Keep away from the trees

There are concerns that waders nesting in open landscapes are threatened by habitat fragmentation, and may be increasingly so in the future by a drive to plant more trees. Plantations obviously remove nesting space used by species such as Lapwing and Ringed Plover but they could also create hiding places for predators that can then target nests in the vicinity. In a 2022 paper in Animal Conservation, Triin Kaasiku and colleagues report on the outcomes of 753 Estonian wader nests in open areas close to forest edges along Estonia’s coastal fringe. Which are the key predators and where do they hunt?

Wide horizons

Open landscapes have been lost at an unprecedented rate over the past century. Warmer temperatures and reduced management of semi-natural landscapes provide ideal circumstances for the establishment of shrub growth and the expansion of forests. At the same time, afforestation campaigns are encouraging tree planting, to generate revenue and perhaps contribute to climate change mitigations. The combined effects of these drivers are exacerbated by the way that these processes increase the fragmentation of these already-threatened open habitats, which have traditionally provided homes for waders such as Dunlin and Curlew.

The coastal strip that used to provide important breeding habitat has nowadays often become fragmented by forest

Waders in Estonia

The semi-natural wet pastures of the Baltic coast have persisted for centuries, formed by the combined effects of floods, winter ice and postglacial land uplift, and through grazing by domestic livestock and wild geese. Together, these processes have created and preserved important wader breeding habitat. Sixty years ago, Estonia’s coastal grasslands used to be full of Lapwing, Dunlin and Redshank, as well as Black-tailed Godwit, Curlew, Common Snipe and Ruff. Since then, farmland abandonment and afforestation, both as a result of commercial forestry and natural succession, have reduced the area of coastal grasslands by about 70%.

The Boreal Baltic coastal grassland habitat is listed as a priority habitat type in Annex I of the EU Habitats Directive. Despite efforts to restore this habitat, by reducing reed and tree cover over the last two decades, most wader populations in these areas have not recovered. This is discussed in a blog about the effectiveness of agri-environment schemes in the same study area (Remote monitoring of wader habitats). Direct habitat loss is part of the problem for Estonia’s breeding waders but increased predation rates may also be a factor. Triin Kaasiku and colleagues have studied this diverse breeding wader community, to learn more about how nest predation varies across this wet grassland-forest system.

Follow that nest

Wader breeding densities at the study sites varied from 7 to 160 wader pairs per km²; the upper end is considered to be a high density, at the European level. Nests were found and then revisited approximately weekly. The Animal Conservation paper describes fully how evidence was used to determine whether nests were successful and to consider the probable causes of failures. Wader nest survival was measured during three breeding seasons (2018-2020).

Lapwing, Ringed Plover, Redshank and Dunlin were the four main species studied by Triin Kaasiku and her colleagues (see table). They accounted for 655 out of the 753 nests, with six other species providing smaller samples. To understand more about predation events, the team deployed camera traps alongside 85 of the nests, all of which were within 1 km of trees and forests.

Coastal flooding can potentially be a serious issue

About 80% of nesting attempts were unsuccessful (526 out of 655) and the outcomes of 14 other nests could not be established. Of the known failures, 89% were lost to predation, while other causes of nest failures included abandonment (6.7%), flooding (2.3%), trampling (0.2%), and others where the causes were unclear (2.3%). The seemingly high nest abandonment rate may also include some nests that were subject to temporary flooding. The results are based on 679 nests that either hatched or where there was evidence of predation.

There were no discernible differences in survival rates for different species. This is in line with findings in an Icelandic study that compared outcomes of open-nesting species (Golden Plover, Oystercatcher and Whimbrel) with those of species that hide their nests (Redshank, Black-tailed Godwit and Snipe). See Where to nest?

Nest losses

The mean daily survival rate (DSR) for nests was 0.929 which, over a combined incubation and laying period of 27 days, predicts that only about 14% of nests survive through to hatching.

DSR is higher further from the forest and where the amount of cover is lower. These two metrics are obviously related but the effects are teased apart in the paper.

From the modelled data, a nest that is only 20 m from forest edge has a 7% chance of being successful (95% CI = 5-11%), while the equivalent figure for one that is 1  km away is about 26% (18-34%).

Similarly, 3% (1-8%) of nests hatch when local forest cover (within 1 km of the nest) is about 50%, compared to 19% (15-23%) in completely open areas.

Nest success was too low across the whole study area – not enough chicks are produced

Predators and predation

Nest cameras were deployed to track what happened to 85 nesting attempts. Although 64 nests were predated, the camera trap only managed to record the nest predator in 41 cases. Of these events, 31 nests were lost to Red Fox, 5 to Golden Jackal, 3 to Raven and 2 to Badger.

Fox predation occurred on average at 217 m (95% CI=161-273 m) from the forest edge but the other mammals tended to predate nests that were further from cover. Predation events by Raven may also be more frequent closer to forest edge, but these events were rare. Based on the recordings of the camera traps, Red Fox detection rate was higher closer to the forest edge but no similar relationship was found with the proportion of forest cover.

In this Estonian study, wader nest survival did not vary with distance from smaller patches of trees or bushes (<30 m wide). Perhaps these patches may be too small for Red Foxes to hide or forage in.

The bigger picture

There is strong evidence, from studies in the UK and elsewhere, that species that breed in open habitats avoid woodland (see the WaderTales blog Mastering Lapwing conservation) and may experience greater population declines in more fragmented landscapes (as discussed in Curlews can’t wait for a treatment plan).

In the Estonian wet grassland-forestry patchworks studied by Triin Kaasiku and colleagues, Red Fox was the most commonly encountered predator operating close to forest edge. It was in these areas that eggs were most likely to be taken. This is in line with some other studies – but by no means all. The authors discuss in detail why different guilds of predators may have different effects in different circumstances and how patterns might be distorted if waders actively avoid nesting near forests or if there are complex predator-prey networks. See Further Reading below.

Predated Lapwing eggs

Hatching success is not high in any of the areas studied in Estonia – even at the lowest forest cover, only 19% of the nests hatch. This result shows that habitat fragmentation may have more severe effects on the open landscape species than previously realised. It may also indicate that more attention should be directed at the high number of generalist predators.

Conservation implications

In Estonia, as elsewhere, landowners are being encouraged to use land to grow food, to deliver biodiversity gain and to lock up carbon. Others can argue whether planting trees is necessarily a good thing for carbon capture, especially if deep peat is drained and cultivated in the process, but forestry is becoming more fashionable. This paper reminds us that piece-meal planning decisions that, for instance, provide grants to one landowner to preserve habitat for declining species of wader and grants for a neighbour to plant trees, are unlikely to maximise biodiversity benefits.

The full paper is available here:

Predation-mediated edge effects reduce survival of wader nests at a wet grassland-forest edge. Triin Kaasiku, Riinu Rannap and Peep Männil. Animal Conservation. doi.org/10.1111/acv.12774

Further reading

Foxes play leading roles in several WaderTales blogs but this selection may be of particular interest:

Tool-kit for wader conservation looks at different ways of reducing predation, particularly by foxes, within lowland wet grassland. The focus is upon issues in the UK.

Can habitat management rescue Lapwing populations? assesses whether the available tools have the power to deliver sustainable wader populations.

Trees predators and breeding waders is a cautionary tale. Reestablishing nesting habitat for species such as Dunlin and Curlew is not just a matter of removing the trees. It may take up to ten years for predator numbers to drop to levels that are associated with an open landscape.

Dunlin: tales from the Baltic is not focused on predation but fragmentation and predation are parts of the story. Veli-Matti Pakanen’s Finnish research is complementary to the studies in Estonia.

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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.

Navigating a vast ocean

The journeys that shorebirds make, as they cross the oceans of the globe, are truly remarkable. Individual birds demonstrate amazing endurance and navigational expertise while in the air for days at a time. Satellite transmitters are providing opportunities to understand how shorebirds refine their flight plans in responses to the wind patterns they encounter. In a paper in Movement Ecology by Jenny Linscott and colleagues, we join the northward flight of Hudsonian Godwits across 7,000 km of the Pacific Ocean and the Gulf of Mexico.

Coping with wind

Conditions encountered en route can dramatically impact the energy that migratory species spend on movement. To complete their journeys across barriers such as oceans, migratory birds need to manage energetic costs by adapting to the wind conditions they encounter. It’s a dynamic environment; no two years are the same and there is often little relationship between local conditions upon departure and what lies ahead, as for instance discussed in the WaderTales blog Rhapsody of Whimbrel.

Migrating Hudsonian Godwits are heading for Alaska

If an individual bird has full knowledge of its location at all times and a fixed destination point in mind, then perhaps it can adjust its airspeed and direction of flight so that it can completely compensate for any lateral drift that is caused by the wind systems en route. Alternatively, it might accept the drift that it experiences or only partly compensate, perhaps gambling that there may be systems ahead that might cause drift in the other direction. It may even over-compensate for the wind patterns it experiences at a particular time, which may be a good idea if there is more of the same weather to come. The authors of the Hudsonian Godwit study explain how these different approaches may integrate to create a movement strategy and summarise what has been found in other studies. Interestingly, it has been suggested that complete compensation (continually adjusting the flight-plan) might not be possible for trans-oceanic flights because there are no landmarks to use as reference points.

Hudsonian Godwits spend the austral summer in coastal Chile and Argentina and migrate northwards in spring, through the midcontinental United States, to breed in subarctic Alaska and Canada. For birds taking off from Chiloé Island in Chile, the first part of the journey takes them across the open Pacific Ocean, the relatively narrow landmass of Central America and the Gulf of Mexico. Godwits making this flight have few or no opportunities to stop, and they traverse several global wind regimes that differ in directionality and strength along the way. The authors predicted that birds would experience drift during their journeys, especially over the featureless open ocean, and increase compensation as they approach North America. Given the vast distances and time spent in flight, it might be expected that the winds experienced during flight should influence the points where individuals cross the coast into North American airspace.

Flock of Hudsonian Godwits on Chiloé Island in Chile – a long way from Alaska

Tracking transoceanic journeys

During the springs of 2019, 2020 and 2021, Jenny Linscott used satellite tracking devices to follow Hudsonian Godwits, as they migrated northward across more than 7,000 km between Chiloé Island, Chile and the northern coast of the Gulf of Mexico.

Prior to migratory departure, two types of solar-powered satellite transmitters were deployed on a total of 54 adults. Technical details are provided in the paper, alongside information as to how the data were filtered and interpreted. This paper focuses on the journey across the Pacific Ocean and the Gulf of Mexico, as birds head towards a relatively narrow refuelling area in North America (the eastern parts of Kansas, Nebraska, South Dakota, and North Dakota).

For any two points along each bird’s route, between Chiloe and the coast of the Gulf of Mexico, it was possible to calculate distance travelled, ground speed, turning angle, and heading. Having removed low-quality data, the team were left with 29 tracks and 689 locations with which to work. They linked each location with the range of possible wind conditions that godwits were likely to be experiencing at the time, depending upon flight height. They then “calculated the total magnitude and mathematical direction of the wind flow for each location at each altitude using vector trigonometry”. Previous studies have shown that migrating waders change altitude in order to find better wind conditions, allowing the team to assign the most likely wind conditions to each godwit location. Please see the paper for details of the modelling and assumption-testing processes.

Results

A total of 24 complete and 5 partial northward journeys were collected from birds migrating north, including repeat tracks from two individuals, which were followed for three years. The sample size was reduced by device failure/malfunctions, presumably some mortality, and by eight birds that oversummered in Argentina (see Teenage Waders and Gap year for sandpipers for information about oversummering shorebirds).  The 25 birds in the study comprised 14 males and 11 females.

• Godwits for which there were complete tracks undertook continuous flights lasting about six days, covering an average of 8,361 km, before making their first stops.
• Godwit ground speeds were best predicted by a strategy in which individuals flew at the altitude offering the most wind support in the preferred direction of movement, but were restricted to altitudes at or below 3000 m. It seems likely that the godwits were mostly flying between 100 m and 750 m above sea level.
• Tracked godwits travelled along paths which showed a close match to a Great Circle line from Chiloé to the North American target area.
• When flight behaviours were analysed, full compensation was the most frequent behaviour, accounting for 41.1% of all observed flight segments. Fewer segments were associated with partial compensation (23.5%), tail winds (8.1%), full drifting (9.4%), or overcompensation (16.0%).
• The prevalence of full compensation remained constant across wind conditions. For example, full compensation was the dominant behaviour under crosswinds to the east (32.0%) and west (45.3%). Full compensation was also prevalent across regions, comprising the largest proportion of behaviours exhibited over the Pacific Ocean (45.1%), while crossing Central America (23.9%), and in the Gulf of Mexico (31.4%).
• There was considerable variation in migration patterns over the Gulf of Mexico and it is suggested that some birds may have been running out of fuel and heading for the nearest land.
• One individual tracked repeatedly over three years completed its crossing of the Gulf of Mexico in central Texas every year. The other bird tracked for three years had no specific point at which it crossed the coast but always ended up in the same spring staging area.

Jenny Linscott and colleagues found little support for their prediction that godwits would tolerate more drift early in their flight and gradually begin to increase compensation as they approached North America. Instead, both fully supported flight (benefiting from tail winds) and full compensation were common soon after leaving Chiloé. Compensation did not increase with distance travelled, was not constrained during flight over open ocean, and did not influence where an individual ultimately crossed over the northern coast of the Gulf of Mexico, at the end of this flight. Instead, the team found a strong preference for full compensation throughout godwit flight paths. Birds ‘knew’ where they were along their route and could judge how to adjust their headings so as to compensate for the drift they were experiencing at any given time. The paper’s Discussion includes more detailed consideration as to how compensation appeared to operate in different parts of the journey and in different wind conditions.

How do shorebirds, flying over the vastness of an ocean ‘know’ where they are, with no island landmarks? Are individuals within migrating flocks picking up on changes in temperature or humidity, that mark passage through broad wind regimes, can they navigate with reference to sun position and stars, are there magnetic cues, or can they interpret surface swell patterns? Perhaps it’s a mixture of several of these skills? It’s pretty amazing!

Learning more

Jenny Linscott and her colleagues found that fully compensating for wind displacement appears to be a critical strategy for Hudsonian Godwits making a long-distance, transoceanic flight. While godwits often followed wind flow in the early stages of this journey, they nonetheless engaged in full compensation more frequently than any other behaviour during the entirety of the flight, across a vast and apparently featureless ocean. These continuous adjustments help to make sure that birds can fly extremely long distances without running out of energy. The team wonder how well future generations will cope with changing wind systems over warmer seas.

The full paper can be found here:

Compensation for wind drift prevails for a shorebird on a long-distance, transoceanic flight. Jennifer A. Linscott, Juan G. Navedo, Sarah J. Clements, Jason P. Loghry, Jorge Ruiz, Bart M. Ballard, Mitch D. Weegman, and Nathan R. Senner. Movement Ecology. https://doi.org/10.1186/s40462-022-00310-z

The Hudsonian Godwits studied here are flying 7,000 km north across the Pacific, but this pales into insignificance when compared to Bar-tailed Godwits that commute between Alaska and New Zealand. The Pacific was once considered a barrier to migration but it is increasingly seen as a conveyor belt. There is an excellent review article by Theunis Piersma and colleagues in Ornithology, the title of which explains its content:

The Pacific as the world’s greatest theater of bird migration: Extreme flights spark questions about physiological capabilities, behavior, and the evolution of migratory pathways. Theunis Piersma, Robert E Gill, Jr, Daniel R Ruthrauff, Christopher G Guglielmo, Jesse R Conklin and Colleen M Handel. Ornithology,  https://doi.org/10.1093/ornithology/ukab086

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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.

Chick squeaks

During the first few weeks of life, wader chicks rely upon their parents to take them to good feeding spots and to look out for potential predators. These youngsters will need to come when called and to freeze or hide when a crow flies over or a fox is on the prowl. A wader chick communicates with its parents too, so that they know where it is and can be alert to distress, but are there more subtle messages being communicated and why do male and female chicks produce discernibly different calls?

Kristal Kostoglou has been listening to the chicks of Red-capped Plovers and Southern Masked Lapwings in Australia and analysing recordings of their calls. Her research is presented in a 2022 paper in Ibis.

Handling waders

Broken-wing distraction display by parent Red-capped Plover

Anyone who studies breeding waders will be aware that parents get very anxious if a chick that is being ringed and measured starts making alarm calls – but how many people have recorded those calls? As Kristal Kostoglou says in the abstract of the paper she has written with her coauthors “We opportunistically recorded calls of chicks while they were in the hand and analysed the calls to determine whether call structure is related to sex or body mass (a proxy for age)”. The results are fascinating.

At the start of the paper, there’s a useful summary of previous studies of the calls made by chicks. It is believed that developmental changes in chick vocalisations can allow parents to assess chick age and/or condition, with evidence that calls get deeper (frequency drops) as chicks get larger. There have been a couple of previous wader studies, relating to Pied Avocet and Eurasian Stone-curlew. In the Ibis paper, Kristal Kostoglou investigated potential relationships of call traits to body mass and sex in chicks of two shorebird species; Red-capped Plover and Southern Masked Lapwing.

The two study populations

The calls of Red-capped Plover chicks were recorded in the Cheetham Wetlands (Victoria, Australia) and the Southern Masked Lapwings were studied on Phillip Island (Victoria, Australia). Parents brood and defend their chicks which fledge at about 35 days (plovers) and 45 days (lapwings) respectively. Nests were found during incubation and most chicks were ringed just after hatching. Each chick was measured & weighed, a recording was made, and a small amount of blood was taken, to establish sex.

Recording the sounds made by a Red-capped Plover chick, as it is weighed and measured

Sound recordings were made using a portable digital recorder and an omnidirectional microphone. The 26 plover chicks made between 1 and 248 calls each and the 95 lapwing chicks produced between 1 and 336 calls. Data were collected from 9 female plovers and 17 males, between from the day of hatching and approximately 4 weeks of age. For lapwings the equivalent figures were 46 females, 49 males and up to 5 weeks of age.

Six call traits were analysed: call duration, the time between calls, entropy, minimum dominant frequency (kHz); dominant frequency range (the difference between a call’s minimum and maximum dominant frequency); and frequency modulation. Please see the Ibis paper for more details and technical information about the analyses. This related paper may also be of interest: Anatomy of avian distress calls: structure, variation, and complexity in two species of shorebird.

Results

For Red-capped Plovers:

• Males were more vocal – time between calls was shorter for males than for females.
• Heavier (and hence older) chicks called more frequently.
• As mass increased, the dominant frequency range of calls decreased (calls became less shrill).

For Southern Masked Lapwings:

• For both sexes, dominant frequency range decreased with increasing body mass (calls became less shrill with age).
• The decline in dominant frequency range was greater in males, resulting in a lower dominant frequency range than for females. This meant that the difference between the calls of the two sexes became more discernible over time.
• Frequency modulation was lower for males than for females.
• As body mass increased, frequency modulation and entropy of lapwing calls decreased.

In the Discussion, the authors consider how a heavier bill and changes to the structure of the upper vocal tract might be linked to the results. It has been suggested that chicks modify sound output to utter more adult-like calls, as they get older.

Red-capped Plover chick

A faster repetition rate of distress calling, as observed in Red-capped Plovers, might encourage parents to provide more defensive support for male chicks, which could contribute to the higher survival of male over female chicks, as reported for several plover species.

For Southern Masked Lapwings, there appeared to be sex-linked differences in calls from hatching, with these differences getting more marked with age. This could mean that either the voice anatomy develops differently or that vocal control is different in male and female chicks.

Conservation implications

Southern Masked Lapwing chicks, at the point of hatching

Both Red-capped Plovers and Southern Masked Lapwings are considered to be of ‘least concern’, according to the IUCN/BirdLife conservation criteria. Many other waders, around the globe, are under various levels of threat, with predator pressure being a significant cause of decline for some species. The ability of adults and chicks to stay together through to the fledging of the chicks could be particularly important, in these cases, with communication being key to success.

Kristal Kostoglou’s study analysed distress calls, which she and others suggest may be under the influence of natural selection. She points out that some non-distress vocalisations, such as contact calls, might communicate further information about the caller’s sex. It is also possible that distress calls may serve to communicate with siblings or with other chicks, not just with parents. In the paper, the authors suggest that future studies could investigate associations between shorebird chick calls and sex, using the full repertoire of chick calls and across species whose adult call repertoires and characteristics vary between sexes. There were few recaptures of chicks in this study and the authors suggests that it would be interesting to observe how calls of individually-marked chicks change over time.

A passing thought

Head-started Black-tailed Godwit chicks have no contact with their parents

One issue that might be considered by others is how chick calls develop if there are no parents in attendance, as we see if chicks are head-started. There is a growing movement to support populations of threatened wader species by removing first clutches of eggs, incubating them and then rearing the chicks in captivity. In their pens, they have no contact with adults and communicate only with each other.

It is clear that head-starting has worked for Spoon-billed Sandpipers, a species that was heading for extinction as discussed here, and the early signs are good for England’s breeding Black-tailed Godwits but do these hand-reared individuals miss out in some ways? Potentially the ‘language’ developed within the family may be important when the chicks are themselves parents? That’s not going to be easy to test! However, perhaps it might be possible to see if there are any differences in the development of calls between hand-reared and parent-reared chicks of the same species?

Communication between parents and chicks helps to keep the family together

In summary

The study published in Ibis provides the first evidence for charadriid chicks of (a) a sex difference in call structure and rate and (b) gradual growth-related changes in call structure and rate, across chicks. The detailed write-up will hopefully be useful in further studies of shorebird vocalisations during growth, which may help further to explain the development and functional significance of all that squeaking!

Vocal traits of shorebird chicks are related to body mass and sex. Kristal N. Kostoglou, Edward H. Miller, Michael A. Weston and David R. Wilson. Ibis. https://doi.org/10.1111/ibi.13055

Red-capped Plover chick seeks to hide in the cracked mud, using the shadow to break up its outline

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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.