In the current report, there are 28 species for which our best long-term trends show statistically significant population declines of greater than 50% over periods of 31–51 years (see Latest long-term alerts).

These are Grey Partridge, Lapwing, Redshank, Woodcock, Snipe, Turtle Dove, Cuckoo, Little Owl, Willow Tit, Marsh Tit, Skylark, House Martin, Willow Warbler, Whitethroat, Starling, Mistle Thrush, Spotted Flycatcher, Nightingale, House Sparrow, Tree Sparrow, Yellow Wagtail, Meadow Pipit, Tree Pipit, Greenfinch, Linnet, Lesser Redpoll, Yellowhammer and Corn Bunting (taxonomic order). 

Meadow Pipit has been added to this list in the current report as the 51-year decline is now greater than 50%; Little Grebe raised a formal alert in both the 2017 and 2019 reports but has been dropped from the list again this year as the trend is no longer statistically significant due to the wide confidence intervals around the estimates.

One further species shows a non-significant decline greater than 50% over a long timescale. Change for Lesser Spotted Woodpecker is non-significant over the longest period but only because data are sparse and monitoring ceased in 1999; a further strong decline has since been logged by Atlas data.

The steepest long-term populations declines we have measured are for Turtle Dove, Tree Sparrow, Nightingale, Grey Partridge, Willow Tit and Spotted Flycatcher, which have all declined by 90% or more since 1967, as, almost certainly, has Lesser Spotted Woodpecker. Turtle Dove shows the biggest decline of any species in this report (98%) and its rate of decline suggests it may soon disappear as a British breeding bird.

These 28 species that have halved in population size outweigh the 21 species found to show an equivalent increase, i.e. a doubling of population size, over similar periods. The gap between the numbers of species halving and doubling over the long-term has decreased by one species in this year's report.

Except for Whitethroat, which has shown sustained, though still limited, recovery following considerable losses in the late 1960s, all but one of these rapidly declining species already benefit from listing as either red or amber Birds of Conservation Concern (PSoB/BoCC4). The other exception is the green-listed Greenfinch, which raises a high alert after a rapid decline in the last ten years, following a period of sustained population increases during the 1980s and 1990s.

Four species listed as amber after the 2015 review (BoCC4) arguably meet red-list criteria for breeding population decline: these are Redshank, House Martin, Willow Warbler and Meadow Pipit. Snipe, which is currently amber-listed though not for population decline, also arguably now meets red-list criteria, although it should be noted that the WBS/WBBS decline reported for this species is based on a small sample.

A further seven species raise lower-level concern, as a result of statistically significant long-term declines of between 25% and 50%. These are Common Sandpiper, Sedge Warbler, Song Thrush, Dipper, Dunnock, Grey Wagtail and Bullfinch. These species are already on the amber list on account of their population declines, except for Song Thrush and Grey Wagtail which are red listed, and Sedge Warbler which for now remains on the green list. Populations Common Sandpiper have been in recent decline, whereas the more sustained recent increases by Song Thrush, Dunnock and Bullfinch have been insufficient to fully reverse earlier declines. Numbers of Sedge Warbler and Grey Wagtail have fluctuated recently.

Two further species which do not raise alerts over the long-term (51-year) period have recorded statistically significant declines of between 25% and 50% over the 25-year period and hence also raise lower-level alerts. These are Oystercatcher and Tawny Owl.

In addition, Curlew (now red listed) has declined by more than 25% (as also shown by atlas data), but raises no formal long-term alert because the confidence intervals around its change estimates are too wide.

Three species with much shorter monitoring histories have also decreased by more than half during just a 23-year period. Two of these are already red listed (Wood Warbler and Whinchat), and the third is currently amber listed (Swift). Set against these three species are eight that have more than doubled over equivalent shorter periods (see Positive changes). In addition, Pied Flycatcher and Wheatear, which also have a shorter monitoring history, declined by between 25% and 50% over a 23-year period. The former species is currently red-listed whereas the latter is green-listed and shows a fluctuating trend over this period. 

Many of the declining species are farmland and woodland specialists, and some of the alerts may therefore relate to common pressures in these habitats which are reflected in the negative trends for both habitats in the UK Biodiversity Indicators, although some farmland and woodland species may be subject to more specific issues which are detailed in the species accounts, for example the Greenfinch decline has been linked to trichomonasis disease. Four species commonly associated with urban habitats (Swift,  House Martin, Starling and House Sparrow) are also declining.