Many Eurasian Curlew migrate directly to their wintering grounds to moult, where they remain until return migration commences (Bainbridge & Minton 1978). As such, staging sites appear to be uncommon (staging sites defined here as per Warnock 2010, as sites with abundant, predictable food resources whereby birds prepare for an energetic challenge requiring substantial food stores and physiological changes e.g. crossing an ocean or mountain chain). However, birds are known to moult in the Wadden Sea before continuing westwards.
Stopover sites may be frequently used by some populations (defined here as per Warnock 2010, as sites used for a short length of time, subsequent to relatively short subsequent flights to their next step, and relatively low fuelling rates and fuel loads). Birds migrating westwards through Sweden are known to occasionally stopover at inland sites and alpine meadows, then coastal sites in Norway, before migrating to the British Isles. These sites are used for short periods of time and their overall migration to the British Isles is rapid (Adriaan de Jong, pers. comm.). Birds that migrate to West Africa pass through France and Iberia.
1.6.4. Winter habitat selection and use
Outside of the breeding season, the species frequents a variety of coastal and inland habitats. The majority are found in coastal areas, where large estuarine mudflats are the preferred habitat (Lack 1986). Lesser-used coastal habitats include sandflats, rocky and sandy beaches with pools, mangroves, saltmarshes, coastal meadows and the muddy shores of coastal lagoons (Johnsgard 1981,Snow & Perrins 1998). Substantial numbers also forage in adjacent grasslands at high tide. Whilst most birds winter on the coast, in certain regions, especially Europe, significant numbers also winter inland (Delany et al. 2009) where the shores of inland lakes, riverbanks, inland grasslands and arable fields are all used (del Hoyo et al. 1996). A UK study suggested that habitat selection may change from coastal to inland sites in response to the onset of hunting (Bainbridge and Minton 1978). Several studies have noted the tendency of short-billed birds, predominately the smaller males, to feed on pastures whilst longer-billed birds, predominately females, feed on mudflats (Cramp & Simmons 1983) where they often form territories up to 1 hectare in size (Cotter 1990). Inland birds are not territorial and frequently feed in flocks, enabling less time looking for predators (Cotter 1990).
The Eurasian Curlew is omnivorous. The following account is mostly adapted from Cramp and Simmons (1983) unless otherwise stated: A range of invertebrate prey items are taken at coastal sites. Polychaetes comprise an important part of the diet, with Nereis, Cirriformia, lugworm Arenicola and sand mason worms Lanice all foraged from the littoral zone. So too are a variety of crustaceans, including crabs Carcinus, shrimps Crangon and amphipods such as Corophium, Gammarus, Bathyporeia and Orchestia. Also included are bivalve molluscs including peppery furrow shell Scrobicularia, clams Macoma and Mya, mussels Mytilus and cockles Cardium. Occasionally small fish are taken, such as the common goby Pomatoschistus microps.
Inland, during the breeding season, the adults and larvae of several beetle Coleoptera families are eaten, such as the ground beetles Carabidae, clown beetles Histeridae, rove beetles Staphylinidae, water scavenger beetles Hydrophilidae and scarab beetles Scarabaeidae. Adults and pupae of various fly Diptera species are taken; importantly the larvae and pupae of crane flies Tipulidae, and various Muscomorpha species. Other invertebrates include earthworms Lumbricidae, grasshoppers and locusts Acrididae, crickets Gryllidae, earwigs Dermaptera, bugs Hemiptera, the larvae of Lepidoptera, caddisflies Trichoptera, dragonflies Odonata, mayflies Ephemeroptera, antsFormicidae, freshwater shrimps Gammarus, woodlice Isopoda and spiders Araneae. Occasionally, vertebrates may be taken, including small fish, frogs Rana and toads, lizards, young birds and occasionally eggs, and small rodents. On wintering grounds, curlews will frequently attempt to steal food from each other and other waders (Cotten 1990).
A variety of plant material is also included in the diet, including mosses Bryophyta, horsetails Equisetum and sea lettuce Ulva. In late summer the berries of various shrubs are consumed, such as bilberry Vaccinium, crowberry Empetrum, blackberry Rubus and cranberry Oxycoccus. The leaves and grain of cereals and grasses Gramineae are also taken.
1.8. Survival and productivity
1.8.1. Population modelling
Grant et al. (1999) described a stable population model for the UK, which assumed (1) zero immigration, (2) breeding at three years old, (3) survival from fledging to 1-year old of between 50-65%, (4) annual adult survival of 88% and (5) annual survival rate from 1-year old to breeding also of 88%. This model requires annual productivity in the region of 0.48-0.62 fledged young per breeding pair.
Valkama and Currie (1999) calculated that 0.79 fledged young per breeding pair was required for their Finnish study population, assuming (1) breeding at 2 years, (2) first-year survival of 47% and (3) second year survival being similar to adults.
Similarly, Roodbergen et al. (2012) described a stable population based on (1) adult survival of 70–90% and (2) juvenile survival of 0.35-0.55 (both taken from Klok et al. 2009) that would require 0.7-1.6 fledged young per breeding pair.
Another productivity study for a population in Westphalia, Germany, suggested 0.41 fledged young per breeding pair was required for population stability (Kipp 1999).
1.8.2. Adult survival, juvenile survival and longevity
Several studies have explored adult and juvenile (post-fledging) survival. Survival is broadly similar between the sexes (Berg 1994) but is lower in juveniles (Bainbridge & Minton 1978). In Finland, adult survival was estimated at 84.4% during 1995-1996 for both sexes combined (Valkama & Currie 1999) which was similar to estimates in Sweden of 82.1% (Berg 1994). Estimates from one UK study placed first-year, post-fledging survival at 47% (lower than other wading birds that have been studied), second-year survival at 63% and adult survival at 73.6% (Bainbridge & Minton 1978).
A more recent UK study, based on long-term ringing data in Wales estimated an average annual survival rate of 89.9% from 1974-2011; a period encompassing both prior to and following a hunting ban in 1982. Estimated adult survival increased slightly following the ban (from 86.9% (se=0.04) to 90.5% (se=0.01)) whilst longevity increased by at least 40%; from 8 years (range 5-10) for a bird hatched in 1974 to 16 years (range 9-22) for a bird hatched in 1982 (Taylor & Dodd 2013).
The same study estimated that the mechanised cockle harvesting which occurred in the winter of 1996/97 had reduced apparent survival from 95% (se=0.07) to 81% (se=0.19) for the two years following dredging. Assuming that this represented an actual impact on survival (and not emigration from the estuary study site) the reduction in longevity was estimated to be considerable; a 5 year (39%) reduction from the pre-dredging estimate of 18 years (Taylor & Dodd 2013).
The longevity record is 32 years (Robinson 2005).
Several studies have shown productivity to be below the threshold required for a stable population, with productivity across Western Europe and Fennoscandia averaging 0.34 fledged young per breeding pair (Taylor & Dodd 2013; Roodbergen et al. 2012).
At two study sites in Northern Ireland, productivity was estimated to be 0.14-0.26 and 0.20-0.47 (Grant et al. 1999). This was deemed sufficiently low to account for the 58% decline in the breeding population recorded between 1987-1999 (Grant et al. 1999, Henderson et al. 2002). Similarly, a 4-year study over 18km2 of arable farmland in southwest Finland found overall productivity to be just 0.32, and concluded this was likely to be responsible for the observed 23% decline in the study area’s breeding population (Valkama & Currie 1998). Mean productivity was estimated to be 0.25 across mixed and arable farmland in Sweden, and again deemed too low to maintain a stable population (Berg 1994). Similarly low productivity has been recorded in Germany; between 0.28 and 0.53 between 1977 and 1986 (Dornberger & Ranftl 1986) and between 1977 and 1990 productivity was recorded at 0.32 in the Upper Rhine Valley (Boschert & Rupp 1993). Between 1991 and 2003 this was as low as 0.05 (Boschert 2004).
1.8.4. Nest survival and causes of nest and chick loss
Nest survival does not appear to be influenced by vegetation height around the nest, nor to clutch laying date (Grant et al. 1999). Like most waders, the Eurasian Curlew typically lays 4 eggs (range 1-7) often 1.5-1.8 days apart (Mulder & Swann 1992, Berg 1992b). Incubation typically lasts around 30 days and starts after the laying of the 3rd or 4th egg (Grant et al. 1999) thereby ensuring that all eggs hatch around the same time.
Nest failure rates are particularly high during the egg-laying period. In four out of five study site years in Northern Ireland, less than 50% of nests survived through to clutch completion (Grant 1997). Studies have shown the frequency of replacement clutches to be highly variable (Valkama & Currie 1999, Berg 1992, Grant et al. 1999). For instance, 76% of first clutch failures were replaced in the Swedish study (Berg 1992) compared to 0% of 10 well monitored nests on the Orkney Islands, UK (Grant et al. 1999). There is evidence to suggest replacement clutches only occur when nest failure occurs during egg-laying or early incubation, and replacement clutches are thought to be less productive (Valkama & Currie 1999). Seven days typically elapse between nest failure and replacement clutches (Grant et al. 1999).
Even successful clutches still suffer partial losses. Grant et al.’s study (1999) found these partial losses result from a variety of sources, including infertility (42.5%), predation (40%), desertion prior to last egg(s) hatching (17.5%) and trampling (10%). As a result of these partial losses, successful nests produced an average of 2.75±0.25 hatched young at the mainland study site in Antrim and 3.02± 0.17 at the island site of Lough Erne.
Several studies have found predation to be the largest source of nest failure. At the Northern Irish study sites, between 1993 and 1995, only 3.6-19.0% of all nests hatched, and nest predation accounted for 85-97% of these failures. A German study found 70 out of 136 nests between 2001-2005 to have been predated (Boschert 2004, 2005) whilst a recent unpublished study in Germany recorded 66% nests lost due to predation (n=35), and three due to desertion and agricultural practices (Natalie Busch, pers. comm.). In Valkama & Currie’s study in Finland (1999), 68% of nests failed to reach hatching with nest predation accounting for 81% of failures. Mainly all nest predation events were attributed to mammals (Red Fox Vulpes vulpes, Racoon Dog Nyctereutes procyonoidesor European Badger Meles meles). Other sources of nest loss in Valkama & Currie’s (1999) study were desertion (3%) and spring farming operations (16%; although nests in arable fields were cane marked, and it was accepted that loss to agricultural operations is higher).
Predation also appears to be the largest source of chick mortality, however less studies have been conducted. In the Northern Irish study (Grant et al. 1999), estimated chick survival (measured from hatching to 31 days of age) ranged from 19.1-38.5% across three study site years, with predation accounted for 74% of chick mortality. Chick survival has also been estimated at 38.3% (Cramp & Simmons 1983) and 20.3% (Boschert & Rupp 1993).
Whilst predation has emerged as the main proximate cause of both nest and chick failure, since the majority of failed breeding attempts occurred during the nest stage, nest predation is thought to be the greatest factor limiting productivity (Grant et al. 1999). An important finding from the Finnish study was that whilst 64% of nests were depredated in the ‘fragmented’ Vammala study site, only 5% were depredated in area of ‘continuous’ Kauhava study site (Valkama et al. 1999).
1.8.5. Chick survival and post-hatching movements
Chicks fledge at approximately five weeks and become independent soon after: it takes over a week between their initial flights and full aerial vigilance (Adriaan de Jong, pers. comm.). Adults and chicks will move around their home range using different foraging habitats. Of 18 broods studied in the North Pennines, UK, the mean maximum distance recorded from nest sites was 374m (Grant 1997). A study in Sweden found movements up to a maximum of 1.5km from the nest site (Berg 1992b).
Table 1: Summary of population estimates from Waterbird Population Estimates 5 (Wetlands International 2012).