This work was made possible with the kind support of The G & K L B Boyes Charitable Trust
Key points
- UK populations of several wader species are rapidly declining, most likely due to low breeding success.
- Predation of eggs and chicks by generalist predators is thought to be a key driver of low breeding success in wading birds.
- GWCT scientists studied 18 pairs of sites in the British uplands to investigate how habitat, predators, and livestock grazing intensity may influence wader abundance and breeding success.
- More wader species were present on grouse moors and were recorded at over twice the density than on non-grouse moors.
- Curlew breeding success was four-fold higher on grouse moors then on non-grouse moors. Similar differences were apparent in other wader species.
- Grouse moors appear to help slow declines by acting as a ‘source’ population for curlew, where surplus fledglings can move to and populate other areas.
- Predation appears to be an important driver of low breeding success in curlew that largely contributes to population declines.
Background
Grouse moor management includes both legal, lethal predator control and management of heather. Reducing the number of generalist predators helps protect red grouse and their chicks, and the cutting or burning of heather keeps vegetation in a healthy, varied structure for grouse. Both these aspects can also benefit other ground-nesting moorland birds.
Around a quarter of the world’s curlew breed in the UK. Since 1990 the UK population has halved, significantly impacting global numbers. Whilst habitat loss and degradation due to agricultural intensification and afforestation have contributed to these declines, reduced breeding success, rather than changes in adult survival, is thought to be driving declines in curlew and other wader species.
In the UK, numbers of foxes and crows are higher than in most other European countries and losses of egg clutches and chicks to these generalist predators is high. On average, each pair of curlew need to produce 0.5-0.6 fledglings a year to maintain numbers. Most European-based studies show that this is not happening, with fledging rates often considerably lower. Rates of decline in curlew and other waders differ between regions of the UK, but greater stability and higher breeding densities are associated with moorland managed for driven red grouse shooting in northern England and parts of the Scottish uplands.
To better understand how to help, GWCT scientists designed a study to try to measure the importance of predator control on grouse moors for breeding curlew.
What they did
The scientists measured wader densities and curlew breeding success in relation to indices of predator abundances within 18 pairs of moorland and moor-edge farmland sites spread across North Wales, northern England, the Scottish Borders, and the Scottish Highlands. Each pair of sites consisted of:
- A grouse moor: A moorland managed for grouse shooting where predators such as foxes, stoats, weasels, and corvids were lethally controlled all year round by full-time gamekeepers.
- A non-grouse moor: Another site with similar habitat, but without gamekeepers and where predators were not lethally controlled.
The team surveyed the entirety of each site, recording the number, location, and behaviour of all breeding waders on five separate visits. The visits were timed to coincide with different stages of the breeding season, i.e. pre-nesting, incubation, and chick-rearing.
They used satellite imagery validated by field surveys to estimate the proportion of different habitat types within each site, and measured vegetation height and composition, together with the level of grazing by cows and sheep. To quantify the predation risk to waders, the team also recorded the numbers of each corvid, gull, and raptor species seen on each site-visit. Fox scats were also counted along tracks and paths at each site to estimate fox activity.
What they found
In their surveys across all study sites, the scientists recorded almost 900 pairs of waders, of which almost half were curlew, a quarter were lapwing, with the remaining quarter comprised of snipe, oystercatcher, golden plover and redshank. They found that, on average, just over four wader species were present on grouse moors compared to three on non-grouse moors, and that on grouse moors waders were recorded at over twice the density as on non-grouse moors. Redshank and golden plover were found on 50% of the grouse moors studied, but only 20% of non-grouse moors.
Curlew accounted for nearly half of breeding pairs recorded in the study and occurred four times more frequently on grouse moors than on non-grouse moors. On grouse moors, it was estimated that two thirds of curlew pairs successfully hatched chicks, compared to just 17% of pairs on non-grouse moors. Curlew fledged 1.05 chicks per pair on grouse moors, almost double the rate needed to keep numbers stable. In contrast, on non-grouse moors they only fledged 0.27 chicks per pair, well below the break-even point to stabilise numbers.
Both curlew hatching and fledging success was lower where corvid abundance and fox indices were higher. Neither gulls or raptor abundances, nor habitat, vegetation type, livestock, and peat depth influenced curlew breeding success. On grouse moors, where gamekeepers managed these predators, crows were on average four-fold fewer and fox signs were three-fold fewer than on the equivalent non-grouse moor in the same block.
The team also found that, whilst habitat measures varied between study blocks in different regions of the UK, they showed no significant differences between grouse moors and non-grouse moors in the same block, and no habitat measure was related to curlew breeding success. This suggests that, whilst curlew attempt to breed in a range of habitat types and conditions, predation is a more important driver of their low breeding success.
Where enough data were available for other waders, the patterns in their breeding success reflected those seen in curlew. For example, lapwing fledged 1.0 chick per pair on grouse moors but only 0.5 chicks per pair on non-grouse moors, whilst 20 pairs of golden plover successfully fledged chicks on grouse moors, but none did so on non-grouse moors. Similarly, 23 pairs of oystercatchers raised chicks on grouse moors, but only two pairs did on non-grouse moors.
What does this mean?
Due to high curlew breeding success on grouse moors, well above that required to maintain numbers, these areas may help slow curlew declines elsewhere by acting as ‘source’ populations, where surplus fledglings can move to and populate other areas. Due to much lower breeding success on non-grouse moors, these areas may act as ‘sinks’, where without recruitment from elsewhere, breeding numbers are not self-sustained and continue to decline.
This study suggests that without lethal control of generalist predators on UK grouse moors, and specifically the control of foxes and corvids, curlew declines may have been more severe. The results complement previous research findings on grey partridge, hen harriers and upland waders that show a clear benefit of predator control for ground-nesting species of conservation concern. To help restore numbers of curlew and other waders, legal predator control on grouse moors should be maintained and encouraged, whilst efforts should be made to make wider uplands landscapes less friendly for generalist predators.
Read the original abstract
Baines, D., Fletcher, K., Hesford, N., Newborn, D. and Richardson, M. (2023) Lethal predator control on UK moorland is associated with high breeding success of curlew, a globally near-threatened wader. European Journal of Wildlife Research, 69:6.