1 Introduction
1.1 Farmland waders
Oystercatcher, Lapwing, Curlew, Redshank and Snipe are birds within the order Charadriiformes and are commonly referred to as farmland waders, due to their usage of agricultural land to ground nest, often in plain site. This nesting strategy requires that a nesting wader continuously surveils their environment for predators. If the nest is threatened, the nesting birds will immediately take flight, relying on the fact that eggs within the nest are cryptically marked so as to avoid detection by predators. A short description of each farmland wader and it’s conservation status is summarised below:
Euraisian Oystercatcher Haematopus ostralegus - primarily lives at the coast, where it also breeds and feeds on bivalves and gastropods. Has over the last 50 years started to breed in-land on farmland, where it’s primary food source is earthworms and insect larvae. Oystercatcher have strong mate and site fidelity, where a single nesting attempt is made per breeding season. They typically lay their eggs in April and unlike other farmland waders, they will sometimes nest close to man made habitat such as buildings or on top of drystone walls. Nesting on flat gravel roofs was first noted in Aberdeen in the 1960s, with these birds often feeding in urban parks and on lawns. Unlike other waders, adults can take food to their young rather than requiring the young to move to areas where they can feed themselves; a feature which enables rooftop nesting. The current IUCN Red List status for the Oystercatcher is near threatened (BirdLife International 2019).
Northern Lapwing Vanellus vanellus - outside of the breeding season, they may overwinter in large flocks in-land as far south as northern Africa. Lapwing feed on insects and earthworms, and have a preference for feeding at night in order to avoid predators. As the Lapwing has a short bill, they can only forage for food near the surface of the soil. They will therefore typically nest in areas that are invertebrate rich. Lapwing often form loose colonies when nesting, that can together drive away predators. Their chicks typically hatch in early May, with replacement clutches typically in June if the first hatch failed. The current IUCN Red List status for the Lapwing is vulnerable (BirdLife International 2017).
Eurasian Curlew Numenius arquata - Scotland’s largest waders generally spend their winters near the coast and return to their Shetland breeding territories to hatch their chicks in early June. This later breeding period than Oystercatchers and Lapwings is associated with a requirement for the nest to be in tall vegetation (c.15cm tall or more). The Curlew can be migratory, often over wintering in Africa or Southern Europe. Though in the milder climate of Scotland it can be resident all year round. The current IUCN Red List status for the Curlew is vulnerable (BirdLife International 2015b).
Common Redshank Tringa totanus - perhaps the least common of the farmland waders. They spend the winter at the coast, particularly on mudflats and estuaries and nest most abundantly on wetland, especially coastal saltmarsh and machair. Pools of shallow standing water are particularly attractive to nesting Redshank, as they feed on the small invertebrates that they support. Their chicks hatch in late May. They are a migratory species, over wintering on Atlantic coasts south of Ireland and Great Britain. The current IUCN Red List status for the Redshank is least concern (BirdLife International 2015c)
Common Snipe Gallinago gallinago - Snipe nest in dense cover of rushes and sedges within wetland habitats, as well as in wet scrapes and bogs in moorland habitats. They feed on invertabrates in areas of shallow standing water. They reside in farmland wetlands throughout the year. Snipe are often difficult to observe and often are only detectable by listening out for the drumming that results from their display flights. The current IUCN Red List status for the Snipe is least concern (BirdLife International 2015a)
1.2 Declines in wader populations
Wader populations are declining across the world (Butchart et al. 2010), and farmland waders are one of the highest wildlife conservation priorities on Scottish farmland. The loss and degradation of natural and semi-natural breeding habitats through changing land use, particularly agricultural intensification, changes in cropping and grazing management is believed to have caused significant declines in farmland wader populations (Thorup 2006). It is generally accepted that the main cause of this reduction lies in low breeding productivity which in-turn is most likely due to agricultural intensification (Wilson, Ausden, and Milsom 2004). A recent paper (Bell and Calladine 2017) reported results of a 25 year study (1990 to 2015) on a typical Scottish farm north of Stirling (a survey area of 7.5km\(^2\)). The authors explain how over this time Oystercatcher have declined by -95%, Lapwing by -88%, Curlew by -67% and Redshank by -87%. The main reason cited for the declines were changes in agricultural practices relating to crop cycles and grazing. This declining trend is reinforced by the annual BTO Breeding Bird Survey for 2017 (Harris et al. 2018). Over a similar time scale as the Stirling study, the UK wide trend between 1995 and 2016 has seen declines for Oystercatcher of -23%, Redshank -41%, Lapwing -42% and Curlew -48%. Snipe were the only species to increase over this time period, by +26%.
1.3 Breeding wader abundance response to farmland habitat
The suitability of farmland habitat for breeding waders is likely to be influenced by the historic conversion of the land from more heterogeneous semi-natural grassland to agriculturally improved grassland through soil drainage, liming, fertilization and re-seeding. These practices can result in a structurally uniform sward with reduced invertebrate and earthworm density (Vickery et al. 2001), though improvement through fertilization and liming (McCallum et al. 2018) may increase the availability of some soil invertebrates. Lowland wet unimproved grassland can provide suitable nesting sites and are a good source of soil invertebrates (Smart et al. 2008), as well as foraging sites for birds breeding on nearby unenclosed moorland (Brown et al. 2015), particularly if fields have high densities of soil invertebrates.
Agriculture is generally thought to depress biodiversity but in some cases it can be beneficial. It has been postulated that there is a beneficial relationship between agricultural improvement of semi-natural land and species alpha diversity (Jóhannesdóttir et al. 2019), but that this relationship is unimodal. Agricultural practices such as liming, spreading animal manure on fields and hay cropping may increase habitat and resource heterogeneity, together with enhancing soil health. This leads to increases in alpha diversity, but beyond a critical level of intensity, alpha diversity quickly falls away due to the habitat homogenisation associated with more intensive and extensive cultivation. For example, a study on the interacting effects of agriculture and landscape on breeding wader populations in Iceland (Jóhannesdóttir et al. 2019) concludes that low intensity agriculture can have a positive effect on breeding wader populations in upland areas where food resources are scarce, but in coastal lowlands wader numbers were seen to decline with increasing cultivation of the land. Further, the proximity and extent of lowland wetlands within Iceland were a particularly important driver of farmland breeding wader abundance. A study to examine how environmental covariates across the UK are associated with breeding Curlew (Franks et al. 2017) found that arable farming, generalist predators and woodland cover were negatively associated with Curlew abundance. The same study showed that Curlew abundance was positively associated with the extent of protected area coverage, gamebird numbers, cooler seasonal temperatures and higher summer rainfall.
A key requirement for breeding waders on farmland is the type of habitat and sward height. Table 1.1 summarises typical habitat and sward height that farmland waders typically prefer for their nesting site.
| Wader | Breeding habitat | Sward height | Breeding season |
|---|---|---|---|
| Oystercatcher | Grassland or arable fields | Short | April - July |
| Lapwing | Grassland, spring crops, fallow land | Bare/short | April - late July |
| Curlew | Heathland, grassland and bog | Long | May - late July |
| Redshank | Wet grassland, wet scrapes | Long | May - late July |
| Snipe | Wetland, standing water | Long | April - July |
With regards to the categorisation of sward height:
- bare - ploughed land with no emergent vegetation. This is typical of spring sowed crops
- short - managed (improved) grassland for grazing, or for mowing to make hayage or silage. Sward is typically less than 5cm tall.
- long - unimproved grassland, fallow pasture, heathland, marsh or wetland. Sward is typically 15cm or more.
1.4 Breeding wader abundance response to landscape composition and scale
The size and composition of a landscape is key to sustaining a long term population of a bird species, especially one that has experienced a recent decline (Macdonald 2019). If a suitable landscape exists at the necessary scale, it will provide security against localised adverse effects such as starvation, predation, inbreeding and bad weather. This in-turn will allow populations, whose number is above a certain threshold, to survive major short-term fluctuations, in abundance (Melbourne and Hastings 2008). Understanding how abundance of breeding waders respond to the scale and composition of suitable habitat could be a major factor in helping to conserve them.
1.5 The Shetland archipelago
Shetland is a subarctic archipelago located in the North Atlantic between Scotland, Norway and the Faroe Islands. The total area of the islands are some 1,466 km\(^2\) and they experience an oceanic temperate maritime climate.Due to the relatively harsh climate, short growing season and poor soil quality, Shetland has relatively low agricultural intensification when compared to the rest of the UK. The traditional crofting approach to farming has created farmland characterised by a mosaic of low intensity cropping and grazing. These factors have helped to create a hotspot for farmland breeding waders; the most recent estimates of the Shetland breeding population are from surveys carried out in the late 1990s: 2,300 pairs of Curlew, 1,740 pairs of Lapwing, 3,350 pairs of Oystercatcher, 1,170 pairs of Redshank and 3,450 pairs of Snipe (The Birds of Shetland 2004).
1.6 Study objective and hypotheses
The objective of this study is to quantify breeding wader abundance response to various environmental covariates, across the archipelago of Shetland. If agricultural practices and landscape scape changes are a significantly associated with the decline in farmland wader populations, Shetland’s lower intensity farming and remoteness, should generate a different response in breeding wader population trends than that seen on the UK mainland. The study will investigate how habitat, topography and landscape pattern are associated with breeding wader abundance on Shetland. Whilst there have been numerous UK studies on breeding wader abundance and habitat associations, none have examined associations across Shetland using a long term dataset. For this study, data from the Shetland Breeding Bird Survey (BBS) from 2002 to 2019 was used (ref), together with a number of spatial datasets that represent environmental covariates.
A number of hypotheses were formulated a priori as outlined in Table 1.2. Here predictions are made as to how the abundance per unit area (density) of breeding waders is associated with a particular covariate. Through the methods of the study, each individual hypothesis was tested for statistical significance.
| ID | Hypothesis | Wader Density |
|---|---|---|
| H1 | Farmland that is managed as improved grassland for silage has a higher pH, possibly due to liming and fertilisation, which in turn is associated with higher earthworm density (McCallum et al. 2016) | Positive |
| H2 | Earthworm density is negatively associated with high organic carbon content within the topsoil, due to increased soil acidity (McCallum et al. 2016). | Negative |
| H3 | Earthworm density is positively associated with higher topsoil pH (between 5.0-7.4), as common earthworm species are acid intolerant. (McCallum et al. 2016) | Positive |
| H4 | Heathland is associated with insects such as Crane fly and their larvae (Tipulidae spp.), and are a major food source for wader chicks | Positive |
| H5 | Semi-natural grasslands are associated with both earthworms and invertebrates, and are a major food source for wader chicks and suitable nesting sites | Positive |
| H6 | Due to a lack of available food sources, peat bogs are not a key habitat for breeding waders | Negative |
| H7 | Higher elevations are associated with lower food source availability per unit area, due to weather induced oligotrophic habitat such as peatland | Negative |
| H8 | Coastal predators such as otters and gulls are likely to reduce breeding wader productivity, so distance from the the sea is positively associated with productivity. Coastal locations are possibly increasingly exposed to extreme weather events during the breeding season | Positive |
| H9 | Bare peat in peatland habitats reduces the extent of suitable nesting and feeding habitats | Negative |
| H10 | Available water capacity within the soil is an indicator of the degree of wetness of the soil which is positively associated with access to invertebrates in the topsoil | Positive |
| H11 | Habitat with EUNIS classes D, E and F with high thematic complexity indicates high habitat heterogeneity, which is associated with higher diversity oand availability of nesting and feeding habitats in close proximity | Positive |
| H12 | Habitat with EUNIS classes D, E and F with high spatially aggregation indicates high habitat homogeneity, which is associated with lower proximity of both nesting and chick feeding habitat | Negative |
Having quantified and evaluated the response of breeding wader abundance against the study hypotheses as outlined in Table 1.2, a population model will be developed to quantify the populations and spatial distribution of farmland breeding waders from 2002-2019, acros Shetland (excluding Foula and Fair Isle).