Corn Crake Crex crex and Skylark Alauda arvensis, while some of our internationally significant (and hence BoCC-listed) seabird populations are not threatened with extinction. 

Table 3.

 Matrix showing the commonalities and differences between the BoCC5 and IUCN2 results. It excludes former breeding, Regionally Extinct and Extinct species. IUCN threat status categories: Critically Endangered (CR), Endangered (EN), Vulnerable (VU), Near Threatened (NT), Least Concern (LC), Data Deficient (DD), Not Evaluated (NE). The highest threat status has been used when two different seasons were assessed for the same species.

Data gaps

We are extremely fortunate in the UK to have a large number of dedicated volunteers contributing data towards established biodiversity-monitoring schemes, such as BBS and WeBS. Without these data, our prioritisation and status assessments would not be possible. Notwithstanding the existence of these rich datasets, gaps in our knowledge do exist. Eaton et al. (2015) documented these in detail and many remain relevant: for example, we lacked robust trends for 20 breeding species including Short-eared Owl Asio flammeus and Scottish Crossbill Loxia scotica. The publication of results from the current seabird census will make a major contribution to our understanding of the status of this group and fill a large knowledge gap. It is the case though that, owing to less species/group-specific monitoring being undertaken (because of lower investment), we are increasingly reliant on ageing information. For example, the last national breeding survey of European Nightjar Caprimulgus europaeus was in 2004, of Red-throated Gavia stellata and Black-throated Divers G. arctica in 2006, Ringed and Little Ringed Plovers Charadrius dubius in 2007, and Merlin Falco columbarius in 2008, while the most recent Winter Gull Roost Survey (WinGS) was undertaken during the winters of 2003/04 and 2005/06. We recommend that increased resources are made available to ensure timely monitoring of these and other species that may otherwise not receive the conservation attention they deserve.

Conclusion

The BoCC and IUCN processes provide a robust framework for targeting conservation actions for birds. Both allow us to track the effectiveness of our interventions for birds and the IUCN assessments offer the prospect of including birds in high-level, cross-taxa Red List-type biodiversity indicators which can help us measure overall conservation effectiveness and thus progress towards national and global targets to halt and reverse biodiversity loss.

These latest reviews, BoCC5 and IUCN2, add to the already extensive weight of evidence that many of our bird populations are in trouble. Our findings reinforce previous reviews and are supported elsewhere. For example, the UK Government’s own Biodiversity Indicators show long-term declines in farmland birds and more recent declines in wintering waterbirds (JNCC 2020). We highlight new species that should now be considered of high conservation concern, such as Leach’s Storm-petrel, Common Swift, House Martin and Greenfinch, but we must not forget the 59 species already on the Red list. With nearly 30% of UK species now Red-listed, further prioritisation may be necessary unless nature conservation action becomes bolder, takes place over a greater scale and is much better resourced. Whilst the need for conservation action is obvious if we are to address the current biodiversity crisis, it is vital that we continue to monitor our bird populations and to make regular, periodic updates to our BoCC and IUCN assessments – we recommend a continuation of the six-year interval – so that we can measure progress towards relevant biodiversity targets and refresh our priorities.

Acknowledgments

We would like to say a huge thank you to the thousands of voluntary and professional surveyors who have collected data on our avifauna, without whom such assessments would be impossible. Special thanks also to the many colleagues who have helped with access to data and provided advice during the assessment process, including Ian Burfield and Anna Staneva at BirdLife International, Teresa Frost and Dario Massimino at the BTO, Andrew Stevenson at NatureScot, Richard Facey at Natural Resources Wales (NRW), Malcolm Burgess, Fiona Burns, Richard Gregory, Ian Johnstone and Simon Wotton at the RSPB, Dave Baines and Phil Warren at the Game & Wildlife Conservation Trust (GWCT), and David Stroud. This study was funded by RSPB and Natural England as part of the ‘Action for Birds in England’ partnership.

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