EN
Milky Stork Mycteria cinerea



Justification

Justification of Red List category

Over the past three decades, the population of Milky Stork has declined probably by more than 50%. Its population is now also relatively small (between 600 and 1,850 mature individuals), such that ongoing declines are of concern and the species is considered to be of relatively high risk of extinction. Accordingly it is considered Endangered. The chief drivers of decline are hunting (especially of eggs and nestlings) and the destruction of coastal breeding habitat, especially in Indonesia where more than 95% of the wild population now resides. Hybridisation with the range expanding Painted Stork Mycteria leucocephala is a potential serious future threat, especially if feral hybrids from Singapore wander more frequently to Sumatra, which hosts the majority of the global population of Milky Storks. The protection of nesting birds is considered the principal conservation action needed, and this has proven immensely successful at reversing trends in other South-East Asian stork species.

Population justification

The population size of this species is considered to be immensely depleted. Countries of occurrence are discussed (approximately) north to south: In Cambodia, it is considered a very rare resident (Goes 2013, CBGA 2019) with most birds at the Prek Toal colony (17 pairs in 2010, 4-5 more recently but stable: Goes 2013, S. Mahood in litt. 2023). In Thailand, small numbers of non-breeding individuals occur in the Pak Thale area (western side of the Bay of Bangkok) and Huai Chorakhe Mak Reservoir Non-hunting Area (eBird 2023), but these likely sum to fewer than 10 individuals, none of which are known to breed in the country and probably originate from the Prek Toal colony in neighbouring Cambodia. In Malaysia, the current situation is opaque. The species is thought to have been extirpated (MNS Conservation Council 2021) or declined to '<5 individuals' (Ismail and Rahman 2016, Puan et al. 2020), but has been subject to various and sporadic reintroduction attempts since 1998. After March 2007, this captive-breeding attempt increased in intensity (Ismail and Rahman 2016) with recent records from Kuala Gula Bird Sanctuary and Kuala Selangor (eBird 2023). However, there is no indication that there is yet a self-sustaining population in Malaysia, despite best efforts, and the population size here (in any instance unlikely to comprise more than 20-30 mature individuals) is not considered part of the wild population. Some birds visiting the west coast of Peninsula Malaysia, however, might be wanderers from neighbouring Sumatra (wild) or Singapore (feral). In the latter, there is a small but frequently-reported population that comprises at least 50 birds (eBird 2023), which sometimes cross over into neighbouring Malaysia. A high percentage of this population are known to be hybrids (with M. leucucephala) (Baveja et al. 2019) and the population is believed to have originated from escapes rather than a deliberate introduction for conservation; they are consequently not considered in the number of wild mature individuals.

The species' stronghold remains in Indonesia, particularly the eastern mangroves of Sumatra, which may now constitute the only viable population (Baveja et al. 2019). In 2009, the first and only comprehensive population estimate of the species in Sumatra since the 1990s (see BirdLife International 2001) was made, recording c.1,600 birds as follows: c.75 individuals in Aceh province, c.500 North Sumatra province, c.350 Riau province, c.100 Jambi province, c.500 South Sumatra province and c.75 Lampung province. Not all of these are likely to have been mature individuals, and there is evidence that the species has continued to decline since then, although perhaps at a rate slower than since the 1990s (M. Iqbal in litt. 2023); the current population is probably somewhere between 800 and 1,600 birds. The population on Java is assumed to be small, with only two breeding sites confirmed in recent years: (1) Pulau Dua, Banten, which has only recently been re-occupied and from which only a handful of birds are known (Noor et al. 2020); and (2) Pulau Rambut Wildlife Sanctuary, which between 2017-2020 hosted 11-70 birds (Y. R. Noor pers. obs., in Noor et al. 2020). In 2017, 73 Mycteria cinerea were also observed at a colony of 20 nests in Indramayu, West Java, although no chicks were observed (Noor et al. 2020). During the ‘Big Month’ citizen science event (conducted in January 2020 comprising 22,054 checklists) across Java and Bali moreover, the species was recorded in only 21 (0.26%) of the 7,935 tetrads (2 × 2 km2 squares) visited, amounting to an estimated 131 individuals based on maximum counts for nine coastal locations on Java and Madura (T. Squires and S. Marsden in litt. 2020). Compiling data, it is considered unlikely that Java hosts more than 50 breeding pairs, although numbers at key sites may be augmented by non-breeding individuals. Finally, on Sulawesi, c.100-150 birds have been observed in Rawa Aopa Watumohai National Park (M. Iqbal pers. obs., 2014) but here too recent data are lacking except for a flock of 10 observed in February 2016 (eBird 2023); there are no records on Sulawesi away from this area, or from Sumbawa, despite historic observations (BirdLife International 2001, eBird 2023).

Adding these total is difficult owing to uncertainty between the ratio of individuals to mature individuals. Overall, the population outside Indonesia is now negligible. On Sumatra, the current number of mature individuals is suspected to lie somewhere between c. 500-1,600 (with scenarios ranging from population stability since censuses in 2009, to continued population reductions). On Java and Sulawesi, the combined total is unlikely to be more than 100-200 mature individuals, thus the global population is assumed to number 600-1,850 mature individuals, with a best estimate of 1,200-1,850 on the basis that declines may recently have slowed (M. Iqbal in litt. 2023).

Trend justification

This species’ population is suspected to have declined rapidly over the past three generations (30 years: 1993-2023) and while rates of decline may have slowed, there is no sign of population recovery, and the species' (now relatively small) population continues to decrease. The trend in each range state is discussed, broadly from north to south: In Cambodia, has evidently always been scarce and confined to a single site, Tonle Sap, which hosts the only inland breeding colony of this species in the world (BirdLife International 2001, Goes 2013). Apparently 15 known pairs in 1996 (Parr et al. 1996), and numbers varied 2002-2012 between two and 17 visible pairs (Goes 2013). Numbers are apparently still stable at 4-5 pairs with a similar number of birds in hybridising pairs with M. leucocephala (S. Mahood in litt. 2023). Over the last three generations there may therefore have been a slight reduction, although numbers now seem stable, and Cambodia has always hosted only a small percentage of this species' global population. In Thailand, there is no evidence it ever bred (BirdLife International 2001) and the number of non-breeding visitors each year is very small, with no plausible hope of elucidating trends; it is consequently not discussed further.  In Peninsular Malaysia, numbers fell from counts of over 100 individuals in 1984, to fewer than 10 birds in 2005, and only a single wild bird in 2010 (Malaysian Nature Society 2005, Li et al. 2006, DWNP 2010). No wild birds remain, and contemporary sightings of this species in Malaysia originate from zoo escapes (either from Kuala Lumpur, or neighbouring Singapore) and (apparently failed) reintroduction attempts (see Ismail and Rahman 2016 for summary; also Puan et al. 2020, eBird 2023).

The majority of the global population now exists in Indonesia, where trends are undoubtedly the most significant in a global context. Here rapid declines are well evidenced in response to intense hunting pressure at nesting colonies and the rapid loss and conversion of coastal habitat. Numbers on Sumatra, which holds more a large majority of the global population, fell from 5,000 birds in 1986 (Silvius 1988, Silvius and Verheugt 1989) to 1,600 in 2009, a 68% decrease (Iqbal et al. 2008, Iqbal et al. 2012). In Java, a wintering flock in east Madura of 170+ birds observed in 1996 had diminished to c.70 birds in 2006, which may be representative of an island-wide decline (B. van Balen in litt. 2013); there are very few recent records from here, except for two birds in February 2021 and seven in January 2023 (eBird 2023), potentially suggesting declines here have continued, but coverage has been very poor. Elsewhere in Java, historic data are too poor to speculate on past and future trends. Trends on Sulawesi are similarly unknown. Since 2009, population declines on Sumatra may have slowed. Data to confirm this are poor, but the population in south Sumatra province, for example, is suspected to have fallen to 300-400 in 2023, down from an estimate of c. 500 in 2009 (M. Iqbal in litt. 2023).

In the early- to mid-1990s, Perennou et al. (1994) and Rose and Scott (1997) estimated a global population of 6,100 birds (not all of which will have referred to mature individuals, and assuming a ratio of 0.5-0.7 of breeding birds, will have been equivalent to approximately 3,000-4,300 mature individuals). The contemporary population size estimate of 600-1,850 mature individuals (with a best estimate of 1,200-1,850) therefore suggests that has been a reduction equivalent to 40-86% over the past three generations. Using a methodology that extrapolates likely rates of decline based only on Sumatran data suggests a decline equivalent to 77% over three generations up to 2009, but with indications that declines, while ongoing, may have slowed slightly since then (M. Iqbal in litt. 2023). Accordingly, the overall rate of population reduction over the past three generations is set at 40-86%, with a best estimate of 50-79%.

Distribution and population

Mycteria cinerea has a small population in Cambodia, with birds occasionally wandering (in the non-breeding season) to Thailand. Historically it occurred naturally in Peninsular Malaysia, but all populations here originate from escapes or unsuccessful reintroduction efforts, with another such population in Singapore. The vast majority of the world's population now exists in Indonesia, with recent records from Sumatra and Java (the strongholds) and Sulawesi (Eaton et al. 2021, eBird 2023), with no recent records from Sumbawa or Buton. It has, perhaps surprisingly given presence in neighbouring Cambodia (and a propensity historically for stork species to wander from here), never been recorded either historically or recently in Viet Nam or Lao PDR (Duckworth et al. 1999, BirdLife International 2001, eBird 2023, Timmins et al. in prep.).

Ecology

It is a predominantly coastal resident in Indonesia and Malaysia, inhabiting mangroves and adjacent, less saline, swamps. It forages on tidal mudflats, in saline pools, freshwater marshes, fishponds and rice-fields. The species has been documented as eating fishes, prawns and crabs (Iqbal et al. 2008, 2009). Birds only occur inland in flooded forest around Tonle Sap lake in Cambodia, from where they disperse in the wet season, possibly to the coast (van Zalinge et al. 2011). Tall, mature trees are important for nesting (Ismail and Rahman 2016). In Prek toal Ramsar site, peak nesting occurs between February and April (Visal and Mahood 2015).

Threats

This species is impacted by a myriad of threats. Historically, the conversion of mangrove habitats to agriculture and development projects caused declines through the direct removal of breeding sites, as well as the associated impact of disturbance. Hunting for food has also exerted significant pressure throughout its range, especially in Indonesia (see, e.g., Iqbal et al. 2008), where it is thought to be the principal cause of population declines in southern Sumatra (Iqbal et al. 2012). There is, however, some evidence that declines may have slowed in these areas (M. Iqbal in litt. 2023), suggesting that these threats may no longer be as acute as they once were. Perhaps the greatest threat to this species in the future is that posed by genetic swamping from M. leucocephala. In Kuala Lumpur, this is clearly a critical risk to reintroduction attempts, while in Singapore there is now conclusive genomic evidence that both species are hybridising freely in a population that is considered to be feral. Given the close proximity of this genetically admixed population to the species' stronghold in Sumatra, there is a risk of further genomic leakage of M. leucocephala into a large percentage of the global population of M. cinerea.

Conservation actions

Conservation Actions Underway
Listed in CITES Appendix I, although international trade is not considered a major threat for this species. The small, stable population at Tonle Sap, Cambodia, is contained within the Prek Toal Ramsar Site (site number 2245) and this site has been subject to nest protection schemes since the early 2000s (Goes 2013, Visal and Mahood 2015). This protection scheme has caused significant rebounds in other threatened and Near Threatened stork populations (Timmins et al. in prep.) although this has not (yet) translated into population gains for this species. In Thailand, feeding sites in the Bay of Bangkok are mostly protected and/or not under threat, with most recent sightings (eBird 2023) in Pak Thale Nature Reserve. In Malaysia, reintroduction attempts have largely been unsuccessful. Between 2007 and 2014, 50 Milky Storks were released in Kuala Gula (Ismail and Rahman 2016) but numbers in this area have dwindled (eBird 2023). In Indonesia, the species is protected and most of its known breeding sites are also (legally, but not necessarily effectively) protected. Surveys of the population in Sumatra took place in 2008-2009 (Iqbal et al. 2012) but future surveys are urgently needed to establish current, and predict future, population trends.

Conservation Actions Proposed
Conduct surveys and research to locate additional colonies, monitor seasonal movements and clarify its ecological requirements. In particular, there is a need to generate population size values for Java and Sulawesi, which may hold critical populations, as well as to repeat the survey undertaken by Iqbal et al. (2012). Monitor numbers and breeding success at all known important nesting colonies. Establish additional protected areas encompassing important nesting colonies and feeding areas, particularly in the Riau, Jambi and Sumatra Selatan provinces of Sumatra. Promote public-awareness initiatives highlighting its conservation importance. Improve captive pre-release training techniques. Maintain and increase public awareness to ensure long term viability of the reintroduction programme. Where relevant or appropriate, instigate and establishment nest protection schemes, which have been successful for other stork species in Cambodia. Implement the removal of hybrids from wild Milky Stork populations, including isolation of hybrids in captivity and releasing genetically pure Milky Stork individuals into the wild (Baveja et al. 2019). 

Identification

92-97 cm. White stork with thick, yellowish bill and blackish flight feathers. Juvenile has paler brown, more streaked head and neck, and darker wing-coverts contrasting sharply with upperparts. Similar spp. Painted Stork M. leucocephalus has black markings on wing-coverts and breast, pink on inner wing-coverts and tertials and more restricted naked head skin.

Acknowledgements

Text account compilers
Berryman, A.

Contributors
Goes, F., Iqbal, M., Li, Z.W.D., Mahood, S., Squires, T., Yeap, C.A., Yong, D. & van Balen, B.S.


Recommended citation
BirdLife International (2024) Species factsheet: Mycteria cinerea. Downloaded from https://datazone.birdlife.org/species/factsheet/milky-stork-mycteria-cinerea on 24/02/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from https://datazone.birdlife.org on 24/02/2024.