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East-West Migration of Endangered Steppe Eagle and other raptors in Thoolakharka watch site, Nepal: Migration count summary -autumn 2019

East-West Migration of Endangered Steppe Eagle and other raptors in
Thoolakharka watch site, Nepal: Migration count summary - autumn 2019
Report Compiled by:
Sandesh Gurung, Sabin K.C., Sanjeev Baniya and Tulsi Ram Subedi
© Himalayan Nature 2019
Citation: Gurung, S., Kc, S. Baniya, S. and Subedi, T.R. (2019). East-West Migration of
Endangered Steppe Eagle and other raptors in Thoolakharka watch site, Nepal: Migration count
summary - autumn 2019 Technical Report 1/2019, Himalayan Nature and Nepalese Ornithological
Union, Kathmandu.
Birds of prey migration have been assessed from different bottleneck migrating sites of the word
(Lott 2002). Raptors breeding in the northern hemisphere of the world, leave their breeding ground
and move towards the southern warmer places to spend winter period. Annual movements of
raptors occur between Eurasia and Africa, North America and South America, Northeast Asia and
Southeast Asia and Australia (Shirihai 1987, 1988; Shirihai and Christie 1992; Meyburg et al.
2003). Migration of raptors in Asia are studied from different watch sites; Thailand, Malaysia
(DeCandido et al. 2004; Lim and Cheung 2008), Japan (Agostini and Mellone 2007) and in Nepal
(DeCandido et al. 2001; Gurung et al. 2004; Subedi et al. 2017). Long term monitoring of the
migrating raptors from the bottleneck area of their migration flyway has been used to estimate the
population status (Bednarz et al. 1990a, 1990b; Titus and Fuller 1990; Kjellén and Roos 2000;
Tholin 2011).
Here in this report we present overview of the raptor migration count conducted in central
west Nepal from the southern boundary of Annapurna Conservation Area (ACA) at Thoolakharka
watch site. In 2019, raptor migration count started from 15th October and continued until 15th of
December. Team of Himalayan Nature; Dr. Tulsi Ram Subedi, Sandesh Gurung, Sabin K.C. and
Sanjeev Baniya made observation for the migration count of raptors from Thoolakharka watch site
at Annapurna rural municipality of Kaski district, Nepal. Thoolakharka is situated at 2050 m a.s.l.
with a majestic view of Annapurna Himalaya Range on the north, Paudurkot to the south, northeast
with a Mardi River valley, east to the Hemja valley and west to the Modi River valley.
Thoolakharka provides 360 view hence we believe that the raptors doesn’t get miss when they fly
using this route. Migratory raptors along with age-class of Steppe Eagle (Photo 1, 2, 3 and 4) was
counted hourly basis starting from 08:00 hr to 17:00 hr. Soaring migrants require thermal energy
to fly (Kerlinger 1989) hence 08:00 hr was selected as starting time for migration count where the
solar intensity starts to increase slowly, 17:00 hr was selected as an end time due of the sunsets
where the thermal energy is supposed to be very low or almost absent. Additionally in our previous
studies (migration count), we did not see raptor’s activity before and after this time. Counting was
not conducted during the rainy days as no raptors found to fly during rain at our watch site. We
spent 62 days (577 hrs.) in the watch site to count the migrating raptors.
In 2019 migration count, 7 780 individuals of raptors belonging to 26 species were counted
at the watch site (Table 1). The highest single day count was on 19th November with total of 410
individuals raptors. The key species to be monitored were Steppe Eagle and the Himalayan
Vulture, the highest numbers of individuals we get every year. As such during 2019 autumn
migration we counted highest number of Steppe Eagles (n = 5 127) which was followed by
Himalayan Vulture (n = 1 677). The single day highest count of Steppe Eagle was 353 (19th
November) and the highest single hour count was on 22nd November at 12:00 - 13:00 hrs. with 114
individuals. On the other hand the highest single day count of Himalayan Vulture was found to be
on 11th of December with 105 individuals. The migration of Himalayan Vulture was recorded to
be lately compared to the Steppe Eagle.
Beside these two species, other migratory raptors included six species of eagle, three
species of buteo, three species of accipiter , four species of falcon, four species of vulture as well
as Black Kite (Milvus migrans ssp. govinda), Black-eared Kite (Milvus migrans sp. lineatus),
Oriental Honey Buzzard (Pernis ptilorhyncus sp. Orientalis- to name just a few.
Table 1: Raptor species observed and their number counted in 2019 autumn migration count at
Thoolakharka watch site central west Nepal.
Oriental Honey Buzzard
Black Kite
Himalayan Buzzard
Black-Eared Kite
Long-legged Buzzard
Hen Harrier
Upland Buzzard
Unidentified Harrier
Unidentified Buteo
Peregrine Falcon
Eurasian Sparrowhawk
Eurasian Hobby
Northern Goshawk
Amur Falcon
Common Kestrel
Unidentified Accipiter
Unidentified Falcon
Steppe Eagle
White-rumped Vulture
Booted Eagle
Egyptian Vulture
Short-toed Eagle
Himalayan Vulture
Indian Spotted Eagle
Cinereous Vulture
Greater Spotted Eagle
Griffon Vulture
Bonelli's Eagle
Unidentified Vulture
Golden Eagle
Unidentified Raptor
Unidentified Eagle
Total of all species 7,780
Photo 1: Sub-adult Steppe Eagle passing from the Thoolakharka raptor migration watch site.
Most of the migration of Steppe Eagles was on latter half of the October to second week
of December. Steppe Eagle is the most notable migrants among all the raptors and the highest
individuals we get, compared to other species. In this watch site most of the raptors passes very
close to the observer, which made possible to categorize approximately 60% of Steppe Eagle to
different age classes, such as juvenile (hatched-year, Photo 3), sub-adult (2nd-4th year, Photo 1, 2,4)
and adult (≥5th year). Age class composition of migrating Steppe Eagle from Thoolakharka
showed that sub-adults occupied the highest percentage compared to juveniles and adults. Out of
total individuals, the juvenile contributed 19.31%, sub-adults 32.71% and adults 8.82%, while rest
39.17% couldn’t be identified (Fig. 1). The percentage of age class composition is almost similar
to the results of Subedi et al. (2013) while contradict with the results of Subedi and DeCandido
(2012) where the juvenile percentage was recorded higher compared to other ages.
Figure 1: Age class distribution of Steppe Eagle counted at Thoolakharka raptor migration watch
site, autumn 2019.
The count data revealed that the peak of migration started from third week of November
and lasted until last week of November. In 21st, 24th, 25th and 29th of November the weather was
worst and the visibility was poor (<1 km) hence we couldn’t locate the migrating eagles. Thus the
graph in between the peak period tended to show the sudden rise and fall in the numbers (wave
pattern, see Fig. 2).
Juvenile Sub-adult Adult Unidentified
Number of Individuals
Steppe Eagle's Age
Figure 2: Migration timing of Steppe Eagle (total number counted and age class categorization for
3119 eagles of known age), X axis shows migration dates and Y axis the number.
Besides these, the arrival timing of Steppe Eagle in a day was mostly high at 09:00 hr -
12:00 hr, after that the migration rate slowly decreased (Fig. 3). One of the most probable reasons
could be the availability of thermal energy at late morning compared to mid-morning and the
eagles that stayed overnight at the places east of Thoolakharka. The less count in the noon time
could be the availability of strong thermal energy close to the slopes of Himalaya range to far north
of watch site that could make us impossible to detect them.
When dense cloud fill the high mountain passes, migrants particularly Steppe Eagles shift
their flight south and into the foothills and lower valleys. For hawk watchers, the best weather
conditions for migration watching at Thoolakharka occur when clouds obscure the famous
Annapurna Range, to our north. At such time, eagles and other raptors often pass directly over the
watch site. The migrants passing from this watch site often prefer to use the thermal present at the
bare watch hill and the agriculture field closer to the observation point. This provides the hawk
watcher along with the other photographers to take beautiful closer photos of raptors which is very
difficult to obtain in other parts of the world’s raptor migration watch sites. One can get photos of
bird from a various angle sometimes even the birds passes below the eye level to the south of
watch site that even helps to produce upper body covert which is quite rare to achieve too (photo
4). Raptors are not afraid of people in Nepal because no one shoots them, nor pays them much
attention. People and raptors share the same agricultural fields that dominate the landscape of this
country, particularly the foothills of the Himalayas.
15-Oct 22-Oct 29-Oct 5-Nov 12-Nov 19-Nov 26-Nov 3-Dec 10-Dec
Figure 3: Arrival timing of Steppe Eagle on hourly basis.
The arrival of Steppe Eagle in the Thoolakharka watch site was recorded from the second
week of October very similar to the finding of Subedi and DeCandido (2012, 2013). We
incorporated the Steppe Eagle data of 2012 - 2019 to compare their population trend, counted at
the Thoolakharka raptor migration watch site. Before this we truncated 10% of the data of each
year to remove the error and total counting days for all the year were made same (15th Oct- 7th
December). The trend line of migrating Steppe Eagle revealed that the observed number of Steppe
Eagle in declining fashion (Fig. 4). If the first year migration count of Steppe Eagle (count year
2012) is considered as baseline population, our results suggest minimum of 22% declines on the
population of this species over last eight years.
Figure 4: Population trend of migrating Steppe Eagle counted at Thoolakharka watch-site during autumn
migration in 2012- 2019.
Number of individuals
Time of day
2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Steppe Eagle number
Photo 2: Sub-adult Steppe Eagle passing by from the Thoolakharka raptor watch site.
Photo 3: A juvenile Steppe Eagle passing from the Thoolakharka raptor watch site.
The migration phenology of Steppe Eagle showed that adult and sub-adult had a peak time
started at the third week of November till to the first week of December while juveniles number
peaked at third week of November (Fig. 5).
Figure 5: Age composition of Steppe Eagles on daily basis crossing the raptor migration watch site at
Thoolakharka, Nepal 2019 autumn.
Photo 4: Sub-adult Steppe Eagle passing very close to the watch site (Note: often you get full frame with a
300 mm lens).
15-Oct 22-Oct 29-Oct 5-Nov 12-Nov 19-Nov 26-Nov 3-Dec 10-Dec
Besides Steppe Eagle, we got good diversity of eagles including both resident as well as
migratory species. Seven species of eagle (including Steppe Eagle) were recorded migrating from
this watch site. Greater Spotted and Indian Spotted Eagle often came with the Steppe Eagle flock
hence making confusion to our research team. As usual a juvenile Golden Eagle made its late
appearance, almost at the end of the migration count, which is a rare migrant to us. Bonellis Eagle
and Short-toed Eagle were also recorded making journey towards west. This watch site has often
acted as a stepping stones for few raptors. Booted Eagle (Photo 5) is one of a kind that has been
often seen spending few days in the farmlands and nearest village of Thoolakharka. Few
individuals of this species make back and forth every day hence such raptors are very important to
identify or else double count can occur. We therefore try to find some of the visible marking on
such raptors for instance, any broken wings, any molted wings or any distinguishable marking so
that one can identify them easily and avoid from double counting. As usual, we got both the dark
and pale morph of Booted Eagle in our watch site.
The resident eagles included Indian Black Eagle (Photo 6), Mountain Hawk Eagle (Photo
7) and Bonelli’s Eagle. Indian Black Eagle was often recorded flying at the canopy of the tree in
search of their food while Mountain Hawk Eagle was often observed on claiming their territory
with a display flight. On 18th October we observed total of 11 unique individuals of Indian Black
Eagle and we presume that is not local population, they might do altitudinal migration. Our
previous observation in lowland during winter also supports our prediction.
Photo 5: Pale morph Booted Eagle which is quite rare in Thoolakharka (we often get dark morph).
Photo 6: A beautiful Indian Black Eagle often comes very close to watch site to gain the thermal.
Photo 7: Mountain Hawk Eagle photographed at the watch site, they are local birds nesting around
Out of nine species of Vulture recorded in Nepal, we recorded eight species of vulture from this
watch site. We considered five species as migrants (latitudinal as well as altitudinal), in total 1 795
individuals made their journey towards west passing from the Thoolakharka raptor migration
watch site. Among them Himalayan Vulture contributed the highest percentage (93.5%) followed
by Cinereous Vulture (2.7%), White-rumped Vulture (2.1%), Griffon Vultures (0.6%) and
Egyptian Vulture (0.22%). In total 0.9% of vulture couldn’t be identified. Red-headed Vulture,
Slender-billed Vulture and Bearded Vulture were the local resident species frequently seen in the
watch site. In addition, flocks of resident Himalayan Vultures regularly pass overhead or at eye-
level. Migrations of vultures were recorded higher at noon time compared to other parts of the day.
The aloft of migration slowly started at the late morning, peaked it up till noon and started to
decline down after 13:00 hr. (Fig. 6).
Figure 6: Arrival timing of migrating Vultures on hourly basis, X axis showing time and Y axis
Similar results were obtained in previous researches by Subedi and DeCandido (2012,
2013) and Subedi (2014). One of the main reasons for such a specific pattern is the availability of
the thermal. Vultures, Eagles and Kites are morphologically similar and thus use similar flight
technique (Spaar 1997). Photoperiod is thought to be the primary mechanism cueing migration
(Gwinner 1996). The solar radiation starts to increase as the day passed from morning towards
noon (Kerlinger 1989; Kerlinger and Moore 1989). Large-bodied soaring raptors are always
conscious about economizing their fuel cost by soaring using high thermal energy to migrate
(Winden et al. 2010). Thermal energy was found to be higher in noon compared to morning and
evening (Vansteelant et al. 2014). The timing of daily peak flights of Himalayan Vulture was
consistent with other studies that revealed most raptors observed during the middle part of the day
relying on thermals and updraft for soaring flight (Maransky et al. 1997; Mellone et al. 2012;
Pannucio et al. 2011; Punnico et al. 2013). Results of this study also match those observed in the
Turkey Vulture migration (Miller et al., 2011; Seeland et al. 2012). Migration of Himalayan
Vulture started to peak up slowly from the third second week of November till to second week of
December (Fig. 7).
Figure 7: Number of Himalayan Vulture counted on respective date during autumn migration
count 2019. X axis shows date and Y axis shows number.
Photo 8: Adult Bearded Vulture photographed from the watch site.
15-Oct 22-Oct 29-Oct 5-Nov 12-Nov 19-Nov 26-Nov 3-Dec 10-Dec
Photo 9: A Juvenile Himalayan Vulture photographed during the migration passage from the
watch site.
Photo 10: Sub-adult Griffon Vulture; a long distant passage migrants in Nepal.
We recorded four species of falcon (Table 1) migrating from Thoolakharka watch site. Among
them, Common Kestrel (Photo 11) was highest in number followed by Eurasian Hobby (Photo
12), Peregrine Falcon (Photo 13) and Amur falcon. On 21st and 22nd we recorded the rare Amur
Falcon migrating from their breeding ground eastern Russia to the wintering ground of South
Africa. Since this year we did not perform count in late September and early October, hence lessen
number of falcons have been recorded as some species are reported to be early migrant. According
to Subedi and DeCandido (2012, 2013 and 2014) the peak time of falcons were recorded before the
third week of October hence we believe we have missed many falcons migrating from Thoolakharka.
Photo 11: Common Kestrel, a common falcon seen in migration in Thoolakharka.
Photo 12: Erusian Hobby in fligh (Note: The short narrow pointed wings with a orange vent).
Photo 13: Peregrine Falcon often appears near the watch site to hunt the bird
Black Kite, Oriental Honey Buzzard, Buteo and Harrier
Both the Black Kite and Black-eared Kite can be seen migrating from this watch site. This year
we observed higher number to Black Kite as compared to Black-eared Kite, in contrast our
previous record shows higher number of Black-eared Kite. Oriental Honey Buzzard held the third
position to pass higher number compared to all raptor species (Table 1). Most of the Oriental
Honey Buzzard we record from watch site were juvenile. We got various morphs including dark,
pale, white as well as rufous morph. This raptor is often harassed by corvids and observed chased
by the flock of Large-billed Crows.
This year we recorded three species of Buteo: Himalayan Buzzard (Photo 14), Long-legged
Buzzard and Upland Buzzard during autumn migration count. Three individuals Hen Harrier were
recorded migrating from Thoolakharka raptor migration watch site, which is another rare species
Photo 14: A juvenile Himalayan Buzzard.
This year we observed three different species of Accipiter. Among them Eurasian Sparrowhawk
occupied the highest number followed by Northern Goshawk and Shikra. We used Nikon D7000
camera with a 200-500 mm zoom Nikor lens to take picture and identify it in case we felt difficult
by our binocular. Besra and Shikra were often seen making a maneuver flight against Large-billed
We thank Japan Fund for Global Environment for the financial support through Japanese Society
for the Preservation of Birds (JSPB). We thank Dr. Hem Sagar Baral, a senior ornithologist of
Nepal, and Sharad Singh, Admin and Finance Director of Himalayan Nature for valuable
suggestion. We thank Department of National Park and Wildlife Conservation (CNPWC) and
Annapurna Conservation Area Project (ACAP) for granting us permit to conduct this study. We
thank our friend and officer at ACAP Rishi Baral for various support. Thank to Dr. Chaiyan
Kasorndorkbua and Thai Raptor Group for continuous encouragement and suggestion during the
count. We express thanks and sincere gratitude to Dr. Toru Yamazaki, Yasunori Nitani, Takashi Fujii
and Mountain Hawk Eagle team and JSPB members from Japan for their constructive suggestion,
untiring help, guidance and their practical suggestions that inspired us to accomplish this work
successfully. We also thank Prakasit Chancharas from Thailand, Jacky Soh and Virginia Cheang from
Singapore for their donation to the project. We also thank Raju Acharya, CEO of Friends of Nature for
his support in our raptor count. Finally we express thank to Hari KC and Australian Camp Guest House
family, Nirmal Gurung and Sunrise Restaurant family for their hospitality and support throughout the
research period.
Landscape to the north of Thoolakharka Raptor migration watch site- The Annapurna Himalaya
Range (Photo: Tulsi Subedi)
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... After 24 Nov. the number of eagles declined and suddenly peaked on 3 rd December with the highest count record (300 birds) in a single day (Fig 2). Since the migration timing and peak time of Steppe Eagle was found to be very similar compared to earlier years autumn count i.e. 2012-2015, 2017, 2018, and 2019 respectively (Subedi and DeCandido 2012, Subedi et al 2013, Subedi and Gurung 2018, Gurung et al 2019. One of the reasons for the highest count of the eagles in a single day towards the end of the season could be the sudden changes in the weather conditions that might have to pressurize the migrating eagles as well as wintering eagles nearby the watch site as a stepping stone to move furthermore west for a warmer area. ...
... The highest individuals were Amur Falcons followed by Eurasian Hobby, Common Kestrel, Peregrine Falcon, Lesser Kestrel and the globally endangered Saker Falcon. Surprisingly, the number of one species (Amur Falcon) in this year was higher compared to other years, however, the total numbers of all falcons are not significantly larger than previous counts (Gurung et al 2019). ...
... . Comparing present numbers with the previous monitoring records (2012 -2019), the numbers of migrating raptors have declined(Subedi and DeCandido 2012, Subedi and DeCandido 2013, Subedi 2014, Subedi and Gurung 2018, Gurung et al 2019. ...
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Counts of migrating Steppe Eagles (Aquila nipalensis) and at least eight other species of raptors were made at Khare, a raptor-migration watchsite in central Nepal, on nine days (27 October-4 November) in autumn 1999. Totals of 821 migrating Steppe Eagles (15.2 birds/h) and 129 other migrating raptors (2.4 birds/h), including the globally vulnerable Lesser Kestrel (Falco naumanni) (0.2 birds/h), were seen at the watchsite. Individuals representing 10 additional species that could not be distinguished as migrants versus local residents also were seen, but were not included in the count. Most autumn migrants at Khare are believed to represent individuals from populations of raptors that breed in central and eastern Asia and overwinter in southeastern and southwestern Asia, the Indian Subcontinent, and Africa. Raptor migration appears to be a regular and predictable phenomenon at the site, leading us to recommend its use by local residents, as a source of ecotourism revenue and as a focal point for environmental-education activity for school children, and by raptor conservationists, as a continentally significant monitoring site.
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ABSTRACr.--Counts of migrating hawks were initiated in 1934 at Hawk Mountain and, except for 1943-1945, have continued to the present. We analyzed the data from 1934-1986 by a standardized sample period appropriate for each species. Counts of several species known to be susceptible to detrimental effects of organochlorine pesticides crashed between 1946- 1971. All of these populations recovered subsequently. The immature-age classes and the species with the shortest generation times recovered first after DDT use diminished in North America. These observations mostly conform to predictions made on the basis of the adverse effects of DDT and metabolites on raptors. We suggest that standardized migration counts reflect qualitative trends of most raptor populations that pass over Hawk Mountain in the autumn. Currently, populations of many species appear to be either stable or recovering from the effects of organochlorines. Possible exceptions are eastern populations of the Red-tailed Hawk (Buteo jamaicensis), American Kestrel (Falco sparverius), Broad-winged Hawk (B. platyp- terus), Red-shouldered Hawk (B. lineatus), and Golden Eagle (Aquila chrysaetos), which may
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During their migratory journey en route to Africa, European birds face a barrier: the Mediterranean Sea. Differently from the majority of birds, Accipitriformes use mostly soaring-gliding flight rather than powered flight. For this reason many species of raptors tend to avoid the water- crossing by following land masses. Different degrees in dependence on soaring flight are shown by different species of raptors. In this paper, we compare the migration of Western Marsh Harriers and juvenile European Honey Buzzards by the means of simultaneous observations at two sites: Mount Olympus (northern Greece) and the island of Antikythira (southern Gree ce). Differences in migration timing, water-crossing behaviour, sex and age classes (in the case of Western Marsh Harriers), show that these species use different migration strategies when crossing the Mediterranean region in autumn. Western Marsh Harriers migrate through parallel flyways, while juvenile European Honey Buzzards migrate island hopping being attracted by landmarks and following leading lines of land masses. A simulation process demonstrated marked dif ferences in the energy consumption rates between the two species, highlighting that the powered flapping flight is particularly more disadvantageous for the European Honey Buzzard rather than for the Western Marsh Harrier. In this case, the morphological features of the studied spe - cies could explain the existence of two different migration strategies.
We used hourly counts of Red tailed Hawks (Buteo jamaicensis) migrating at Hawk Mountain during the autumns of 1992, 1993, and 1994, to examine the possibility that the extent to which time of day and local weather parameters affected the numbers of birds seen at the site varied over the course of autumn migration. Data were analyzed separately for early-, mid-, and late season periods of migration. High versus low windsM, following versus opposing winds, low versus high relative humidity, and high versus low barometric pressure were associated with increased hourly passage rates of Red-tailed Hawks Relative humidity had a greater effect during early-season migration. Wind speed and wind direction had greater effects during late season migration. We suggest that shills in the extent to which weather affects the numbers of Red-tailed Hawks seen at Hawk Mountain Sanctuary result from seasonal shifts in the species' dependence on thermal versus slope soaring.
Using simple regression, pooled-sites route-regression, and nonparametric rank-trend analyses, we evaluated trends in counts of hawks migrating past 6 eastern hawk lookouts from 1972 to 1987. The indexing variable was the total count for a season. Bald eagle (Haliaeetus leucocephalus), peregrine falcon (Falco peregrinus), merlin (F. columbarius), osprey (Pandion haliaetus), and Cooper's hawk (Accipiter cooperii) counts increased using route-regression and nonparametric methods (P < 0.05). Northern harrier (Circus cyaneus) counts increased based on the nonparametric method (P < 0.01) but not the route-regression method (P > 0.10). We found no consistent trends (P > 0.10) in counts of sharp-shinned hawks (A. striatus), northern goshawks (A. gentilis), red-shouldered hawks (Buteo lineatus), red-tailed hawks (B. jamaicensis), rough-legged hawks (B. lagopus), and American kestrels (F. sparverius). Broad-winged hawk (B. platypterus) counts declined (P < 0.05) based on the route-regression method. Empirical comparisons of our results with those for well-studied species such as the peregrine falcon, bald eagle, and osprey indicated agreement with nesting surveys. We suggest that counts of migrant hawks are a useful and economical method for detecting long-term trends in species across regions, particularly for species that otherwise cannot be easily surveyed.
DETERMINACIÓN DE PATRONES DE MIGRACIÓN DE RAPACES EN UN PAISAJE EXTENSO Cada otoño, decenas de miles de rapaces sobrevuelan Hawk Ridge en Duluth, Minnesota, en su migración hacia el sur, pero las rutas migratorias que llevan a Hawk Ridge son desconocidas. Para resolver este tema, contamos rapaces migratorias entre mediados de agosto y mediados de noviembre de 2008 en 24 puntos de observación a lo largo de ocho transectas perpendiculares a la línea de costa entre Duluth y la frontera entre Minnesota y Canadá. Nuestros objetivos fueron determinar las rutas migratorias sobre un área extensa (>2000 km2) e identificar cómo estos movimientos se vieron afectados por el clima, la hora del día, la estación y las características del paisaje. Un total de 4303 rapaces de 14 especies diferentes fueron contadas durante la época migratoria de 2008. Análisis exploratorios sugirieron que las rapaces migratorias se concentraron cerca de la línea de costa norte del Lago Superior, particularmente durante el mediodía cuando los vientos tienen dirección oeste. La altura media de migración difirió entre rapaces de vuelo planeado (especies de Buteo y águilas) y accipitéridos, con >40% de rapaces de vuelo planeado observadas a alturas mayores a los 100 m sobre el dosel de los árboles y ≥30% de accipitéridos observados a alturas menores a los 100 m sobre el dosel arbóreo. Modelos de análisis mixtos identificaron los factores significativos (P < 0.05) asociados con la migración de rapaces total: dirección del viento, hora del día, temperatura y viento antecedente (número de días en los que el viento no tuvo un componente oeste previo a los días de observación; R2 = 0.23). Los factores significativos asociados con la migración de rapaces de vuelo planeado incluyeron dirección del viento, hora del día, temperatura e intervalo estacional (ej., temprano o tarde en la estación migratoria; R2 = 0.17). Aquellos asociados con la migración de accipitéridos incluyeron hora del día, temperatura, viento antecedente, dirección del viento e intervalo estacional (R2 = 0.29). Con la creciente popularidad del desarrollo de la energía eólica, se necesita información respecto de las rutas migratorias para evitar conflictos entre las aves y las turbinas. La metodología y el diseño de este estudio proveyeron medios para cuantificar la magnitud, las fechas, las rutas y las condiciones meteorológicas asociadas con la migración de rapaces sobre un paisaje extenso.
Every autumn, large numbers of raptors migrate through geographical convergence zones to avoid crossing large bodies of water. At coastal convergence zones, raptors may aggregate along coastlines because of convective or wind conditions. However, the effect of wind and thermal convection on migrating raptors may vary depending on local landscapes and weather, and on the flight strategies of different raptors. From 20 August to 14 October 2008 and 2009, we studied the effect of cloud development and crosswinds on the flight paths of raptors migrating through the eastern Black Sea convergence zone, where coastal lowlands at the foothills of the Pontic Mountains form a geographical bottleneck 5-km-wide near Batumi, the capital of the Independent Republic of Ajaria in southwestern Georgia. To identify key correlates of local aggregation, we examined diurnal variation in migration intensity and coastal aggregation of 11 species of raptors categorized based on size and flight strategies. As reported at other convergence zones, migration intensity of large obligate-soaring species peaked during the core period of thermal activity at mid-day. When clouds developed over interior mountains and limited thermal convection, these large obligate-soaring species aggregated near the coast. However, medium-sized soaring migrants that occasionally use flapping flight did not aggregate at the coast when clouds over the mountains weakened thermal convection. Numbers of alternate soaring-flapping harriers (Circus spp.) peaked during early morning, with these raptors depending more on flapping flight during a time of day with poor thermal convection. Small sparrowhawks (Accipiter spp.) aggregated at the coast during periods when winds blew offshore, suggesting aggregation caused by wind drift. Thus, weather conditions, including cloud cover and wind speed and direction, can influence the daily rhythm and flight paths of migrating raptors and, therefore, should be accounted for before inferring population trends from migration counts. Efectos del viento, convección térmica y variación en estrategias de vuelo, ritmos diarios y rutas de vuelo de aves migrantes en la costa del mar negro en Georgia Cada otoño un gran numero de rapaces convergen en zonas geográficas a lo largo de su migración para evitar cruzar largos cuerpos de aguas. En las zonas costeras de convergencia las rapaces se pueden agregar a lo largo de la línea costera debido a la conectividad o condiciones del viento. Sin embargo, el efecto del viento y de las convecciones térmicas en las aves rapaces migratorias pueden variar dependiendo de las condiciones locales del paisaje y el clima, y en la estrategia de vuelo de las diferentes rapaces. Del 20 Agosto al 14 Octubre 2008 y 2009, estudiamos el efecto de la formación de nubes y vientos cruzados en las rutas de vuelo de las rapaces migrantes a través de la zona de convergencia en el mar negro del este, en donde las tierras bajas costeras y el pie de monte de las montañas del Ponto forman un cuello de botella geográfico de 5 km de ancho cerca de Batumi, la capital del la republica independiente de Ajaria en el sur de Georgia. Para identificar correlaciones claves de agregaciones locales, examinamos variación diurna en la intensidad de la migración y la agregación costera de 11 especies de rapaces que difieren en su tamaño y vuelo. Al igual que otras zonas de convergencia, la intensidad de migración de grandes especies planeadoras por obligación aumentan al medio día, mayor periodo de actividad térmica. Cuando las nubes se forman sobre el interior de las montañas y limitan la convección térmica, estas grandes especies planeadoras se ven obligadas a agregarse cerca de la costa. Sin embargo, planeadores migrantes de tamaños intermedios que ocasionalmente agitan sus alas para volar no exhibieron agregaciones costeras cuando las nubes sobre las montañas debilitan la convección térmica. Los números de individuos de Circus spp. que alternan entre planear y agitar las alas para su vuelo tiene un pico temprano en la mañana, en donde estas rapaces dependen más en agitar las alas para su vuelo durante la hora del día con pobre convección térmica. El pequeño Accipiter spp. se agrega en las costas durante los períodos cuando los vientos soplan hacia afuera de la costa sugiriendo una agregación causada por la deriva del viento. Así, las condiciones climáticas, incluyendo cobertura de nubes y velocidad y dirección del viento, pueden influir los ritmos diarios y las rutas de vuelo de las rapaces durante la migración y, por lo tanto, se deberían tener en cuenta antes de inferir tendencias poblacionales basadas en conteos durante la migración.