Causes of change

There is strong evidence that deer grazing is having a negative effect on Nightingale numbers. Conditions on the wintering grounds, such as changes in habitat, are also likely to have carry-over effects into the breeding season. Several studies have highlighted the benefit of habitat management for this species, involving coppicing and control of deer numbers to promote the heterogeneous vegetation structure that Nightingales need.

Further information on causes of change

Several hypotheses have been put forward to explain Nightingale decline and are the subject of ongoing BTO research: these include reduction in coppicing, maturing of scrubland and conifer plantation, an increase in deer and their browsing pressure, higher predation pressure, reduced food quality, pressures on migration and deterioration of conditions on the wintering grounds (Fuller et al. 1999, 2005). Wintering habitat of British birds is being investigated by fitting geolocators to Nightingales (Holt et al. 2012b). Habitat deterioration on the wintering grounds may result in greater winter mortality or in birds arriving on the breeding grounds in poor condition (Ockendon et al. 2012). The potential roles of predation and reduced food quality have been little studied (Holt et al. 2012b). There is strong evidence, however, that increased browsing by deer has had a negative effect on Nightingale numbers.

Nesting Nightingales typically require closed-canopy scrub or young woodland, with bare ground under the canopy for feeding, but also area of low thick vegetation, generally associated with secondary succession and early regeneration after coppicing (Hewson et al. 2005, Wilson et al. 2005b). Canopy height in territories occupied by Nightingales is usually less than four metres in height (Wilson et al. 2005b). A study based in Cambridgeshire found that territory distribution peaked on areas where scrub height varied between three and five metres (Holt et al. 2012c). Nests are built on or close to the ground, in a thick field layer that will provide cover for nests and a refuge for newly fledged young. Scrub structure seems more important than its species composition, and the ideal habitat is probably a dome of increasing vegetation heights, with a crown of vegetation dense enough at the centre to create bare ground underneath, and a gradient of ground-cover towards the edges where the species can nest (Wilson et al. 2005b).

The structural diversity of woodland can be readily reduced by suspending coppicing and rotational cutting, as well as by increased grazing pressure from deer (Fuller et al. 1999). A study based on BBS results from 1995 to 2006 found a negative correlation between the abundance of deer and Nightingales at a regional level, with the species declining the most where deer population increase had been greatest, and modelling suggested that deer alone could have caused a decline of 14% in Nightingales over this period (Newson et al. 2012). Experimental approaches have demonstrated the effect of deer browsing on Nightingale numbers at site level: an exclusion experiment carried out over nine years found that Nightingale territory density within deer exclosures rose to ten times that of the rest of the wood, while radio-tracked Nightingales spent more time inside the deer exclosures than outside (Holt et al. 2010). Mist-netting confirmed that more Nightingales were present within the exclosures than in control plots, although the sample of birds was small (Holt et al. 2011). These findings fit with results across a wider range of breeding bird species that require low vegetation in woodland (Gill & Fuller 2007).

Woodland-scrub mosaics appear to be important breeding habitats for Nightingales, with implications for conservation practice (Holt et al. 2012c).

Information about conservation actions

The habitat requirements for Nightingale are well understood, and habitat management actions such as coppicing and control of deer numbers can be taken at a local level to promote the heterogeneous vegetation structure that Nightingales need. Woodland-scrub mosaics appear to be important breeding habitats for Nightingales (Holt et al. 2012c). A management advice sheet has been produced by the BTO and gives full details about the precise management requirements for Nightingales. Management plans should maximise the area of shrub at the vigorous thicket stage, typically involving rotational cutting on a 10-15 year cycle, preferably using reasonable sized blocks to create a coarse mosaic of larger patches instead of many small and widely dispersed patches of different ages (BTO 2015).

Although the habitat requirements are understood and can hence be applied locally, regional or national policies to encourage habitat management for Nightingales may be required to provide such habitat at a sufficiently wide scale and on an ongoing basis in order to enable the decline of this species to be stopped and reversed. Migrating Nightingales are attracted by singing males, hence one strategy as part of a wider landscape scale approach to conservation management for Nightingales would be to focus on providing this new habitat adjacent to existing sites.