With Curlew populations in free-fall across much of the British Isles, researchers are trying to understand the reasons for poor breeding performance. At the same time, several groups are trialling emergency interventions, such as predator control and predator exclusion, to try to boost the number of fledged chicks. Sharing knowledge is crucial, so it’s great that a 2020 paper by Natalia Zielonka and colleagues in Bird Study adds to our understanding of nest-site selection and the reasons for nesting failures.

Breckland Curlew

When we think of Britain’s breeding Curlew, the traditional image that comes to mind is moorland, where displaying birds deliver their haunting, bubbling call across upland heather moor and sheep pasture. In lowland East Anglia, in the east of England, things are very different. Some Curlew nest on Breckland heaths, which are structurally similar to moorland, but you can also find nests in sugar-beet field, in military training areas and around airfields.

The Eurasian Curlew is now categorised as Near Threatened by IUCN & BirdLife International, due to populations declines (see Is the Curlew really near-threatened?). Figures from the Breeding Bird Survey for the period 1995-2017 show that the situation in England (30% decline) is less bad than Scotland (down 61%) or Wales (down 68%). The species is now too thinly spread to be monitored in Northern Ireland but we know that the breeding population in the Republic of Ireland dropped from 3,300 pairs to just 138 pairs in 30 years (more in Ireland’s Curlew Crisis). In this context, the hot-spot in Breckland (see left-hand map from Bird Atlas 2007-11) is significant, as is the fact that there are four 10-km squares in East Anglia where an increase in density was noted between 1988-91 and 2008-11 (right).

Problems for Curlew

The main driver of UK Curlew decline is low breeding productivity, attributable to predation and reduced quality of breeding habitats (see review by Franks et al., summarised in Curlews can’t wait for a treatment plan). Research has informed habitat provision and demonstrated that legal predator control (of foxes, crows, stoats and weasels) can increase Curlew breeding success and abundance – but not in all cases.

Much of the research underpinning the above review was conducted in upland areas. What is happening in the flatlands of East Anglia and might any differences explain the apparent resilience – or even growth – of this population? Most lowland Curlew breed on dry grasslands and heathland, where physical ground-disturbance is increasingly advocated as a land management technique for other rare, scarce and threatened species, such as Stone-Curlew and Woodlark. How do these interventions affect breeding Curlew in the same areas?

Study site

Natalia and her colleagues studied Curlew in two extensive grass-heath sites that differed in predator density and management. Across both sites, vegetation structure had been modified, using ground-disturbance plots, as part of a wider multi-taxa experiment, aimed at increasing biodiversity and supporting key species such as Woodlark (paper in Ibis) and Stone Curlew (paper in Biol. Conserv.). This disturbance involved creating 2 to 4 hectare plots of deep- or shallow-cultivated ground within the wider grassland/heathland environment.

Before the experiment began, and based on previous research elsewhere, it was assumed that Curlew would avoid physically-disturbed areas, given that Curlew have been shown to prefer to nest in rougher habitats with longer grass swards. Wherever the Curlews decided to nest, it was predicted that nest survival would be higher on the site with lower predator density, that most clutches would be taken at night and that success would decrease through the season.

The study was carried out in 2017 and 2018 across the Stanford Military Training Area (STANTA) and Brettenham Heath. Both sites contain extensive areas of dry grassland and grass-heath, surrounded by arable farmland and woodland. Generalist predator control on STANTA was largely carried out around Pheasant release pens, with little or none in most of the surrounding, open arable farmland and woodland. In contrast, Brettenham Heath was subject to continuous predator control across the whole site and in surrounding arable farmland (but not woodland). Brettenham Heath is also enclosed by a two-metre high deer fence with a single electric strand set 50 cm above the ground. There is more about the two sites in the paper (link below).

Across both sites, 64 experimental ground-disturbance plots were established in early 2015 and subsequently disturbed annually, using a variety of management techniques. Areas on STANTA that might have contained unexploded ordinance could not be disturbed – and were trickier to survey! In both years, nests were located between mid-April and late June, by visiting any area where Curlew had been seen and looking for adults sitting on, or walking back to, a nest.

Evidence of nest success

To determine the date and timing of nest failure, temperature sensors were placed under the eggs. Nests were remotely checked every three-to-seven days, to confirm adults were still incubating, and the scrape was visited once a week to record any predation events (e.g. partial clutch predation). From three days before the predicted hatch date, nests were remotely monitored daily to accurately determine their fate. After hatching, the nest site was visited every three-to-five days, to observe adults and chicks from a vehicle, continuing until the chicks fledged or the breeding attempt had failed.

Three types of evidence can help to reveal the outcome of a nesting attempt:

Small chicks or alarming adults are seen in the vicinity of an empty nest.

Small shell fragments are found in the nest cup, indicating that chicks have hatched.

The temperature sensor reveals that adults kept the eggs warm for the whole incubation period.

A sudden drop in nest temperature can reveal the timing of predation and hint at the culprit. Other studies have shown that nocturnal visits are usually by foxes or badgers, or occasionally hedgehogs. In 2018, infra-red cameras, triggered by movement, were used on ten nests.

Where to nest?

Unexpectedly, given that breeding Curlew in the uplands are usually associated with areas with longer grass, Breckland pairs were five- to six-times more likely to select disturbed plots than undisturbed grassland. Nearly half of nests were located on disturbed grassland across both years, which only occupied about 8% of the grassland area. Curlew are long-lived and site-faithful, so disturbed plots may have been created within already-established breeding territories. Whilst physical-disturbance interventions are unlikely to bring birds in from the wider landscape, this study shows that nest placement was more likely to occur on disturbed grassland within a pair’s home range. This suggests that local-scale management can influence nest placement within established breeding sites.

Disturbed grassland is characteristically bare and short compared to uncultivated grassland. Curlew may have placed nests in this habitat because it allows greater vigilance and/or because there is a greater abundance of some important prey for chicks than the surrounding grassland (information in paper by Hawkes et al.). Most nests were on shallow-cultivated plots (created with a rotary rotovator), with few on the barer deep-cultivated plots (created with an agricultural plough). There was taller vegetation and more ground cover in the shallow-cultivated plots.

Nest survival rates

Over the course of the two summers, 44 Curlew nests were monitored, of which 32 failed. 29 nests were predated, one was trampled by livestock, one was mown and one pair deserted (and then renested). Mean overall nest survival probability from start of incubation to hatching was 0.70 ± 0.18 SE at Brettenham Heath (where there was intensive predator control and fencing) and 0.16 ± 0.06 at STANTA (where predator control was patchy and the site was open). Overall breeding productivity was 0.16 ± 0.01 SE fledged chicks per nesting attempt. As predicted, daily nest survival rate decreased through the course of the nesting season.

The figure for STANTA represents low nesting survival, especially when compared to other lowland sites in England, and re-nesting following failure appeared infrequent. Productivity here is likely to be substantially below that required to maintain a stable population. Importantly, nest survival across the two sites was not influenced by ground-disturbance, which suggests that this management intervention did not increase nest exposure to predators.

Identifying the predators

Of the ten 2018 nests with nest cameras:

Three nests survived through to hatching.

Four were predated by fox (one during the day and three at night).

One nest was predated by an unknown predator (following camera malfunction).

One nest was predated by a sheep (two out of four eggs remained but incubation was not resumed and the other eggs were later taken by a crow).

A single-egg, late-season re-nesting attempt was abandoned three days after camera deployment (egg was later taken by a crow)

There was no effect of nest cameras on daily nest survival rate.

Parent Curlews removed some of the temperature loggers but there were sufficient data to identify the timing of predation events for 23 nests. Of these, 17 events were during the night (13 nocturnal, four crepuscular) and six during the day. This nocturnal timing of nest predation was consistent with mammalian rather than avian predation, with camera traps and other evidence suggesting that foxes were the main perpetrators.

As expected, from the levels of predator control, nest survival was lower at STANTA than Brettenham Heath. The latter site was both fenced and subject to lethal fox control, delivering a breeding productivity well above that considered necessary for a sustainable population of Curlew. It is possible that a few fenced sites and others with high levels of predator control might be disproportionately responsible for the fact that Curlew appear to be doing better in the Brecks than in other areas.

Conservation implications

The key finding of this project is that physical ground-disturbance, which is advocated as a conservation measure within lowland dry grassland and grass-heath for many rare, scarce and threatened species, also provides suitable Curlew nesting habitat, with no reduction in nest survival. Implementing ground-disturbance, particularly through shallow-cultivating, in areas with few or no mammalian nest predators, could provide a useful management tool for attracting breeding Curlew to safer areas.

An intervention to help Stone Curlew and Woodlarks was never designed to assist Curlew. Indeed, there was a prediction that Curlew would actively avoid areas that had been rotovated, in order to create bare patches in which the target species could nest and feed. In a rare case of serendipity, experimental research by Rob Hawkes, Paul Dolman and others has delivered a way of encouraging Curlew to nest in relatively small plots (2-4 hectares) around which it may be possible to run an electric fence. One of the big questions “How can we protect Curlew nests when they have such big territories?” might have become easier to answer. If ‘Curlew plots’ can be created within known territories, or even areas that seem good for Curlew, then they can potentially become the focus for protection.

Having spotted that Curlew seemed to be attracted to disturbed areas it is great that Natalia Zielonka was able to study this population, in order better to understand constraints that seem to be restricting productivity. Her research was undertaken as part of her MSc in Applied Ecology and Conservation at the University of East Anglia,

Further reading

The paper at the heart of this blog is:

Placement, survival and predator identity of Eurasian Curlew Numenius arquata nests on lowland grass-heath. Natalia B. Zielonka, Robert W. Hawkes, Helen Jones, Robert J. Burnside & Paul M. Dolman.

Bird Study. DOI 10.1080/00063657.2020.1725421

Graham (@grahamfappleton) has studied waders for over 40 years and is currently involved in wader research in the UK and in Iceland. He was Director of Communications at The British Trust for Ornithology until 2013 and is now a freelance writer and broadcaster.