Ecology of the eastern population of the Siberian Crane Leucogeranus leucogeranusin the taiga section of its flyway: habitats, foraging, trophic links with Elk Alces alces, and
threats
Victor G. Degtyarev 1 & Sergei
M. Sleptsov 2
1 Head
of Laboratory, 2 Junior
Research Worker, Institute for Biological Problems of Cryolithozone,
Siberian Branch, Russian Academy of Sciences, 41 Lenin Ave, Yakutsk,
677980, Russia
1 dvgarea@yandex.ru
(corresponding author), 2 sleptsov@mail.ru
Abstract: Stopovers along the flyway of the
Siberian Crane in the taiga zone of northeastern Asia
have not been previously described. In this study, we provide the first
investigation of the stopovers of its eastern population in the taiga zone
(2006–2011). Seven spring
stopovers at bogs were investigated for 10–24 hreach. The birds spent 5–6 hr sleeping at night, while 54–74 % of their time was
spent foraging outside the sleep time budget. Based on excreta samples, only 5/109
cranes successfully obtained adequate food during stopovers at typical bogs. Favorable foraging conditions were found at the rare
shallow lakes or at Elk Alces alces salt licks where the foraging efficiency was 16
times greater than at bogs unaffected by ungulates. Elk activity led to areas of peat
exposure in bogs where the availability of edible grass rhizomes and readily
extractable sprouts were increased for the Siberian Crane. Investigations at three autumn stopovers
at poor forage riverbeds lasted for 0.3–11.0 hrand were interrupted by boating activities, as were 22 other reported
stopovers. Cranes allocated a high proportion of their time to foraging, which
they commenced immediately after landing. However, the cranes failed to acquire any significant sustenance during
the overwhelming majority of taiga stopovers. Our investigation showed that the
eastern population was not specialized in foraging at typical taiga
wetlands. Humans indirectly
aggravated the naturally harsh migration conditions by overhunting Elk on the
flyway and intensive boating activities during the most intense migration
period.
Keywords: Bog, Critically Endangered, foraging success, time budget, salt lick,
stopover.
doi: http://dx.doi.org/10.11609/JoTT.o3467.4871-9 | ZooBank:urn:lsid:zoobank.org:pub:9FF60369-87FD-4D5A-9880-4B2F32EE3BCC
Editor: Crawford Prentice, Nature Management Services, Jalan, Malaysia. Date of publication: 26 October 2013
(online & print)
Manuscript details: Ms # o3467 | Received 29
December 2012 | Final received 10 July 2013 | Finally accepted 09 October 2013
Citation: Degtyarev, V.G. & S.M. Sleptsov (2013). Ecology of the eastern population of the Siberian Crane Leucogeranus leucogeranusin the taiga section of its flyway: habitats, foraging, trophic links with Elk Alces alces, and threats. Journal of Threatened
Taxa 5(14): 4871–4879; http://dx.doi.org/10.11609/JoTT.o3467.4871-9
Copyright: © Degtyarev& Sleptsov 2013. Creative Commons Attribution 3.0 Unported License. JoTTallows unrestricted use of this article in any medium, reproduction and
distribution by providing adequate credit to the authors and the source of
publication.
Funding: The
work was supported by the Russian Foundation for
Basic Research (the project No. 11–04–00130) and the UNEP/GEF
project GF/2712–03–4627.
Competing Interest: Authors declare no competing
interests.
Author
Contribution: VGD conceived the investigation project,
carried out the field work, assembled and analyzed the data, and prepared the manuscript. SMS carried
out the field work.
Author Details: Dr. Victor G. Degtyarev was awarded PhD (zoology) and DSc (nature conservation) in 1987 and
2000 respectively. He studies cryogenic wetlands, waterbirdsecology and resources, endangered species (mostly Hooded and Siberian
Cranes). Sergei M. Sleptsov is preparing
his thesis on tundra waterbirds. He has carried on ongoing research on optimal nesting sites of the eastern
population of Siberian Crane in the tundra (the Indigirka River basin).
Acknowledgements: We
thank I.D. Pavlov, A.G. Takhvatulin, and N.V. Bushkov for their assistance during our field studies, and
A.E. Pshennikov for his assistance in wetland
quantification, and L.V. Kuznetsova and V.I. Zakharova for their work in plant identification, and G.W.
Archibald for his valuable suggestions and improvement of this manuscript.
For figures, images, tables -- click here
Introduction
The Siberian
Crane Leucogeranus leucogeranusPallas, 1773 is one of the largest flying birds (Image 1) and in 2000 it was
listed as Critically Endangered (Birdlife International 2012). This species is mainly
represented by its eastern population of approximately 4,000 individuals. The Siberian Сrane’s current conservation status was justified by the
expected alteration or loss of wintering grounds for the eastern population
after the development of the Three Gorges Dam in China (IUCN 2011). Studies of the eastern population have
provided some basic knowledge on the species and its population distribution,
including the breeding biology, major migration route, wintering habitat
conditions, and principal conservation threats (Johnsgard1983; Potapov & Flint 1987; Potapov1992; Meine & Archibald 1996; BirdLifeInternational 2001; Kanai et al. 2002; Wu et al. 2009). However, details of the Crane’s
population ecology have not been fully described. In particular, the
characteristics of flyway stopovers, as well as the environmental conditions
and ecology of the staging wetlands in northeasternAsia remain unknown. For
conservation purposes, the lack of such knowledge makes it difficult to assess
or predict the population’s reaction to probable disturbances during the
pre-migration period.
The Siberian
Crane is known to be omnivorous and ‘the most specialized crane in terms of its
habitat requirements, and the most aquatic, exclusively using wetlands for
nesting, feeding, and roosting, preferring wide expanses of shallow fresh water
with good visibility’ and it prefers such habitats in high and low latitudes (Meine & Archibald 1996). The breeding and wintering habitats
differ significantly from the taiga wetlands and it is not clear how this
highly specialized population can utilize them, or if it can display omnivorous
foraging behaviour. To understand
the impact of any disturbance that might affect the pre-migration period, e.g.,
the development of the Three Gorges Dam in China (Wu et al. 2009), it is
important to determine how the flyway environmental conditions allow the
population to compensate for lipid reserve shortages. We hypothesized that stenotopy would not allow the population to find favorable foraging conditions during taiga stopovers.
In this paper,
we describe and discuss the habitat use, foraging conditions, relationships
with Elk Alces alces,
foraging success, and the correspondence between the taiga wetlands and the
ecological requirements of the eastern population of the Siberian Crane, as
well as the impacts of anthropogenic activity along its flyway in northeastern Asia during spring stopovers. This is the first study of Siberian
Crane stopovers in the taiga zone.
Materials and Methods
The studies
were conducted during 2006–2011 in the basin of the Aldan River
(northeast Asia) (Fig. 1). Wetlands
along the centerline of the flyway were investigated
between 560N and 630N. The stopover study area (the lower
catchment area of the Maya River, center: 5904’N
& 134030’E) was selected based on existing data on the flyway
used by the eastern population (Kanai et al. 2002), which contained two areas
that differed in their degree of human activity.
The studies of
the wetlands aimed to determine their typology and structure, as well as
an assessment of whether they satisfied the habitat requirements (shallow and
muddy area, with wide expanses and typical food resources) of the Siberian
Crane’s eastern population. We
visited 133 wetland points, which corresponded to the categories of lake, bog,
or lake marsh-meadow and lake-bog complexes. We covered 1500km by motorboat or
rafting along the main water bodies and five tributaries. Based on a direct inventory of the
wetlands in the study area, which was created using ESRI ArcView GIS 3.3 and
Landsat 5TM satellite images (30m ground sample distance; acquisition dates, 29
July and 14 August 2010), we roughly quantified the water courses of the
wetlands utilized by the study population.
We tested the
match between the taiga wetlands and the habitat requirements of the study
species based on known ecological characteristics of the population (Potapov & Flint 1987; Meine& Archibald 1996; BirdLife International 2001)
and by a study of Crane foraging in the Momoge Nature
Reserve (northeast China) in October–November 2007 and ongoing research on the population’s optimal nesting sites
in the tundra (the Indigirka River basin), which revealed technical details of
fishing, earthworm searching, plant object extraction, or collection, and the
functional structure of the nesting territory.
We monitored
three bogs, three floodplain lakes, and three riverine areas for five years
during the population’s most intense migration period, and on two occasions we
performed observations at two other bogs. Systematic observations were made at distances of 100–150 m
from the time of arrival of the cranes until they left. We also conducted in depth examinations
of two bogs after spring stopovers, where the number of cranes resting and the
duration of their stay were known. We also measured the time spent sleeping at night. The time allocated to other activities
was quantified using scan sampling. One pair, one family, and six flocks of 11–31 individuals were
scanned at 15-minute intervals. The basic diet was determined based on foraging
marks, fecal samples, and visual observations of
foraging. In addition, we observed
and recorded the conditions in eight wetland locations, which had been reported
with some degree of certainty as stopover locations during interviews.
The field data
on stopover locations was supplemented with interviews with 26 reliable local
informants.
Results
Wetlands utilized by the population
The Siberian
Cranes stopped at marshes around flood plain lakes, at river gravel spits or
flats, and at bogs that differed in their distribution, total area, and density
(Table 1), as well as extremely rare shallow lakes. All the habitats were
<0.5km2 in size.
Gravel spits
and flats are formed mainly by riverbed outcroppings from low water
levels. The majority of the river
systems in the study area were mountainous or semi-mountainous, so low water
levels were rarely observed in the autumn during the period of intense
migration, while they were never observed in the spring. The water levels and
the shallow water available for Cranes fluctuated significantly, because these
factors were dependent on hydrological regimes. Thus, areas located on mountain
tributaries or the upstream edges of large river islands contained no resources
for cranes to feed upon. The land
and shallows on the downstream edge of large river islands contained sparse
aquatic vegetation, which included Pondweed Potamogeton spp., algae growing on gravel and submerged arboreal substratum, and
(rarely) Water Horsetail Equisetum arvensetubers. Small shoals of whitebait
(juveniles of common fish species) occasionally approached the shallows. The main streams were intensively used
for boating activities. During the
period of intense migration in 2010 and 2011, at least one motorboat passed the
stopover study site per hour between sunrise and twilight. All of the stopover
locations along the main stream were passed by boats at distances of <300m.
Marshes
occurred around floodplain lakes in gently sloping lake depressions, which were
surrounded by unforested lands covered by water
meadows. This type of marsh formed
a patch with an area up to 3,000m2 (at one end of an oblong lake) or
a belt up to 20m wide (between an open lake and a meadow). The marsh vegetation
was dominated by sedges, Carex spp.The lakes, meadows, and marshes formed an elongated area with an area of ≤0.2km2. Adequate forage for Siberian Cranes in
the marshes included the roots of hydrophilic sedges and horsetails. In years with wet springs, the flood
marshes were covered entirely by water and were unsuitable as stopover
points. The potentially
suitable habitats were limited by the high water levels of flood plains in
valleys surrounding large rivers. Many flood plain lakes served as hunting grounds for waterfowl during
spring.
Bogs were
ageing successions of cup-shaped lakes, which ranged from a lake surrounded by
a narrow bog belt to a larger, continuous blanket bog that completely covered a
water body. The resulting
irregularity meant that the surface included dry and wet plots, and pools. The vegetation was composed of Hypnum moss, sedges, and horsetails. During the migration period, no insects
or other small animals were observed in any noticeable quantity. Frogs spawned during early May in pools
of melting snow, before leaving them.
Bogs were
widespread throughout the modern taiga flyway. The majority of bogs were flooded but
not submerged. Until the end of
May, the bog surface was on the ice layer, which bulged with the pressure of
water entering the frozen bog from the upper layers, as found in the central Yakutian Plain (Degtyarev2007). Bog areas that were
difficult for humans to access tended to retain a high local density of Elk,
which had very important effects on the foraging conditions for the Siberian
Crane. From the spring to autumn,
Elk used the bogs as foraging grounds, salt licks, and as shelters from
bloodsucking insects. The intensive
activity of Elk resulted in damage to the topsoil and herbage, exposing
peat slush in the bog. In these
areas, there was an increased availability of grass rhizomes, which were
consumed by Siberian Cranes. The
abundance of green sprouts on the bog disrupted by elks during the spring
provided further sustenance. Numerous root fragments resulted from the frequent movement, hoofing,
and other activities by Elk. In
addition, their combination with early warming of the uncovered peat by solar
radiation produced superficial, weakly-rooted and
readily extractable sprouts. Bogs
and lake-bog complexes were rarely used as fishing or hunting grounds.
We found only
three shallow lakes in river valleys where the Siberian Cranes could forage
throughout this area. Two lakes
were located at the population’s migration centerline,
where Siberian Cranes could forage by wading during the autumn stopover. These were fishless, oval-shaped lakes
with quaking bogs along the edges. One of these was reportedly a regular stopover location for several
hundred Siberian Cranes. In
2011–2012, this lake almost perfectly met the foraging habitat
requirements, i.e., it was relatively spacious (600x800 m in area),
shallow (~0.2mm depth), with light bottom sediments and well-developed aquatic
vegetation. In 2011, it had frozen
by 25 September.
Spring stopovers
All of the
investigated spring stopovers were located on bogs (Table 2), where one
(Stopover No. 2) was in an area with a high local ungulate density and Elk
utilized the bog intensively as a foraging ground and salt lick (Image 2). The remaining stopovers were located in
the main stream valley, which had a lower Elk density
and less significant impact of ungulates on the bogs.
The Siberian
Cranes landed at the observed stopover locations at 16.50hr, 18.00hr, 18.40hr,
19.50hr, and 21.50hr, before leaving at 08.30hr, 08.33hr, 08.36hr, 10.20hr, and
16.30hr respectively. At other
stopover locations, flocks were recorded landing at 18.00hr, 18.35hr, and 19.50hr,
before leaving at 10.45hr, 09.30hr, and 11.00hr respectively. After landing, the Cranes immediately
began foraging at their maximum intensity and they continued until they slept
at night. They flocked in the bog
centre, occupying no more than 25% of its area and maintaining their distance
from the woodlands surrounding the bog. At Stopover No. 2, the Cranes occupied the area that was most
extensively affected by ungulates (mostly Elk), foraging for the most part on
plots of uncovered peat and peat mud measuring 1–60 m2 in
area with a total of about 700m2. Three Elk intruded in the flock’s
foraging area and grazed or ate peat matter for four hours nearby, but the
cranes were apparently undisturbed.
Visual
observations of foraging activity indicated that Siberian Cranes plunged their
bills into the peat to extract food from the Hypnummoss layer. They also picked up
small objects from the bog surface. Less frequently, moss plucking was followed by pulling
rhizomes from the peat. While foraging at the border between the lake and bog surfaces, the
cranes sometimes removed peat pieces from the water and extracted then ate
rhizomes from those pieces. No
activities resembling hunting were recorded. The foraging left marks, with
holes in the moss layer measuring 2.5–4.0 cm in diameter and up to
12cm deep along the stems of sedges and Red Cotton-grass Eriophorum russeolum. The subterranean parts of grasses
(rhizome, root-neck, and 3–4 cm of the stem) were pinched off and
absent, while the surface parts protruded from the holes. At Stopover No. 2 (the salt lick), we
found holes measuring 0.5–1.5 cm in diameter and up to 3cm deep,
with bill-tip prints among dense Siberian Crane and
Elk footprints, while fragments of green sprouts were observed on the exposed
peat and peat mud. The Cranes also
turned over the winter nests of voles. The foraging area at Stopover No. 3 had frog spawnon the surface, but the Siberian Cranes did not feed on it.
The Siberian
Cranes defecated only at Stopovers No. 2 and No. 7. At Stopover No. 2, the
Cranes left 27 droppings with a bright green colorand friable consistency. Microscopic analysis of the fecal samples
showed that the excrement samples contained vegetable fiberswith a few seed membranes or undigested seeds. At Stopover No. 7, the Cranes left five
droppings with a brown color and watery consistency,
which contained only fibers. In both cases, the droppings included
about 1–2 ml of uric acid excreta.
Autumn stopovers
During the
study period in the autumn, the migrating Siberian Cranes did not stop at the
monitored bogs, but they did stop at riverbeds with low water levels during
2010–2012. The stopover by a
46-member flock observed on 29 September was interrupted by a passing motorboat
15 min after the flock had landed (at 17.30hr). The flock allowed the boat
to approach within 500m. The flock
had landed on a low gravel flat, with sand and silt
plots at the downstream edge of the island.
A second
stopover, by an 11-individual flock (including two juveniles) on a gravel spit,
was observed from 2 October to 3 October between 22.20hr and 07.20hr. After landing, the flock slept for seven
hours then began to forage in a creek, shallow silt pool, and gravel. After
50 min, the flock was frightened away by a passing motorboat. The flock
flushed when a boat suddenly appeared from an island 100m away.
Landing was
accompanied by the piercing calls of juveniles. Visual observations of foraging behaviors showed that the cranes picked away at forage in
the creek and shallow silt pool, or on the dry gravel surface. One individual tried three times,
without success, to catch moving objects in the shallows. The foraging area (about 7,000m2) consisted of a shallow silt pool (3,000m2) and a dry gravel
surface, which was crossed by two 20–40 cm wide creeks. An examination of the foraging area
revealed several pondweed stems with pinched-off root-necks and pluck marks in
mats with attached algae on the riverbed gravel, and an arboreal substratum
that had been uncovered by the decrease in the water level. Whitebait werethe only potential targets for fishing attempts in the shallows. At this stopover, the Cranes left five
uric acid excreta measuring about 3–4 ml, without excrement.
A third
stopover by a pair with a juvenile was observed on 7–8 October from
18.45hr until 06.20hr, on a gravel flat by the
island. These Siberian Cranes were
frightened off three times by motorboats passing at 15 to 20 min
intervals. After each such
incident, the cranes flew around for approximately 5–6 min and moved
2km away, before returning to the same place. The juvenile emitted piercing
calls. Their night sleep commenced
at 19.50hr and was interrupted by the first passing motorboat at 06.20hr. During disturbances, the cranes allowed
each of the boats to approach within 300–400 m. The stopover location in silt and sand
was overgrown with Water Horsetail while sediment scouring made plant tubers
available to the cranes.
Interview results
Local
informants reported sightings (between 1980 and 2012) of Siberian Crane flocks
containing up to 50 individuals (about 100 individuals on one occasion, about
200 individuals on one occasion, and about 300 individuals on two occasions),
as well as families (3) and solitary juveniles (2) and adult (1) near the main
river (20 reports during the autumn and two in the spring), mountain
tributaries (four reports in the autumn), a flood plain marsh (one report in
the spring), bogs (three reports in the spring and one in the autumn), and a
shallow lake (six reports in the autumn). In the riverine areas, cranes were
frightened away by passing motorboats at a distance of
200–500 m. The spring
stopovers on the main river gravel spits occurred at low water levels, before
seasonal ice drifting and floods. There were two records of solitary juveniles
immediately before the main river froze; the informants considered it likely
that these individuals died shortly thereafter. After visiting 12 reported
stopover locations, we found that a marsh, two shallow lakes, two bogs, and
four areas by the main stream were typical habitats, which were utilized by
Siberian Cranes as described above. However, six of the areas by the main stream were gravel or sand spits
that were devoid of any vegetation.
Discussion
All of the
taiga wetlands used by the Siberian Cranes were small- or
medium-sized and they were integrated with continuous woodlands, while
only the areas by the main rivers could be regarded as wide expanses when the
area of the water was taken into consideration. This species is specialized for
expansive habitats with wide visibility, so they were not able to exploit
foraging conditions that were favorable to other
species adapted to the taiga wetlands. For example, they never foraged at a mossy larch forest edge during
spring stopovers, where the Hooded Crane G. monachusfed on earthworms or at a narrow, dried up watercourse containing fish in
numerous shallow pools during early autumn stopovers, which were used by the
Grey Heron Ardea cinerea,
or berry plantations in woodlands used by the Middendorff’sBean Goose Anser fabalis middendorffii. However, the population forages for
earthworms and fish of different sizes throughout the breeding range, as well
as for berries.
The bogs were
numerous and scattered throughout the migration pathway, and they were regular
stopover locations. By contrast,
riverine grounds and flood plains were subject to flooding during the period of
intense migration. Furthermore, the
distribution of such habitats was restricted by the extent of flood plains and
the migration pathway crossed through only a limited area. In addition, the bogs yielded sufficient
forage for migrating Siberian Cranes. However, this resource was not optimal in terms of availability. When
feeding on vegetable matter in optimal foraging conditions, Siberian Cranes
readily extract rhizomes or tubers from light bottom sediments in shallow
water, wet mud, or marshes. In the taiga bogs, however, the
forage (predominantly roots) is buried in peat below a continuous Hypnum moss layer. Such conditions force Siberian Cranes to
use foraging techniques that are not normally observed in this species, such as
making holes in the moss layer and pinching off rhizomes. The closest known behaviorobserved at nesting sites involves pinching off the rhizome after plucking the
moss from around it. As a result,
food extraction was impeded and the food intake was reduced, as shown by the
foraging data for Stopovers No. 1 and Nos3–7. Overall, the majority of
individuals failed to achieve any significant sustenance, despite intensive
foraging. Based on the excreta collected, only 5/109 Siberian Cranes managed to
obtain sufficient food during Stopovers No. 1 and Nos3–7.
A different
level of foraging efficiency was identified at Stopover No. 2, despite its
comparable time budget. The
quantity of excrement showed that the majority of 15 individuals may have eaten
until satiated and they evacuated their digestive tracts twice, while their
total weight showed that their foraging efficiency was 16 times greater than
the foraging efficiency during the most successful of the stopovers by the
group that was described previously. This high foraging efficiency resulted from the high local density and
activity of ungulates, especially Elk, which utilized the bog intensively for
foraging and salt licks. Weakly-rooted sprouts were the primary forage for Siberian
Crane at salt lick stopovers because they were readily extracted from the
substratum. hence, it was clear that Elk activity dramatically affected
the functional significance of stopovers. Cranes mainly rested overnight at bogs lacking any Elk impacts and they
consumed insignificant quantities of food despite intensive foraging. By contrast, the bogs affected by Elk
substantially compensated for the energy costs of migration. In autumn, Siberian Cranes could also
find favorable foraging conditions on the extremely
rare shallow lakes.
Consequently,
Siberian Cranes did not encounter their habitual foraging conditions while
migrating across the taiga zone. All of the dominant types of wetland found along the current flyway in
the taiga zone failed to meet the specialized ecological requirements of the
eastern population and they provided severely limited forage. In the available habitats, the cranes
encountered foods that were either highly restricted
in quantity or difficult to forage. Migrating cranes made use of a few variations of their foraging methods
used at their nesting sites. At first
sight, making holes in the moss layer to extract grass subterranean parts was a specific foraging technique determined by the
relatively dense moss layer of the taiga bogs. On closer examination, however, both
holing and plucking while foraging on moss bogs consisted of the same movements
employed by Siberian Cranes in lake shallows at low latitudes, where they dig
for tubers from light bottom sediments. It is noteworthy that despite its omnivory,
the Siberian Cranes did not feed on frog spawn, but they reacted without
success to whitebait movements and winter nests of voles that more accurately
conformed to their stereotypical food.
Significantly,
Siberian Cranes did not search for stopover locations with favorableforaging conditions. Migrating Cranes made stopovers 1–4 h before
twilight. If areas were devoid of
appropriate wetlands immediately before nightfall, they continued to fly during
the twilight and dark. Thus, the
selection of a stopover point was random and the Siberian Cranes only stopped
when night began to fall. That
accounts for only four stopovers being registered during six years at the three
bogs monitored in spring, and during the spring of 2010, 20% of the eastern
population passed through the study area, but migrating cranes did not stop
near the salt lick in the area where the productive stopover (No. 2) was
recorded in 2009. The high
proportion of waking time allocated to foraging and the immediate commencement
of foraging after landing showed that Siberian Cranes arriving at stopovers
needed to replenish their energy requirements. This was also suggested by the piercing
calls of juveniles, which aimed ‘to solicit feeding by their parents’ (Meine & Archibald 1996). The unfavorableenvironmental conditions that dominate the taiga wetlands in the Siberian
Crane’s flyway accounted for the winter migratory patterns of satellite-tracked
individuals from the eastern population (Kanai et al. 2002), which consisted of
a fast passage over the taiga zone, with its poor forage, and long stopovers at
low latitude wetlands, which provided good foraging conditions. The majority of the population could
migrate via the taiga zone, having accumulated sufficient fat reserves during
the pre-migration periods at wintering or breeding grounds. This is exemplified by data obtained on
the fat reserves of three migrating Siberian Cranes. Two adult cranes that had traversed at
least 20% of their entire migration distance to the wintering grounds before
dying of lead poisoning in the autumn (Pshennikov et
al. 2001) still had 20–25 % body fat at autopsy. On 4 November 1996, an
adult bearing the PTT No. 25328 was killed in the wintering range, which
weighed 5.6 kg (Hu 1998). On 29 July, about three months earlier (Kanai et al.
2002), we had weighed the same individual in the breeding range at 7.15kg. Prior to the start of the migration
(middle of September–early October), it must have gained several hundred
grams. Thus, the crane lost approximately 2kg (30% of body mass) while
migrating to the wintering grounds. We challenge the assumption that arduous migrations are the principal
factor that causes natural mortality in juveniles passing over the taiga for
the first time, especially in years with unfavorablepre-migration conditions.
The harsh
natural conditions of taiga stopovers for the migrating eastern population are
affected unfavorably by human-related
activities. In the study region,
the Elk population has been overhunted in all accessible areas, especially the
main river valleys where intensive hunting has decreased the Elk density to the
point that bogs are no longer impacted significantly by this ungulate. Thus, a stopover on bogs altered by Elk
is currently unfeasible for most of the eastern population of Siberian Crane
migrating along the main river valleys. High boating activity has also affected
the population’s migrating conditions. In years with low water levels, when the population makes stopovers
along the main rivers, these stopovers are interrupted by
boating. This impact
appeared abruptly about 20 years ago, when reliable and powerful outboard boat
engines came onto the national market.
We do not
envisage any practicable methods to mitigate the disturbance to Siberian Cranes
on the main river stopovers. The
width of the river in the region, especially at low water levels, impedes any
attempt to pass a stopover without disturbing the cranes. Motorboats are the only mode of
short-distance transportation that are entirely
accessible to the locals. Purchases
of boat engines are rising steadily throughout the region, so disturbances will
become more frequent. Furthermore,
management of the Elk population to maintain an appropriate population density
in the region can scarcely be addressed at present.
The beneficial
effect of ungulate activity on the primary habitat of the Siberian Crane in thenortheastern Asian part of the flyway suggests that
appropriate measures can be undertaken to improve stopover foraging
conditions. Simple procedures aimed
at exposing peat formation in bogs along the Siberian Crane migration routes
might facilitate the passage for weaker Cranes (in the worst-case conservation
scenario). This would be identical
to impact on ungulates on this substrate, so this method is ecologically
permissible. Such wetland management
procedures could be implemented in at least four regional nature reserves along
the taiga flyway (Fig. 1). Considering the random character of stopover range selection, any
improvement to the foraging conditions in certain bogs would be worthwhile if
combined with luring techniques based on decoys and callers. Migrating Siberian Cranes could also be
lured to draw them away from areas near the main river stopovers to appropriate
nearby wetlands, where they will not be disturbed by boating.
Conclusion
Thus, the
eastern population of the Siberian Crane is not specialized for foraging in the
taiga wetlands. Its stenotopy limits any possibility of omnivoryduring the overwhelming majority of taiga stopovers. This population probably
developed without any pressure from the environmental conditions found in taiga
wetlands. Humans indirectly
aggravate the naturally harsh conditions for migration by overhunting Elk along
the flyway and by intensive boating during the most intense migration
period. Consequently, the passage
through the taiga zone may be the most demanding point in the annual cycle for
the eastern population. The
possession of sufficient fat reserves has critical importance in such
conditions.
These findings
provide new insights into the problem of the degradation of wintering
habitats. The loss of important
wintering habitats or any disturbances during the pre-migration period will
reduce the reproductive capacity of large birds (Ankneyet al. 1991). But the majority of
the eastern population of Siberian Cranes will inevitably be threatened by
difficulties when passing through the taiga zone, especially when the migrants
pass over the two mountain ranges in the area. The vulnerability that results from the
eastern population’s ecological specialization will make the passage through
the taiga harder for weaker individuals. In this worst-case conservation scenario, stopover management might
facilitate the passage of Siberian Cranes, including the improvement of
foraging conditions.
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