A moderately rapid to rapid reduction in the population of Lesser Yellowlegs is estimated to be taking place with three-generation reduction rates from migration count data exceeding 50% and Breeding Bird Survey (BBS) data for well-covered core regions estimating declines of 23%. There is evidence of potentially unsustainable harvest levels at migration and non-breeding sites which is the greatest current threat, though conversion of migratory stopover and non-breeding habitat may also be contributing to declines. Confidence in the precision of the estimated decline rate is uncertain, and a potentially significant and variable proportion of the population is not available for sampling by the migratory surveys. Moderately rapid reductions are also evident from eBird data and for the small proportion of the non-breeding season population covered by the Christmas Bird Count. Given the lower rates of reduction evident from other sources, the best estimate for the three-generation rate of reduction is considered to fall between 23% and 49%. The population remains large, estimated at 650,000 mature individuals and the range is very large. Based on the rate of population reduction, Lesser Yellowlegs is assessed as Vulnerable.

This migratory shorebird breeds from extreme eastern Siberia, Russia, through much of Alaska, United States of America (USA) south of the north slope, and through north central Canada from the coastal Northwest Territories, Yukon, northern British Colombia east to Hudson Bay and James Bay (Tibbitts and Moskoff 2020, Fink et al. 2023). The majority of the population spends the non-breeding season in south-eastern South American wetlands, including at high elevation in the Andes, and around the coast of northern and northeastern South America, though a proportion remain in southern USA, in the Caribbean and along the Pacific and Gulf coasts (Tibbitts and Moskoff 2020, McDuffie et al. 2022, Fink et al. 2023). Those breeding in eastern Canada stop off on southbound migration initially on the Atlantic coast of Canada, then many make a direct multi-day transoceanic crossing through the Caribbean to important areas along the north-eastern coast of South America. From here some continue to sites in Brazil, Argentina and Paraguay (McDuffie et al. 2022, Tibbitts and Moskoff 2020). Variable numbers from this source take a less oceanic route along the eastern seaboard of the USA at sites along the east coast of the United States (McDuffie et al. 2022). Those breeding further west in central Canada and Alaska pass through the Prairie Potholes region in the midwest of the USA before many again pass through the Caribbean and northern South America on their way again to the south-eastern South American wetlands, though especially those from Alaska pass through the Gulf of Mexico and Central American countries south along the Andes (McDuffie et al. 2022). During the pre-breeding northbound migration the Mississippi alluvial plain is a key stopover area for birds that winter in the south eastern South American wetlands, but many also stop in Mexico (McDuffie et al. 2022).

The global population is estimated to be at 650,000 mature individuals (Andres et al. 2012), though a modelling approach suggested 7.6 million mature individuals in Canada alone (BAM 2020). However, COSEWIC (2020) consider this likely overestimates true density because of birds approaching observers during point counts. But it is conceded that densities in the favoured habitat of 5.6 per square kilometre are similar to the 2-3 pairs per square kilometre generated by territory mapping surveys (Cooper 2004, COSEWIC 2020). The species is not covered by the Program for Regional and International Shorebird Monitoring (PRISM) surveys as it breeds further south, but the observation that these systematic surveys typically increased estimates significantly does suggest the current population size may be conservative.
Earlier estimates were substantially lower: Morrison et al. (2006) give a population estimate of 400,000 birds, with a range of 300,000–500,000.

Lesser Yellowlegs breeds around wet habitats in the boreal forest and taiga zone (Tibbitts and Moskoff 2020). All individuals migrate, many travelling to southern South America to winter in freshwater and saline wetlands, estuaries, saltmarshes and wet agricultural fields (COSEWIC 2020, Tibbitts and Moskoff 2020, McDuffie et al. 2022, Fink et al. 2023). Often occurs in human-made habitat during migration and in the non-breeding areas, including salt-extraction sites and sewage treatment lagoons (Tibbitts and Moskoff 2020). It is considered monogamous and breeding can commence from one year old; there is typically a single clutch of four eggs (Tibbitts and Moskoff 2020).
Feeds primarily on aquatic and terrestrial invertebrates, occasionally taking small fish and seeds (Tibbitts and Moskoff 2020).
Habitat loss and degradation, largely through conversion to agricultural use or intensification in the key areas highlighted (especially south-eastern South American wetlands and the north-eastern coast of South America) are inferred to be causing a continuing decline in available habitat and habitat quality (COSEWIC 2020). There has been a rapid increase in the proportion of the Argentine Pampas converted to annual crops, especially soy beans with a 45 % in cultivated area between 1990 and 2006 (Miñarro and Bilenca 2008, COSEWIC 2020).

The large distances travelled by this fully migratory species over the course of the annual cycle expose it to a variety of threats, from habitat shifting due to climate change in northern breeding areas to habitat loss and degradation and hunting during migration and in non-breeding areas. Possibly the most severe threat is due to unsustainable harvest, predominately during the post-breeding migration in the Caribbean and northern South America (AFSI 2016, 2020, COSEWIC 2020, Tibbitts and Moskoff 2020). A minimum of 18,316–46,940 individuals are estimated to be harvested annually (Rivera-Milán et al. 2023), based on the following values; Martinique 1,118–5,741; Guadeloupe 1,982–2,513; Barbados 6,100–9,150; Guyana 1,283–2,386; and Suriname 7,833–27,150. Additional take may occur in Brazil and French Guiana (Rivera-Milán et al. 2023). A preliminary Potential Biological Removal (PBR) estimate for the species was set at 79,450 ± 20,562 (Watts et al. 2015), but this has been updated using a Prescribed Take Level framework which estimates a lower range between 5.000 ± 1,945 (SD) individuals to 38,910 ± 13,238, suggesting that the species is indeed being overharvested (Rivera-Milán et al. 2023). For Guyana, Andres et al. (2022) found levels of hunting have likely decreased and at a minimum has not increased in the past two decades.
Reductions recorded in northern South America have at least in part been ascribed to a loss of suitable intertidal mud habitats following the clearance of mangroves in Suriname and Guyana (Lesterhuis 2021, Andres et al. 2022). Population level impacts are likely negligible, with individuals expected to relocate although this is expected to represent an overall reduction in available suitable non-breeding habitat. Coastal defence work has also reduced the area of suitable intertidal habitat within the very large range of the species (COSEWIC 2020), but it is not clear that the amount of this habitat is a limiting factor for the species. However there has been considerable loss and degradation of wetland habitat largely through conversion to agricultural use or intensification in other key non-breeding and migratory staging sites, especially south-eastern South American wetlands (COSEWIC 2020). Much has been lost to the planting of crops, for example there has been a rapid increase in the proportion of the Argentine Pampas converted to annual crops, especially soy beans with a 45 % in cultivated area between 1990 and 2006 (Miñarro and Bilenca 2008, COSEWIC 2020).
Climate change is thought to be causing significant changes to the breeding habitat that may already be affecting reproductive success, principally through the drying of wetlands (Carroll et al. 2011). There may also be a temporal mismatch in peak prey abundance and breeding timings as noted for other boreal nesting waders which may impact reproductive output (Galbraith et al. 2014), but this has not been demonstrated to affect the species as yet.
Native predators, notably Red Fox Vulpes vulpes, have expanded their range and are now more abundant within the breeding range of Lesser Yellowlegs (COSEWIC 2020). It is suspected that rates of nest predation and potentially rates of adult mortality during breeding have increased as a result, but there is no evidence of this as yet and the severity of the threat is considered unknown (COSEWIC 2020).

A minimum of 18,316–46,940 individuals are estimated to be harvested annually (Rivera-Milán et al. 2023), based on the following values; Martinique 1,118–5,741; Guadeloupe 1,982–2,513; Barbados 6,100–9,150; Guyana 1,283–2,386; and Suriname 7,833–27,150. Additional take may occur in Brazil and French Guiana (Rivera-Milán et al. 2023). A preliminary Potential Biological Removal (PBR) estimate for the species was set at 79,450 ± 20,562 (Watts et al. 2015), but this has been updated using a Prescribed Take Level framework which estimates a lower range between 5,000 ± 1,945 (SD) individuals to 38,910 ± 13,238, suggesting that the species is indeed being overharvested (Rivera-Milán et al. 2023). For Guyana, Andres et al. (2022) found levels of hunting have likely decreased and at a minimum have not increased in the past two decades.

Conservation Actions In Place
Protected in U.S. and Canada under the 1927 Migratory Bird Treaty Act. Hunting is illegal in many countries within the species range, though there is allowance for subsistence hunting in Brazil and for indigenous people in Canada and the USA (AFSI 2020). In Trinidad and Tobago a ban on all waterbird hunting was enacted in 2019 (AFSI 2020). On Martinique a bag limit (of 20 individuals per day) was imposed for the first time in 2020 (Birds Caribbean 2021). Harvest in Barbados was limited to 10,000 birds annually from 2012 as a result of initial findings into hunting pressure (Wege et al. 2014).
Migratory stopover habitat is largely identified and protected under the Western Hemisphere Shorebird Reserve Network (WHSRN), although this offers no legal protection. 
The Atlantic Flyway Shorebird Initiative (AFSI) has identified and undertaken a series of measures to address unsustainable harvest in the Caribbean and northern South America (AFSI 2016, 2017, 2020). An initial Potential Biological Removal estimate was generated (Watts et al. 2015) and subsequently a revision using a Prescribed Take Framework has been developed (Rivera-Milán et al. 2023). A preliminary estimate of the harvest in Guyana has been made (Andres et al. 2022), allowing an assessment of annual harvest level against likely sustainable take (Rivera-Milán et al. 2023).
Lesser Yellowlegs is monitored via the North American Breeding Bird Survey (BBS) (Ziolkowski Jr. et al. 2022) although this covers only a small proportion of the breeding range; by the Christmas Bird Count (Meehan et al. 2022) which does not cover the main wintering areas; by the International Shorebird Survey during migration (Brown et al. 2001, Smith et al. 2023); and partly covered by the International Waterbird Census during the non-breeding season (Wetlands International 2023).

Conservation Actions Needed
Improved monitoring of key non-breeding sites is required.
The biological and social aspects of harvest across the countries for which there is significant harvest in the species need to be fully assessed (AFSI 2016). In particular further research into harvest levels is needed in Suriname, Brazil, French Guiana, Guadeloupe and Saint-Martin and Saint Pierre and Miquelon (AFSI 2020). Increase communication among stakeholder organizations and agencies to develop a regional approach to managing shorebird harvest (AFSI 2016). Efforts are needed to convince national governments in key countries to invest in cooperative shorebird conservation efforts, linked to multinational agreements to secure the status of migratory species. The provision of sufficient salary, equipment, travel, capacity, and training is needed to effectively enforce existing laws and policies where shorebird harvest is illegal. Where harvest is legal, enforcing seasonal regulations and monitoring voluntary compliance is required. Based on assessment results from areas where shorebird harvest is legal, regulations and policies addressing harvest management components, such as the number of licensed hunters, daily and seasonal bag limits, and season timing and duration, may need to be developed to ensure a sustainable harvest. Harvest of shorebirds of conservation concern may also be restricted through enactment of policies, regulations, domestic legislation, or international agreements (AFSI 2016). Consult with the Barbados Wildfowlers Association on further lowering bag limits for the species, and delaying the start of the season to reduce the take of adult birds (Wege et al. 2014). Establish and maintain No-Shooting Reserves by purchasing and restoring defunct shooting swamps on Barbados and private wetlands on other islands or by establishing no-shooting reserves on public lands: if the latter are established there is the need to devise means support their long-term maintenance (AFSI 2016).
Biological and social harvest assessments can also provide information to develop potential incentive schemes to reduce hunting, where appropriate, through promoting economic alternatives or identifying substitute food sources (AFSI 2016).