Nobody can yet fully explain the rapid decline of Nightingales in the UK but there is a mounting body of research and strong evidence for some factors. These include: deer browsing, development of nesting grounds, less coppicing, changes to Nightingale wintering grounds, and climate change.
Pesticides and other human factors may also play a part by reducing food availability, and Nightingales may have subtle habitat requirements that we have yet to discover (see Other Possible Causes).
(For a scientific BTO report summarising what is known about Nightingale habitat requirements see here)
- Deer eating them out of house and home
Deer, especially muntjac and roe deer, eating the young growth of coppice (see below) tree shoots and other plants, which leaves woodlands dominated by a few tough and unpalatable grasses. Muntjac escaped from captivity in Bedfordshire and have now spread across the whole range of Nightingales. They are small and hard to survey but Game Conservancy ‘bag’ figures show a 12% annual increase.
Although some may have come from Woburn Park, many more muntjac are believed to have escaped from Whipsnade Zoo in Bedfordshire from where they have spread across S England and the Midlands in the last two decades.
Above: Game Conservancy ‘bag’ figures for Muntjac
Importantly, they remove the very dense cover at low levels which are the favoured habitat for nesting and concealment for nightingales (and some other warblers).
Research by the BTO and the University of East Anglia in woods browsed by deer has shown that Nightingales became concentrated in the small areas where deer were fenced out. Another BTO study found that deer browsing has significantly reduced numbers of the Nightingale and the Willow Tit, another once common but now rare British bird.
- Direct Destruction of Nightingale homes
There is no doubt that ‘tidying up’ of thickets and scrub, and the outright destruction of other places where Nightingales used to nest including woods, has had a negative effect on the population and if allowed to continue, will make them even rarer.
The biggest single current threat is at Lodge Hill in Kent (now subject to a Public Inquiry) where Medway Council has backed development for 5,000 houses on land that supports 85 singing Nightingales (the biggest single Nightingale site in Britain and over 1% of the whole population). The land is owned by the MoD and the developer is Land Securities. Over 12,400 people objected along with Natural England, the National Trust, and the RSPB (see RSPB page and Maidstone RSPB Group).
The Kent Wildlife Trust and other local groups such as Medway Countryside Forum have been actively campaigning to save the site for Nightingales.
At Twyneham in West Sussex a large proposed housing development would affect many woods and Nightingale habitats –Henfield, Twyneham and Wineham Woods, – and is opposed by many local conservation groups. (Website Locals Against Mayfield Building Sprawl – http://lambs.org.uk/west-sussex-nightingales-singing-for-survival/)
Helen Crabtree a BTO local volunteer said: “The area in question has some fantastic habitat for wildlife and for birds in particular. Not only are Nightingales relatively abundant in the hedges and copses, but there are breeding Skylarks, Yellowhammers, Linnets and sometimes Lapwings. These are all farmland birds that are suffering declines in Sussex.”
If you know of other sites under threat please let me know. Most ‘minor’ sites are simply cleared without needing planning permission and don’t get recorded.
- Less Coppicing
Birds are strongly affected by the three-dimensional structure of woodland.
Coppicing is an ancient form of woodland management or ‘woodsmanship’, which was traditional from the Neolithic until the early C20th. Trees are cut to near ground level and then allowed to regrow, providing a continuous crop of wood as ‘poles’ of different sizes. This also creates a cycle of light and shade as the regrowth increases, mimicking natural changes in a forest, and encouraging a variety of woodland plants and animals. It also creates a ‘thicket’-like stage a few years after coppicing, which provides conditions much liked by Nightingales. This is one of the main reasons why Nightingales were once so common in British woods.
Above: early purple orchids growing inside an area of recently cut coppice at Foxley Wood in Norfolk. When new shoots from the coppice ‘stool’ seen in the mid-ground grow up, they will form a dense ‘thicket’ of the sort which is attractive to Nightingales for nesting.
Above: the same ‘compartment at Foxley wood is on the right (the trees bent over are hazel pegged to the ground to help create new ‘stools’, as they will take root). On the left is an area cut some years earlier, now at the dense thicket stage. Such areas at Foxley have to be fenced to keep deer out. Norfolk Wildlife Trust has been trying to attract Nightingales back to Foxley Wood since 2006. (It has not been heard there since the 1960s).
The decline of coppice management in England (mostly Ancient Woodland) has played a major role but not the only one, in the decline of the Nightingale. Rob Fuller and Martin Warren explained in a 1993 study:
In the late 1800s, coppicing went into the long decline which continued until the middle of the 20th century. Between 1900 and 1970 there was at least a tenfold reduction in the area of actively coppiced woodland in Britain. This was brought about by the collapse of its traditional markets. Demands for firewood diminished as coke and coal became alternative fuels, and this led indirectly to an increasing preoccupation with plantation forestry.
Coppicing died out first in the north and steadily contracted towards the south-east until by the 1960s active coppice was heavily concentrated in Kent and Sussex. Much of the present-day active coppice is very different from the traditional mixed coppices of the English lowlands. Underwood was once rarely grown without standards [conventional shaped trees], but now the area of simple coppice is more than double that of coppice-with-standards. In medieval coppices few standards were allowed to grow on for more than three coppice rotations. The number of standards in a wood varied enormously, with up to 40 per acre, usually of uneven age.
Nowadays, standards are often allowed to grow much larger, up to 150 years. Another change is that approximately half the area of coppice is now of sweet chestnut, much of which was probably planted in the last 200 years.
Andrew Henderson, who has worked on several Nightingale research projects says: “Sweet chestnut coppice is less suitable for nightingales for several reasons, including its rapid growth and open structure, which fails to create the dense low cover; its suppression of other species (though occasionally birch and/or bramble can make chestnut coppice suitable, though only in poor stands from a silvicultural point of view); and possibly its poor invertebrate community (though often wood ants are present)”.
Since the 1980s research has shown that Nightingales particularly favour dense woody vegetation for nesting territory. It likes dense vegetation below head height – up to 1.5m – but also bare ground to feed on. One study noted that in woodlands containing Nightingales horizontal visibility is on average 4.1 m while in woodlands without Nightingales the average is 8.6 m’ but that ‘Nightingales additionally have a requirement within territories for areas with little or no ground vegetation, an average of 61% bare ground in the centres of territories. In England, they also like their thickets to be near ‘patches of mature trees’, and ‘coppiced woodlands with coppice aged 3-9 years after cutting provide optimal structure for territories’.
Neglect of coppice, coupled with browsing near ground level by deer has undoubtedly had a negative impact on Nightingales but it is hard to explain all the losses by this factor alone, as there are now woods where great effort has gone into improving the habitat but the birds are still disappearing.
Andrew Henderson comments: ‘There still are many Nightingales in coppice (and correct management in the right areas can still boost numbers there). It’s true that there has been a major shift in Nightingale habitat use from coppice to scrub, but it is far from complete”.
- Changes to Nightingale wintering grounds
Our Nightingales fly south to winter in Africa and spend winter in the ‘humid’ zone rainforest, south of the Sahel. Most are thought to winter in the far west of West Africa in areas such as Senegal, Gambia, and Guinea. Many such parts of Africa are now being developed for intensive agriculture and forest is lost. As in Britain, birds will no longer find the food and places they need to survive. Other British summer migrants using this region including the Willow Warbler, the Garden Warbler and the Cuckoo are also in decline. More here.
The State of the UK’s Birds (2014) found that 73% of British summer migrants wintering in the ‘humid zone’ of Africa such as Sierra Leone, Gambia, Burkina Faso and Senegal are in decline.
This interacts with the next possible cause (below)
- Climate Change
Changing climate has always affected the British Nightingale population, which is right at the NW edge of the species’ range. Historical accounts report it coming-and-going, generally reaching further north and west in warmer years or decades.
With the general warming of Britain’s climate now caused by human-made pollution (global warming), many plants are flowering earlier, many southern species (especially insects) have moved north, some birds are nesting earlier, and some migrant birds have arrived earlier in spring.
At first sight, the puzzle is that Nightingales, a ‘southerly’ bird in Britain, are not spreading north and west but retreating south and east, and, becoming rarer in the south and east at the same time.
Research has shown that about a third of 65 bird species analysed in Britain were nesting earlier in 1995 than in the 1970s, with egg-laying dates advancing by an average of almost nine days, and this was due to warmer springs. Studies on the continent have shown that insect-eating birds in woods such as Pied Flycatchers, are suffering ‘reduced breeding success because, as a result of spring warming, they are now nesting later than the peak availability of their main caterpillar prey’.
So their food such as moth caterpillars, are synchronized with leaf-break on trees but the birds which winter in tropical Africa are not leaving any earlier on their long-distance migration.
The BTO also says
‘Species overwintering in the tropical Humid Zone of west and central Africa [have] declined more rapidly than migrants wintering in other regions. This might be because Humid Zone species have not advanced their return migration dates as much as species wintering further north, and are therefore not keeping pace with earlier springs in the UK. Humid Zone species affected included four on the Birds of Conservation Concern Red List : Turtle Dove, Tree Pipit, Spotted Flycatcher and Cuckoo’. While ‘generalist’ adaptable species that winter in this zone like House Martin ‘did well no matter where they wintered’ ‘habitat specialists, such as Nightingales and Garden Warblers that rely on understorey and scrub, are likely to be adversely affected by accelerating habitat changes in their wintering grounds, compounding similar effects on their breeding grounds’.
In other words, climate change may be throwing everything ‘out of synch’ for Nightingales, and at the same time, loss of habitat may be hitting them on both the wintering grounds in Africa and the breeding grounds in Britain.