Formerly placed in genus Erolia. Specific name formerly listed as testacea, but ferruginea has priority. Form described as C. paramelanotos (“Cox’s Sandpiper”) shown to be a hybrid between present species and (almost certainly) C. melanotos; C. (Pisobia) cooperi (“Cooper’s Sandpiper”) probably a hybrid between present species and C. acuminata. Monotypic.

Recent monitoring data have shown that this widely distributed species has probably declined by 30-49% over the past three generations (15 years). The exact causes of declines are unknown, but are likely to include habitat loss and degradation (particularly on stopover and wintering grounds) and climate change impacts (particularly affecting breeding productivity), as well as disturbance and hunting.

The species breeds across Arctic Siberia from the Chosa Bay to Kolyuchinskaya Gulf (north Chukotskiy Peninsula) (Russia) (Lappo et al. 2012), and winters from sub-Saharan Africa through the Middle East and south and south-east Asia to Australasia (van Gils and Wiersma 1996).

Populations breeding in Western Siberia and wintering in West Africa are estimated to total are estimated to number 300,000-400,000 birds, while those breeding in Central Siberia and wintering in south-west Asia, East and southern Africa probably total an additional c.400,000 (van Roomen et al. 2022). A further 200,000 birds are estimated wintering in South Asia, although this may be an overestimate. The East Asian-Australasian Flyway population was estimated to be c.90,000 in 2016 (Hansen et al. 2016, 2022; Mundkur and Langendoen 2022), but has probably declined sharply since then. In total, the global population size is estimated at c.990,000-1,090,000 birds. To account for considerable uncertainty in the calculations of these estimates (with added uncertainty on the extent to which populations have declined since they were made), the total global population is placed in broad bracket of 700,000-1,200,000 birds. Approximately 60-80% are suspected to be mature individuals, leaving a population of 420,000-960,000 mature individuals.

This species breeds on slightly elevated areas in the lowlands of the high Arctic especially on southward-facing slopes, as well as along the coast and islands of the Arctic Ocean (Johnsgard 1981, del Hoyo et al. 1996). It shows a preference for open tundra with marshy, boggy depressions and pools (del Hoyo et al. 1996, Snow and Perrins 1998, Lappo et al. 2012) from melting permafrost and snow (Snow and Perrins 1998). The nest is a cup positioned on the margins of marshes or pools, on the slopes of hummock tundra, or on dry patches in Polygonum tundra (del Hoyo et al. 1996). Breeding success can vary year to year, with prime years yielding a higher number of juveniles (Australian BirdLife 2017). The Victorian Wader Study Group and Australasian Wader Studies Group for example recorded a higher percentage of juveniles (c. 48%) during 2016-2017 (Australian BirdLife 2017). In the winter the species chiefly occurs on coastal brackish lagoons, tidal mud- and sand-flats, estuaries, saltmarshes (del Hoyo et al. 1996, Snow and Perrins 1998), exposed coral, rocky shores and tidewrack on sandy beaches (Urban et al. 1986), and also inland on the muddy edges of marshes, large rivers and lakes (both saline and freshwater), irrigated land, flooded areas (del Hoyo et al. 1996), dams (Urban et al. 1986) and saltpans (Khomenko 2006). Abnormal rainfall in inland sites may additionally create temporary wetlands that provide additional suitable habitat (Geering et al. 2007, Dhanjal-Adams et al. 2018). On the breeding grounds the diet of this species consists mainly of insects, such as the adults, pupae and larva of Diptera (e.g. midges, craneflies (Johnsgard 1981)) and beetles, as well as bugs and leeches (del Hoyo et al. 1996). In the winter its diet consists of polychaete worms, molluscs, crustaceans (such as amphipods, brine shrimps and copepods), and occasionally insects and seeds (del Hoyo et al.1996). It is a full migrant, moving long distances by well-travelled routes (del Hoyo et al. 1996, Snow and Perrins 1998).

On the breeding grounds, breeding success of this species is apparently strongly correlated with lemming cycles (Underhill 1987, Summers et al. 1998) and it has been suggested (e.g. de Fouw et al. 2018) that recent falling numbers may be a reflection of collapsing lemming cycles, probably driven by climate change in the Arctic (Wauchope et al. 2017). Moreover, climate change has been identified for several shorebird species as a threat that is driving reduced breeding productivity by a number of mechanisms (e.g. Meltofte et al. 2007, Eikelenboom 2016, Kubelka et al. 2018); more research for this species specifically is needed.

The species is exposed to additional threats on its staging and wintering grounds, particularly habitat degradation and conversion. At Barr Al Hikman, Oman, where up to 9% of the Asian-East African Flyway has been reported to overwinter, planned urban developments, the aquaculture (for shrimp) industry, oil refineries and disturbance from fishing industries and falcon hunting were all identified as threats by de Fouw et al. (2018) and these almost certainly apply to other important staging and wintering sites in the broader region. The extent of hunting in this region is poorly known, but shorebirds are occasionally targeted (Brochet et al. 2016). The species is threatened on the south-east coast of India (Point Calimere) by illegal hunting (bird trapping), reservoir and marshland habitat alteration by salt-industries, and habitat degradation by diminishing rainfall (changing the salt regime) (Balachandran 2006). Rapid land use change and loss of mudflats are also major threats here (S. Jarwade in litt. 2020). It is also threatened at Walvis Bay in Namibia, a key wetland site in southern Africa, by habitat degradation (e.g. changes in the flood regime due to road building, and wetland reclamation for suburb and port development), and disturbance from tourism (Wearne and Underhill 2005). This species is susceptible to avian influenza (Melville and Shortridge 2006, Gaidet et al. 2007) and avian botulism (Blaker 1967, van Heerden 1974) so may be threatened by future outbreaks of these diseases.
Significant numbers of C. ferruginea migrate down the Yellow and East China Seas where habitat loss and hunting are plausibly significant threats. Astonishingly, the extent of reclaimed land along the Yellow Sea coastline now exceeds the extent of remaining intertidal mudflat (Murray et al. 2014, IUCN 2023), suggesting that this may have contributed greatly to declines in this species. The rate of habitat lost to land reclamation has slowed since a peak around 2013, in particular in recent years due to the promised near-ceasing of land reclamation in China in early 2018 (Melville 2018); however, recent satellite data suggests that the extent of intertidal mudflat has continue to decrease, in large part due to continued reclamations in the Korean Peninsula (IUCN 2023). Mudflats have also continued to degrade in suitability for this species and other shorebirds because of the invasive alien Spartina alterniflora cordgrass, which appears to be spreading; since 2020 this threat has probably driven greater declines than planned land reclamation. In addition to land reclamation, coastal development and a rapid increase in the human population along the coastline of the Yellow Sea has led to widespread degradation of mudflats and coastal habitats used by this species; IUCN (2023) (and references therein) cite the follows as additional drivers of mudflat loss and degradation: 'processes such as changes in sediment supply, loss of coastal vegetation associated with development...erosion, redistribution of sediments due to storms, and compaction and subsidence (sinking) caused by subsurface resource and groundwater extraction are also likely to be factors'. Offshore windfarms may have a so far unquantified impact on birds due to collisions (unlikely) and displacement and disturbance of feeding birds (more likely). Disturbance on mudflats by fishermen and others using the mudflats may also be reasonably considered a threat. Hunting is also possibly a significant threat impacting this and other shorebird species on passage (Gallo-Cajiao et al. 2020).

Hunting is a probable threat to this species, as the hunting of shorebirds across the East Asian-Australasian Flyway is not uncommon (see, e.g., Gallo-Cajiao et al. 2020, Yong et al. 2022). The extent of hunting in the Arabian Peninsula region is poorly known, but shorebirds are occasionally targeted (Brochet et al. 2016). This species was recorded in one of the seven trade datasets evaluated by Donald et al. (2024).

Conservation and Research Actions Underway
This species is not listed on the annexes of the European Union (EU) Birds Directive, but is covered by the general protection regime provided by Article 1 of the Directive to all naturally occurring wild species in the EU. It is listed on Annex II (strictly protected) of the pan-European Bern Convention, Annex II of the Convention on Migratory Species (CMS) and Annex II of the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA). Across the EU, 374 Natura 2000 sites have been designated for the conservation of this species on passage and in winter (EUNIS 2024). It is designated as a priority species for international conservation measures by the AMBI CAFF program of the Arctic Council. Concentrations of the species when breeding and on migrations are protected in the Ust-Lensky (Yakutia) and Daursky (Chita oblast) nature reserves, the Novosibirsk Islands federal nature reserve, and a number of regional protected areas in Yakutia. It is federally listed as Critically Endangered in Australia due to rapid population declines. Some habitats along flyway routes are protected. Active management is also underway at some Australian sites to combat cord grass invasion, as well as protective measures implemented to reduce disturbance at roost sites (Clemens et al. 2020).The Australasian Wader Studies Group, along with BirdLife Australia's Shorebirds 2020 Project and committed volunteers continue to monitor migratory shorebirds within the East Asian-Australasian Flyway (Vine and Maurer 2016). China is investing hundreds of millions of dollars to clear tidal mudflats of Spartina cordgrass, with the aim of eliminating 90% of it by 2025; locally, this has already been successful, for example at Chongming Dongtan National Nature Reserve (Stokstad 2023). The governments of China and South Korea have halted new projects that require or depend upon land reclamation, and across the Korean Peninsula the number of coastal wetlands has increased; moreover in China and South Korea, key areas for migratory shorebirds (including C. ferruginea) have been identified and prioritised by government and two World Heritage nominations have been made on these grounds (IUCN 2023). Although data on scale or impact are sparse, IUCN (2023) consider that "support for coastal wetland conservation and wise-use appears to have increased over the last decade". There has been a substantial increase in the number of volunteer groups and NGOs helping to monitor the migrations of shorebirds and other waterbirds, as well as increased media coverage and special events, demonstrating enhanced awareness among coastal communities (see also EAAFP 2016). North Korea became a Party to the Ramsar Convention and joined the EAAF Partnership in 2018. In 2021, the 'Regional Flyway Initiative' (RFI) was set up by the Asian Development Bank, with technical support from BirdLife International, with the aim of protecting and restoring priority wetland ecosystems and the associated ecosystem services they provide in the East-Asian Australasian Flyway (EAAF), the most threatened flyway globally. The Initiative is slated for implementation in 10 East, South and Southeast Asian countries:  Mongolia, China, Bangladesh, Viet Nam, Cambodia, Lao PDR, Philippines, Thailand, Malaysia and Indonesia, all of which are used by C. ferruginea on northbound and southbound migrations between breeding and wintering areas. The RFI will mobilise large-scale financing to support the protection, sustainable management and restoration of at least 50 priority wetlands across ten Asian countries, with an initial financing commitment of $3 billion from the ADB (BirdLife International 2022). Over time, the RFI aims to enhance and expand the existing efforts in conserving and managing priority wetlands identified on the basis of supporting globally significant congregations of migratory waterbirds, and  leverage on collaborative opportunities with stakeholders including national governments, civil society organisations, communities, regional organisations like the East Asian-Australasian Flyway Partnership.

Conservation and Research Actions Proposed
Conduct research to better understand the species' dependence on key migratory staging sites, and increase knowledge of the impacts of disturbance (Anon. 2015). Protect remaining intertidal habitats across the species' range (including the Yellow Sea) to prevent further habitat loss and degradation (Yang et al. 2011, Anon. 2015), and work with governments along the East Asian – Australasian Flyway to prevent destruction of key migratory staging sites (Anon. 2015). Manage important sites to reduce the impact of disturbance (Anon. 2015). Continue and expand monitoring schemes. Raise awareness of the species. Incorporate requirements for this species and others into coastal planning (Anon. 2015). Develop climate change adaptation plans and strategies (Clemens et al. 2020). Implement protection of Australian non-breeding habitats, involve indigenous owners in management of local land, and protect stopover sites (Clemens et al. 2020).