Grassland management for Stone-curlew

Detailed studies of a small number of Stone-curlews, breeding in Breckland in the east of England, give some clues as to how to provide the right habitat mix for these big-eyed, nocturnal waders. Increasing structural diversity, by ploughing and/or harrowing areas of grassland, can create an attractive network of nesting and foraging sites for breeding and non-breeding adults.

In a 2021 paper in Animal Conservation, Rob Hawkes and colleagues from the University of East Anglia, RSPB and Natural England give us insights into the daily lives of Stone-curlews nesting in the dry grasslands of East Anglia. By fitting five individuals with GPS tags and following their movements they were able to establish which habitats are used at different stages of the breeding season.

The Stone-curlew is the only migratory member of the thick-knee family. England is at the north-western limit of a breeding range that stretches east to the steppes of Kazakhstan, with birds wintering in southern Europe, North Africa, the Arabian Peninsula and the Indian sub-continent. Most English birds spend the winter months in Spain, Portugal, Morocco or Algeria but a small number are known to cross the Sahara. Stone-curlews return to East Anglia in March and April.

Unmodified grassland in the Brecks of East Anglia provides limited feeding opportunities for Stone-curlews

Creating heterogeneity within areas of grassland by ploughing and/or harrowing patches

During the twentieth century, numbers of Stone-curlew across Europe fell significantly, as mechanized farming expanded. The East Anglian population had dropped to fewer than 100 pairs by 1985 and it took huge conservation efforts to increase this to 200 pairs. Breeding birds are now typically to be found in sparsely-vegetated ground, often in spring-sown crops, on dry heathland or in semi-natural grassland areas, including those used for military training. Preferred food items, such as earthworms, soil-surface invertebrates, slugs and snails, are easier to find in areas of bare and broken ground than in thick grass, so grazing is an important part of conservation action on heaths and in grassland areas.

Each of these square treatments covers a hectare

The UK’s migratory Stone-curlew population has received a huge amount of conservation support, on the back of detailed studies of the species’ breeding ecology (Green et al. 2000).  Tremendous efforts have been made to maximise chick production in farmland, with conservation staff and volunteers working with farmers to monitor breeding pairs, so that they can protect nests and chicks during crop-management operations. In the long term, however, such interventions are too labour-intensive to be sustainable. Can equivalent benefits accrue if more Stone-curlews nest successfully in semi-natural grassland, where the costs of conservation subsidies are lower than in intensively farmed arable cropland?

The study that is reported in the 2021 paper in Animal Conservation took place in an extensive area of semi-natural grassland (nearly 40 km²) that is surrounded by a mosaic of arable farmland. The aim was to understand whether ploughing or harrowing patches of grassland can provide suitable foraging areas, and which other habitats are important.    

Tracking Stone-curlews

GPS tag attached to the back of a Stone Curlew

Stone-curlews are predominantly nocturnal feeders so some form of remote tracking device was needed to understand their movements. GPS loggers were fitted to five adult Stone-curlews during the breeding season. Individuals were caught at night using small, beetle-baited, elastic-powered clap-nets or in the daytime using nest-traps. Each individual was fitted with a 5.2 g solar-powered nanoFix Geo PathTrack GPS tag and an external whip antenna. GPS data were downloaded to a remote base station through a radio connection. Tagged birds were visited at least once a week to establish whether they were still nesting, if they had chicks or had finished breeding.

Where did they go?

One, two and three hectare plots of disturbed ground had already been created within Breckland’s grassland and heathland areas, prior to this study, as described in the paper and discussed in the blog Curlews and foxes in East Anglia. It was already clear that these plots were favoured as nesting sites but does disturbed ground also provide additional feeding opportunities for adult birds – including pairs nesting nearby on arable fields?

Using a telescope to point the base station ‘reader’ at a bird wearing a GPS logger

Three male and two female Stone-curlews were tracked for more than nine weeks (67 to 102 days), yielding 510 GPS fixes during nesting and 1371 post-breeding. There were some fixes in the pre-nesting phase and also during the chick-rearing phase but too few to be considered for analysis. Analytical methods are detailed in the paper.

During the nesting period, what was presumed to be the off-duty bird of each pair was found within 1 km of the nest on 90% of fixes. The mean distance from the nest during daytime feeding was about 100 m but, at night, tagged birds travelled five times as far, on average. They travelled furthest when heading for pig-fields and manure heaps.

• Relative to closed, undisturbed grassland, nesting Stone-curlews were two- to three-times as likely to forage on disturbed-grassland during night and day, but especially during the day.
• Night and day, ‘sugar beet or maize’ fields were used more than unmodified-grassland but similar to disturbed-grassland.
• Nocturnally, Stone-curlews were ten-times as likely to use ‘pig fields or manure heaps’, when compared to undisturbed grassland.
• The furthest distance travelled was just over 4 km, which is further than previously thought.

Stone-curlews will fly a long way to feed in pig fields

In the post-breeding period, tagged birds travelled up to 13 km to forage, with 90% of nocturnal foraging locations found to be within 5 km of day-time roosting sites.

• Tagged birds were approximately 15-times as likely to use either disturbed-grassland or arable fallows when compared to undisturbed grassland.
• Use of the category ‘pig fields or manure heaps’ was nearly as strong (factor c. 10), in comparison to undisturbed grassland.
• Open crops in the categories ‘sugar beet or maize’ and ‘vegetable or root crops’ were also used more than undisturbed-grassland (factor c. 2).

Conservation messages

In England, there has been a long-term focus on trying to maximise Stone-curlew productivity within arable farmland, especially in East Anglia but also in Wessex. As suggested earlier, this is expensive – foregone production requires high subsidies and interventions by conservation staff are time-consuming. A French study, written up in Ibis by Gaget et al., seriously questions the viability of Stone-curlew populations within intensively managed arable farmland, even with conservation support. Given these problems, should conservation efforts focus on supporting and building up grassland populations?

In a previous tracking study, thirty years previously, Green at al. found that short semi-natural grassland provided suitable foraging habitat for Stone-curlews. Much has changed in Breckland in the intervening period, with the collapse of the rabbit population and an increase in the amount of outdoor pig-rearing. As the short swards of rabbit-grazed grassland have disappeared, Stone-curlews seem to have increasingly taken advantage of alternative opportunities offered in pig fields.

Previous attempts to replicate the grazing efforts of rabbits have involved increasing livestock numbers but this study shows that physical ground-disturbance interventions immediately and effectively create alternative foraging habitat. The authors suggest that multiple areas of disturbed-ground, close to the edge of large grassland blocks, can provide a network of nesting and foraging habitats, whilst allowing access to a mixture of feeding opportunities in the surrounding arable farmland. Of course, nobody is suggesting that intensive outdoor pig-rearing is a positive addition to the habitat mix, as it involves nutrient run-off, ammonia leakage into fragile plant-rich heathland, and pelleted feed attracts corvids (which are known to predate eggs).

The effect of creating ploughed or harrowed plots is almost immediate, in terms of prey availability. In more detailed studies of the effectiveness of slightly different ground-disturbance options, the research team found a strong selection preference only one year after the treatments were first implemented. In the longer term, these areas may become increasingly important, if young birds recruit to the local population and seek out these features. The researchers suggest that a bespoke, ground-disturbance agri-environment option might open up new breeding opportunities within semi-natural grasslands that are currently dominated by tall, closed swords. 

In England, conservation of dry grasslands and heathlands has tended to focus on the preservation of charismatic plant communities, an approach that may have been too gentle and conservative for other taxa. Whilst this study demonstrates that adding pockets of disturbed ground appears to benefit Stone-curlews, previous studies, conducted by this research team, showed benefits for Woodlark (Hawkes et al. 2019), Eurasian Curlew (Zielonka et al. 2019, summarised in Curlews and foxes in East Anglia), and rare, scarce or threatened dry-grassland invertebrates (Hawkes et al. 2019). Similar disturbance techniques have been shown to potentially benefit other grassland-breeding waders, such as Mountain Plovers (Augustine & Skagen 2014) and Upland Sandpipers (Sandercock et al. 2015) in North America, and Sociable Lapwings in Kazakhstan (Kamp et al. 2009).

Paper

This is a summary of a 2021 paper in Animal Conservation:

Effects of experimental land management on habitat use by Eurasian Stone-curlews. Robert W Hawkes, Jennifer Smart, Andy Brown, Rhys E Green, Helen Jones & Paul M Dolman.

Each summer, farmers, volunteers and conservation staff work together to monitor and protect nesting Stone-curlews on farmland, grasslands and heaths in eastern and southern England. Thanks to all of these people, the number of pairs of Stone-curlew in England is holding steady, at over 300 pairs. Progress was reviewed at a conference in 2017, as you can read in this layman’s report and this technical report. More guidance for landowners can be found here.

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WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.

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Oystercatcher Migration: the Dad Effect

What determines whether some birds migrate and others do not? This question is fundamental to understanding how migratory systems change over time but the causes of individual migratory behaviours have proved difficult to isolate.

Verónica Méndez and colleagues are studying Icelandic Oystercatchers, some of which remain in Iceland for the winter but most of which migrate across the Atlantic to Ireland, Britain and mainland Europe. In a 2021 paper in Scientific Reports they show that a chick’s migratory behaviour seems to align with the behaviour of its father but not its mother. What can explain this pattern?

The story so far

The Icelandic Oystercatcher study system has already featured in three WaderTales blogs. The first was Migratory decisions for Icelandic Oystercatchers. This focused upon the key questions that Verónica Méndez and colleagues from the universities of Iceland, East Anglia (UK) and Aveiro (Portugal) are trying to answer.

• Why do some Oystercatchers migrate when others don’t?
• Is it the same birds each year?
• Do resident or migrant birds have an advantage when it comes to choosing a territory and raising chicks?
• Do chicks follow the same migratory patterns as their parents?

When the first blog was written, in 2015, eight colour-ringed Oystercatchers had been seen in Ireland and the UK, and five had been seen wintering in Iceland. Fast forward to the next blog in 2018 – Mission impossible? Counting Iceland’s wintering Oystercatchers – where counts showed that over 11,000 Oystercatchers spend the winter in Iceland. Using colour-ring sightings of resident and migratory birds, the research team concluded that this total is about 30% of the whole Icelandic population. The other 70% fly south across the Atlantic each autumn, with no individuals yet observed to change what they do between years.

In the third blog – Which Icelandic Oystercatchers cross the Atlantic? – some patterns were starting to emerge.

• Females and males are equally likely to migrate.
• Size does not matter – small and big birds are equally likely to migrate
• There are regional patterns across Iceland, with birds breeding in the west being most likely to be resident.
• Birds do not pair up assortatively – residents don’t pair up with other residents before the migrants return, for instance.

Family ties

In most species of waders, parents protect their chicks and take them to suitable feeding areas but they do not actively feed them. Parental care in European Oystercatcher includes foraging for food and bringing it back to the chicks. This is why it is possible for Oystercatchers to nest on the roofs of buildings (Oystercatchers: from shingle beach to roof-top), where they are out of the reach of ground predators.

Focusing on chicks

To be able to understand the relationship between migratory behaviour in adults and their chicks, you need to be able to mark and then attempt to follow all of the members of a family. Adult Oystercatchers generally keep the same mates and nest in the same areas year after year, enabling the establishment of marked population of birds in different parts of Iceland. Between 2015 and 2018, a total of 615 incubating adults were caught. By following the outcomes of nesting attempts and then monitoring the growth of chicks, the research team also managed to individually mark 377 chicks.

Three colour-ringed chicks. Where will they go?

The success of the whole project relied heavily upon winter sightings of marked birds within Iceland and in Ireland, the UK and continental Europe. Through a network of volunteer observers reporting sightings of marked individuals across the wintering range, the migratory behaviours of 227 of the 615 colour-marked adults and 50 of the 377 colour-marked chicks had been identified at the time that this paper was written. In addition, it was possible to infer the migratory behaviour of 353 marked adults using measurement of isotope ratios (δ¹³C and δ¹⁵N) of feathers that were grown in the winter (as described here).

The analyses in the paper by Verónica Méndez and her colleagues are based upon 42 marked chicks of parents for which the migratory behaviour of both parents is either known or can be inferred from isotopic signatures. These chicks all fledged successfully and were seen during the winter period, either in Iceland or having crossed the Atlantic. In three cases, two chicks from the same broods are known to have behaved in the same way. More data have become available since the analyses, all confirming the same patterns.

Results

It is possible to imagine a scenario in which late or slow-growing Oystercatcher chicks might be more likely to stay in Iceland than their more mature counterparts – simply by developing too late to gain enough resources to cross the Atlantic. Analysis of hatch dates and growth parameters did not suggest the existence of such a link, as described in the paper.

This young Oystercatcher was spending its first winter on the coast of western Iceland

The interesting finding of this study is the link between the behaviour of parents and chicks. Data generated by observations of colour ringed individuals (adult and chicks) and from isotopes (adults) established 21 chick/parent associations.

• Of the sixteen chicks raised by migrant mothers, eight migrated and eight remained in Iceland.
• Of the five chicks raised by resident mothers, three migrated and two remained in Iceland.
• All ten of the chicks raised by migrant fathers migrated from Iceland.
• Of the eleven chicks raised by resident fathers, one migrated and ten remained in Iceland.
• Seven chicks that fledged from pairs with one resident and one migrant parent adopted the migratory behaviour of the father.

This is pretty compelling evidence that chick migratory behaviour is associated with paternal (and not maternal) migratory behaviour!

What does this mean?

There is no evidence of genetic control of migratory destinations and both Oystercatcher parents care for chicks, so what mechanism could produce such strong paternal but not maternal effects?

The authors suggest that the migratory behaviour of individual oystercatchers may be linked to social interactions they experience during the post-fledging period. In shorebird species, such as Oystercatchers, mothers commonly depart before the chicks fledge, or at about the same time. Fathers often provide parental care for longer and this extended period of the parental bond may underlie the link between paternal and juvenile migratory behaviour in Icelandic Oystercatchers. Despite being able to fly and feed independently, juvenile Oystercatchers in Iceland have been seen begging for food several months after fledging, suggesting that some parents (most likely fathers) may care for youngsters much longer than in other species.

This Iceland-ringed Oysterctatcher was photographed in Guernsey in January 2021. It departs at the start of February each year.

Under this extended-care system, a chick that is being look after by a resident male may well become a resident, simply by following dad. As autumn arrives, the youngster can follow his parent when he moves to the coastal mudflats where resident Icelandic Oystercatchers spend the non-breeding season. Autumn turns to winter and the chick is destined to be a resident.

Is it possible to explain a similar link for migrants? As the breeding season comes to an end, migrant fathers leave their breeding areas and head south, across the Atlantic, leaving fledged youngsters to fend for themselves. Groups of youngsters gather together in flocks which also include adults that are feeding up in preparation for migration. Although not influenced by their own fathers, chicks may follow the cues of other migratory adults, thereby creating the patterns seen in this paper.

Most of the chicks included in these analyses were early-fledged birds, simply because earlier nesting attempts tend to be more successful. The research team were unable to detect any significant effect of fledging date on migratory behaviour but they do not rule out the possibility that late-fledging individuals lack the time or resources to undertake a migratory journey, irrespective of paternal behaviour.

The broader context

Migratory behaviour typically arises in seasonal environments, allowing individuals to exploit peaks of resource abundance in distinct locations across the world. Rapid shifts in the distribution and migration phenology of many migratory species present challenges to site-based conservation strategies. There is an urgent need to understand the processes that influence individual migratory behaviour, in order to attempt to predict species’ responses to environmental change.

The findings in this paper suggest that the social interactions experienced by individuals can directly influence the development of their migratory behaviour, and that the extent and timing of parental care may be key in shaping individual access to these social interactions. You can read the full paper here:

Paternal effects in the initiation of migratory behaviour in birds Méndez V., Gill, J.A., Þórisson, B., Vignisson, S.R., Gunnarsson, T.G. & Alves J.A.

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WaderTales blogs are written by Graham Appleton (@GrahamFAppleton) to celebrate waders and wader research. Many of the articles are based on published papers, with the aim of making shorebird science available to a broader audience.