Introduction

The Lesser White-fronted Goose Anser erythropus is the smallest of the so-called “grey” geese of the genus Anser (BirdLife International 2018). Excluding threatened taxa, grey geese are traditionally used for subsistence and sport hunting in Eurasia. Arctic nations especially continue to consider geese as a sustainable source of fresh meat in spring. However, hunting bans in many European countries, Republic of Korea and Japan have allowed the various species of grey geese to become part of agricultural landscapes. In contrast, several species of grey geese in China prefer to winter on wetlands with typically low levels of human use, rather than exploiting agricultural lands that are densely populated by people and their livestock (Denget al. 2018). Since 1994, following rapid population reduction,A. erythropus has been globally list as Vulnerable in the IUCN Red List (BirdLife International 2018).
Three sub-populations can be distinguished: Fennoscandian, West Asian, and East Asian, with potential overlap of the breeding grounds between the West and East Asian sub-populations (Jones et al . 2008). Aarvak and Oien (2018) note that the Fennoscandian sub-population appears on the brink of extinction with only 30-35 pairs left, despite captive breeding and restocking in Finland and Sweden during 1981 – 1999 (Ruokonen et al . 2000; Andersson and Holmqvist 2010). The number of the West Asian sub-population assessed from counts at stop-over sites during autumn migration has risen from an estimated 10,000-21,000 in early 2000s (Fox et al . 2010) to 30,000-34,000 in 2015 (Cuthbert and Aarvak 2016) and perhaps as high as 48,580 ± 2,820 in 2017 (Rozenfeld et al . 2019). However, this increase could be attributed to additional survey efforts for A. erythropus at previously infrequently or un-visited staging sites in Kazakhstan. The most recent estimate of the East Asian sub-population is 14,000-19,000 individuals (Jia et al. 2016), accounting for around 25% of the global A. erythropus population (Jia et al . 2016 and Rozenfeld et al . 2019). The eastern sub-population of A. erythropus extends from the Taymyr Peninsula eastward to Chukotka region (Morozov 1995; Morozov and Syroechkovski -Jr 2002; Lei et al . 2019a), and in common with other subpopulations, is declining (BirdLife International, 2018). A range of threats, including habitat loss and degradation along the migration route and on the wintering grounds proposed to fragmentation of the formerly continuous breeding range, have all been identified being responsible for past population declines (Madsen et al . 1984; Grishanov 2006; Morozov 2006). In addition, illegal and accidental hunting (i.e. the genuine confusion with the similar looking Greater White-fronted Goose A. albifrons , a species that can be hunted legally in Russia) are also threats to population viability.
Quantitative knowledge of a species spatial distribution is the cornerstone for its effective conservation. Due to the remoteness and restricted accessibility, historical observations of the summer range of the East Asian sub-population are rather scarce (Ruokonen et al . 2004, Morozov 1995; Morozov and Syroechkovski -Jr 2002; Lei et al . 2019a) Further, there are no systematic surveys covering the potential range of eastern sub-population of A. erythropus (Fig. 1). Current knowledge on the breeding distribution and habitat preference of A. erythropus is therefore limited (Egorov and Okhlopkov 2007, Solovieva and Vartanyan, 2011, Degtyaryev et al.2014). In the last 25 years, ornithologists generally considered that the East Asian A. erythropus had a patchy breeding distribution, and the number, position and shape of those areas changed as new knowledge was acquired from occasional visits to remote sites in East Siberia as illustrated in Figure 1. Furthermore, an intensive multi-year survey in the area adjacent to the breeding grounds along the Rauchua River, West Chukotka, helped locate a number of breeding/molting groups and separated broods, suggesting that the entire survey area was populated by A. erythropus (Fig. 2). This suggests that a single survey in one year, the usual method employed to study distribution of geese in remote areas of East Siberia (Egorov and Okhlopkov 2007, Solovyeva and Vartanyan 2011), may not allow for an effective understanding of the summering distribution, limiting potential conservation actions for the species.