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Global Ecology and Conservation 28 (2021) e01655
Available online 1 June 2021
2351-9894/© 2021 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Vocal behavior of the Mountain Hawk-Eagle Nisaetus nipalensis
in Taiwan
Yung-Kun Huang
a
, Hou-Chun Chen
b
, Po-Jen Chiang
c
, Yuan-Hsun Sun
d
,
*
a
Institute of Bioresources, National Pingtung University of Science and Technology, Ping-Tung 912, Taiwan
b
Central Animal Facility, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
c
Formosan Wild Sound Conservation Science Center Co., Ltd., Taoyuan 326, Taiwan
d
Institute of Wildlife Conservation, National Ping-Tung University of Science and Technology, Ping-Tung 912, Taiwan
ARTICLE INFO
Keywords:
Mountain Hawk-Eagle
Nisaetus nipalensis
Vocalization
Raptor
Taiwan
Temporal variation
ABSTRACT
The Mountain Hawk-Eagles (Nisaetus nipalensis) is a large raptor native to Asia. Despite the wide
distribution of this species, their vocalizations have been described only occasionally, and no
studies have provided detailed outcomes for other researchers to follow. This study aimed to
obtain systematic and comprehensive results on the vocalizations of the Mountain Hawk-Eagle
and a preliminary understanding of its vocal behavior in terms of temporal changes. This study
performed a road transect survey along two forestry roads between January 2006 and December
2008 in Taitung County. Two nests were found and observed upstream of the Dazhu and Fang-
shan streams during the breeding season in 2007 and 2010, respectively. All vocalizations, vocal
behavior, and behavior context were recorded. Mountain Hawk-Eagles were discovered to pro-
duce at least seven types of vocalization depending on context and structure. The ying call was
recorded throughout the year and was made by both sexes. Some vocalizations were recorded
only during the breeding season or could only be identied around the nest area; for example, the
female begging call, juvenile begging call, and alarm call. From the road transect survey and
observation of nests during breeding seasons, we discovered temporal variation in the call rate of
the Mountain Hawk-Eagle. The ndings and recordings made in this study provide insight into the
vocal behavior of the Mountain Hawk-Eagle, but further investigations are needed.
1. Introduction
Vocalizations are a type of communication widely used among avian species (Gill, 1994). Most related studies have focused on
passerine species (Crouch and Mason-Gamer, 2019; Magoolagan and Sharp, 2018, Raihani and Ridley, 2007; Shieh, 2004; Weir, 2018),
with relatively few studies investigating the vocalizations of raptors. Research on raptors has included basic descriptions of vocali-
zations and vocal activity (Ishtiaq and Rahmani, 2005; Leonardi et al., 2013; Odom and Mennill, 2010; Penteriani, 2001, 2002),
individuals (Grava et al., 2008; Grivas et al., 2009; Holschuh and Otter, 2005; Yee et al., 2016), sex (Odom and Mennill, 2010), species
identication (Gjershaug et al., 2020; Gjershaug et al., 2008; Rasmussen et al., 2012), temporal variation in and factors related to
vocalizations (Braga and Motta-Junior, 2009; Clark and Anderson, 1997; Penteriani, 2001), and methods for censusing populations
(Bogardus and Hatch, 2007; Burnett and Sieving, 2016; Hannah, 2009; Henneman et al., 2007; McClaren et al., 2003; McLeod and
* Corresponding author.
E-mail addresses: ykhuang50@gmail.com (Y.-K. Huang), ysunbird00@gmail.com, ysun@mail.npust.edu.tw (Y.-H. Sun).
Contents lists available at ScienceDirect
Global Ecology and Conservation
journal homepage: www.elsevier.com/locate/gecco
https://doi.org/10.1016/j.gecco.2021.e01655
Received 1 March 2021; Received in revised form 29 May 2021; Accepted 29 May 2021
Global Ecology and Conservation 28 (2021) e01655
2
Andersen, 1998; Mosher and Fuller, 1996; Navarro, 2005). The majority of published articles have focused on nocturnal species; few
have related to diurnal species. Hence, knowledge and understanding of the vocalizations of diurnal species are largely absent,
especially for rare species.
The Mountain Hawk-Eagle (Nisaetus nipalensis) is a large diurnal raptor that generally lives in forests and is currently present in
Japan, Taiwan, China, Vietnam, Thailand, Bangladesh, Nepal, India, and Sri Lanka (Brazil, 2009). The species is typically considered a
comparatively silent bird of prey, and reports on the vocal behavior of this species are insufcient. Two studies have described the
vocalization of the bird and the contexts of vocal behavior, but no quantitative data or sonograms were obtained (Morimoto and Iida,
1992; Tsai, 2007). Some articles have reported the production of sonograms of the Mountain Hawk-Eagle, but no actual measurements
or quantitative data were taken (Gjershaug et al., 2008; Kabaya and Matsuda, 1996; Rasmussen and Anderton, 2004). None of the
studies have systematically and comprehensively presented and described the vocalizations of Mountain Hawk-Eagles. Consequently,
conducting further study on the basis of existing ndings is difcult. For example, Gjershaug et al. (2008) concluded that N. n. kelaarti
is a species independent from N. kelaarti on the basis of their ndings of differences in morphological features and vocalizations. The
vocal data provided by Gjershaug et al. (2008) are not considered to be quantitative or systematic, undermining their results and
suggestions (BirdLife International, 2012). Although recent studies based on phylogeny have supported the split of N. kelaarti from
N. nipalensis into a distinct species (Lerner et al., 2017), it has also received general support. However, there is still a problem with
insufcient vocal research on Mountain Hawk-Eagle or Legge’s Hawk-Eagle (IOC World Bird List; The eBird/Clements Checklist of
Birds of the World; Legge’s Hawk-Eagle Nisaetus kelaarti). Hence, the possible vocal repertoire of the Mountain Hawk-Eagle, including
quantitative data, should be established before further questions can be answered.
This study aimed to systematically obtain all vocalizations of the Mountain Hawk-Eagle. Moreover, the bird’s vocal behavior
variation over different temporal scales was also analyzed. To obtain high-quality audio les of vocalizations, observe voice-related
behavior at close range, and initially record the vocalizations of eaglets, we observed and recorded near a nest during the breeding
season without causing any disturbance in addition to surveying along easily accessible forestry roads.
2. Materials and methods
The study site was located in the southern part of the Central Mountain Range in Taiwan (Fig. 1). The area included Pingtung and
Fig. 1. Location of study area in southern Taiwan.
Y.-K. Huang et al.
Global Ecology and Conservation 28 (2021) e01655
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Fig. 2. Call types and spectrograms for the Mountain Hawk-Eagle in Taiwan: (a) ying call, (b) female begging call, (c) juvenile begging call type 1, (d) juvenile begging call type 2, (e) trill, (f) parent-
offspring contact call, (g) alarm call, and (h) high-pitched call.
Y.-K. Huang et al.
Global Ecology and Conservation 28 (2021) e01655
4
Table 1
Measurements of each type of vocalization made by the Mountain Hawk-Eagle.
Call type n Measurement
Start frequency (Hz) End frequency (Hz) Peak frequency (Hz) Duration (s) Range (Hz)
Flying call Call 1 rst note 75 1879.14 286.5 2360.77 263.5 2360.77
a
263.5 0.16 0.09 481.63 175.81
Call 1 s note 70 2799.48 296.3 3178.23 254.56 3178.23 254.56 0.17 0.08 378.75 107.38
Call 2 rst note 59 1948.6 313.12 2411.21 301.11 2411.21 301.11 0.06 0.04 462.61 122.95
Call 2 s note 57 2905.78 383.92 3217.44 294.42 3217.44 294.42 0.08 0.04 311.66 131.73
Call 3 rst note 40 1874.89 206.54 2357.21 227.64 2357.21 227.64 0.07 0.04 482.33 103.84
Call 3 s note 38 2830.09 268.4 3131.59 250.46 3131.59 250.46 0.08 0.06 301.5 88.93
Female begging call 5 2601.7 23.41 2472.25 116.18 2725.48 19.7 0.12 0.02 253.23 120.56
Juvenile begging call Type 1 108 3203.16 506.82 3187.49 561.51 3511.06 539.15 0.19 0.09 307.9 187.61
Type 2 10 3853.65 385.89 3863.57 352.84 4239.09 306.09 0.17 0.02 385.44 147.66
Trill 11 1510.59 231 2120.57 91.52 2169.36 57.66 0.91 0.52 609.98 170.21
Parent–offspring contact call 1 1363.47 N/A 2095.8 N/A 2968 N/A 0.74 N/A 1604.62 N/A
Alarm call First note 3 2413.76 149.91 3234.76 349.16 3594.71 450.91 0.1 0.03 1180.94 595.48
Second note 3 2790.58 219.77 2944.95 42.97 3128.75 278.41 0.09 0.03 338.18 183.1
Third note 1 2231.74 N/A 2996.16 N/A 2996.16 N/A 0.2 N/A 764.42 N/A
High-pitched call 15 5020.19 355.14 4569.08 473.51 6105.28 647.38 0.06 0.03 1536.2 409.24
a
The peak frequency of the ying call appears at the end frequency, so these two measurements are the same.
Y.-K. Huang et al.
Global Ecology and Conservation 28 (2021) e01655
5
Taitung Counties, and the total area was 6290.09 km
2
. The percentage of land occupied by forest in these two counties is approxi-
mately 56.27% and 81.64%, respectively (Taiwan Forestry Bureau, 2016).
We performed a road transect survey to record the vocal behavior of the Mountain Hawk-Eagle in areas where this bird is
commonly found and along roads with favorable conditions. The road transect survey was conducted along the Lijia and Yianping
forestry roads in Taitung County between January 2006 and December 2008 (Fig. 1). The length of road transects along the forestry
roads was 5 km—the 12–17 km markers on Lijia forestry road and the 12–17 km markers on Yianping forestry road, respectively. The
survey was conducted on at least 3 days in each month. Vocal and nonvocal behavior was recorded on each survey day between 06:00
and 16:59. The survey was not conducted on days with adverse weather conditions (rain or strong wind). When surveying along a
forestry road, the researcher walked at 2 km/h or drove slowly and turned back when they reached the end of the transect. When the
researcher heard or saw a Mountain Hawk-Eagle, they stopped moving and made observations and recordings until the vocalization
had stopped or the eagle had left, at which point they continued moving. We also considered that the forestry roads would collapse
from earthquakes or extreme weathers (such as typhoons), and the engineering unit was closed for repairing the road, which made it
impossible to conduct an investigation in some months. Therefore, our investigation has been carried out for 3 years to achieve at least
3 survey days of effort per month.
Two nests were also found and observed: one in the upstream region of the Dazhu stream, with observation made between March
and June 2007, and another upstream of the Fangshan stream, with observation made between March and May 2010. A blind tent was
constructed 30–70 m away from each nest. Each nest was observed on at least 7 days a month, and observation was performed from
05:00–18:59 each day. All vocalizations, vocal behavior, and behavior contexts were recorded through observation inside the blind
tent.
The call types and number of call events were recorded by the researcher. When the same type of call was continually produced and
the silent interval between each call was less than 30 s, the calls were counted as one call event. If different call types were produced
continually within a 30 s period, all calls were counted as a single call event; however, the call type details were noted in the records.
We normalized the number of call events recorded by the variation in the number of call events in each hour of the day and the
variation in the number of call events in each month of the year to obtain the hourly call rate (no. of call events per hour) and daily call
rate (no. of call events per day), respectively. We did not distinguish between call types in the analysis of the hourly call rate variation,
but we did distinguish each call event into different call types in the analysis of the daily call rate variation. Call types that were
recorded fewer than 10 times were not included in the analyses. The recorded call combinations were considered separately to
calculate the frequency. For example, a ying call +female begging call was counted as one ying call and one female begging call.
When analyzing nest observations during the breeding season, the data from only a complete observation day were employed. The call
audio recordings from the nest observation were used to distinguish between adult and juvenile vocalizations.
We divide the months of the year into breeding season (Feb-Jun), non-breeding season (Jul-Oct), and pre-breeding season (Nov-
Jan) according to the breeding cycle of Mountain Hawk-eagles (Sun et al., 2009; Sun, 2007; Tsai, 2007). Use the Kruskal-Wallis H test
to test whether there are differences in the daily call rate of different call types recorded in the line transect survey between the
aforementioned 3 seasons. Similarly, test the main call types of adults and juveniles recorded in the nest observation, and whether
there are differences in the daily call rates between the months (Mar-Jun) in the breeding season. We also tested whether the hourly
call rates from the two surveys are different at various times of the day. The level of signicance was set to
α
=0.05.
All vocalizations of Mountain Hawk-Eagles were recorded using a digital recorder and a digital camcorder (Sony M-10 and DCR-
HC90, respectively; Sony Inc., Minato, Tokyo, Japan). All video data were transformed into uncompressed video, and the audio tracks
were extracted and then saved as waveforms. Five call parameters were selected and analyzed: start frequency, end frequency, peak
frequency, duration, and frequency range of the note. Sound les were analyzed using sound analysis software (Raven Pro 1.4, Cornell
University).
3. Results
3.1. Call types
On the basis of call structure and the context in which a call was heard or recorded, seven types of call were concluded to have been
recorded. Samples of seven call types were successfully collected.
(1) The ying call was the most commonly heard call during the study. This call comprises a certain number of note sets, with each
set consisting of two notes. The frequency of the rst note is lower than that of the second note (Fig. 2a, Table 1). Two- and
three-set ying calls were the most common calls recorded during the study. The Mountain Hawk-Eagle does not produce ying
calls in only one particular season; ying calls are audible throughout the year, and both males and females make this call. The
juvenile occasionally makes this call as it becomes a edging. Several occasions of Mountain Hawk-Eagles producing this call
were recorded, and these recordings were mostly of individuals during ight. For example, the call was made when the male
was performing the courtship display, a breeding pair was cooperatively hunting Formosan rock macaques (Macaca cyclopis; the
caller was in ight), and the male was bringing food back to the nest. Sometimes, the male also produced it while delivering food
to the adult female. Adult females also produced a ying call when they perched on a tree trunk and were mobbed by other birds
such as the Eurasian Jay (Garrulus glandarius). The call events were longer on such occasions. The longest call event lasted more
than 40 min. Most of the call les recorded in this research were obtained from the nest observation, with a few recorded during
the road transect survey.
Y.-K. Huang et al.
Global Ecology and Conservation 28 (2021) e01655
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(2) The female begging call contained several single repeated call notes, with up to 16 notes within a single call. However, the
female sometimes produced another type of female begging call comprising a set of two short notes (Fig. 2b, Table 1). Female
begging calls were often heard during the breeding season, particularly when the adult female was near the nest. No recordings
of this call were made outside of the breeding season. On some occasions, after the female uttered this call, a male responded
with a ying call and approached the female or returned to the nest (n =8). In a few of these recordings, the male was
conrmed to be carrying prey. This call also appeared in an incident in which a breeding pair was cooperatively hunting for
macaques. In most contexts, the call was made by a female perched near the nest and a male did not appear or respond (n =18).
Therefore, we speculate that this voice may not be only a female begging call but also the exclusive contact call function through
which a female communicates with its mate. The female begging call les recorded during the nest observation were used for
this analysis.
(3) The juvenile begging call was similar to the female begging call; it mainly contained a repeated single note (Fig. 2c-d, Table 1).
However, the frequency was higher than that of the adult female begging call. The structure of the call changed as chicks grew
up, with more variety introduced to the call. Chicks normally produced this type of call when the parent brought food; the call
was accompanied by a lowering of the head or a sideways stare at the parents and a lowering of the body, with the wings slightly
opened (Fig. 3). The chick began to produce this call a couple minutes before the adults returned to the nest. Like the begging
call of the adult female, the juvenile begging call analyzed in this study was recorded during the nest-side observation.
(4) The trill was a long rapid modulated call (Fig. 2e, Table 1). It was normally recorded along with the ying call, but it was
sometimes also recorded on its own. This type of call was usually recorded when an adult male performed the courtship display
in the prebreeding season. One recording of this call was made when the researchers tried to search for a nest from the side of the
forestry road; an adult was perched on a conspicuous branch and uttered a trill near the researchers (March, 2007). One author
also recorded one trill when a breeding pair was hunting Formosan rock macaques.
(5) Parent–offspring contact call: The structure of the parent–offspring contact call was similar to the rst note of the ying call but
of longer duration (Fig. 2f, Table 1). Most parent–offspring contact calls were either produced independently or combined with
other types of call such as the ying call, female begging call, or trill. This type of call was recorded only during the breeding
season, between March and June, but more often in May–June. Adults made this call during the breeding season, and juveniles
also made this call before they became edglings (at 70 days old). Of the 15 records of this call type, two records were of a
faraway call, and two were made in dense fog; thus, the identity and behavior of the caller and receiver could not be determined
for these recordings. Eleven recordings were made during the nest observation. In these recordings, the caller was one of the
parents (conrmed to be an adult female in three recordings); in nine recordings, the receiver may have been an almost edging
or edging, and the juvenile subsequently uttered a ying call or begging call to respond. We did not record this call type clearly
(because of the long distance and the call not being used often).
(6) Alarm call: This type of call was always accompanied by a ying call. Adults usually produced a ying call followed by one or
two alarm call notes. The alarm call sounded like “hge-hge-” with a low-frequency aspirated sound (Fig. 2g, Table 1). Almost no
alarm calls could be recorded because they occurred quickly. These calls were observed on only two occasions. The call was rst
observed when adults ew close to the blind tent in which the researchers were hidden in 2007; the caller was perching on a
branch 3–5 m above the blind tent and was faced downwards—that is, toward the researcher—and was watching and calling.
The observer also heard similar vocalizations from one handled individual while handling and restraining the birds during the
banding process in 2010. The call comprised a low frequency.
(7) High-pitched call: This was an ultrahigh-frequency call type. This type of call was not heard to be made by the wild individuals
observed in this study; it was only found to be made from a captive-raised individual. Most context of high-pitched calls from
this caller was quite indistinct and obscure. The caller just called briey when it perching on the branches in the cage. The other
species in the adjacent cages before and after this calling had no obvious behavior, and the keepers who were active in the open
Fig. 3. Female Mountain Hawk-Eagle and a chick begging for food.
Y.-K. Huang et al.
Global Ecology and Conservation 28 (2021) e01655
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area outside this cage were at least 20 m away. But only once was the dedicated keeper entering the individual’s cage and
placing the eagle’s food, it briey uttered high-pitched calls and walked on the same branch. The other time was when the
researcher was setting the blind tent in front of this cage, this individual made two call events within 10 s, and ew briey from
the branches at one end of the cage to the branches at the other end between the call events. The exact function of this call
remains unclear (Fig. 2h, Table 1).
3.2. Road transect survey
A 178-day road transect survey was conducted in Taitung over a period of 3 years. Points of view to the effort of each month, the
average number of survey days per month was 14.83 days [4–27, standard deviation (SD) =5.86]. In total, 84 vocal events were
recorded; the type of call could not be determined for 32 of these events because of the long distance between the researcher and the
caller. For the other 52 vocal events, clear recordings of the type of vocalization were obtained.
Mountain Hawk-Eagles were commonly found to produce more than one call type within a single call event. The combinations
included ying calls plus trill calls, ying calls plus female begging calls and trills, ying calls plus parent–offspring contact calls, and
female begging calls plus trill calls. Among these combinations, ying call plus trill was the most common call combination within the
call events. Of the 52 clear vocal events, ying calls were the most common vocalizations (n =19), followed by ying calls plus trill
calls and trill calls alone. Nearly 77% of the vocal events recorded during the road transect survey (40/52) involved ying calls.
The road transect survey revealed that the hourly call rate in a day varied (Fig. 4). After the Kruskal-Wallis test, it was indicated that
there is a signicant difference in the hourly call rates between the hours of a day (H =31.685, df =11, p =0.001). The hourly call rate
peaked at 11:00, the average hourly call rate during this period was 0.11 times/hour, and the rate was lower at dawn and dusk.
Regarding the four outlying data points revealing remarkably high hourly call rates in Fig. 4, three were recorded during the mating
display period (November–January) before the breeding season, and one was recorded during the breeding season (March).
The monthly variation in the daily call rate for the two most common call types (ying call and trills) is illustrated in Fig. 5. The
difference in daily call rates of the ying call and the trill between seasons has reached a signicant difference (Kruskal-Wallis test,
H=17.992, df =2, p=0.001; and H =12.657, df =2, p=0.002). The daily call rate was highest between November and January for
both ying calls and trills; it peaked in December, reaching 1.17 and 0.67 times/day for the two call types, respectively. The daily call
rate from February to October was lower, regardless of the type of call.
In 26 of the veried vocal events, visual observation was achieved. Most of the occasions on which Mountain Hawk-Eagles pro-
duced calls involved ying-related behaviors including soaring and diving. Mountain Hawk-Eagles produced the ying call plus till
combination when they performed the courtship display from November to December (n =6).
3.3. Observation during the breeding season
Nest observation was performed for a total of 51.5 days in 2007 at the Dazhu stream and in 2009 and 2010 at the Fangshan stream.
The nest site at the Dazhu stream (2007 only) and at the Fangshan stream (2009, 2010) belonged to different breeding pairs. Although
the nest sites at the Fangshan stream in 2009 and 2010 were in different nest trees, because they were very close (the straight-line
distance is about 300–500 m), we assume that they may be in the same breeding area. However, since we did not tag these in-
dividuals (leg band tags or wing tags), and there is considerable hunting pressure in the local area, we cannot conrm whether they
were the same breeding pair. We recorded 521 call events: 366 call events from adults and 155 call events from juveniles. The dif-
ference in the hourly call rate of adults in each hour of the day reached a signicant difference (Kruskal-Wallis test, H =47.215,
df =13, p=0.001), but the difference in the hourly call rate of juveniles between time periods was not signicant (H=20.478, df =13,
p=0.08). From the observation results obtained near nests during the breeding season, the hourly call rate (Fig. 6) of adults (green
line) peaked at 08:00 (average: 1.18 times/hour) and then gradually decreased until 18:00. The call rate of juveniles (orange line)
Fig. 4. Hourly call rate at various times of the day from the road transect survey (n =144).
Y.-K. Huang et al.
Global Ecology and Conservation 28 (2021) e01655
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peaked at 07:00 (average: 0.73 times/hour) and then gradually decreased; a second small peak occurred at 17:00 (average: 0.43 times/
hour). The average hourly call rate for chicks was 0.08 times/hour before 18:00. The average daily call rate was 10.29 times/day.
Five types of calls (ying call, female begging call, parent-offspring contact call, trill, and juvenile begging call) and 11 call
combinations were observed during the breeding season, but we only analyzed four call types for which more than 10 recordings were
obtained. Throughout the breeding season, the average daily call rate for adults was 8.24 times/day (4.55–10.55, SD =2.69). The daily
call rate was highest in April and May (9.9 and 10.55 times/day, respectively) and lowest in March (4.55 times/day; Fig. 7). During
this period, the highest adult call rate was that for ying calls, followed by female begging calls. Adults made more parent–offspring
contact calls from May to June. The daily calls rate of the ying call is not signicantly different from month to month (Mar-Jun)
(Kruskal-Wallis test, H =4.485, df =3, p=0.214). However, the daily call rate of the female begging call, the parent-offspring contact
call, and the trill are signicantly different among months (Mar-Jun) in the breeding season (H =16.61, df =3, p=0.001; H =14.25,
df =3, p=0.003; H =24.94, df =3, p=0.001, respectively).
The average daily call rate for juveniles was 3.85 times per day (0.18–9.00, SD =3.95), and the daily call rate gradually increased
from March (0.18 times/day), with the highest call rate occurring in June (9 times/day; Fig. 8). The majority of calls made by chicks
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
flying call
trill
Fig. 5. Monthly variation in daily call rate for two types of call of Mountain Hawk-Eagles recorded in the road transect survey (n =144).
Fig. 6. Hourly call rate of the Mountain Hawk-Eagle during the nestling period (n =144).
Y.-K. Huang et al.
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were juvenile begging calls, followed by ying calls. The daily call rate of the juvenile begging call varied from month to month (Mar-
Jun) (H =18.095, df =3, p＝0.001). The daily rate of juvenile begging calls increased from 0.18 times/day immediately after
hatching in March to 7.33 times/day in June. The daily call rate of the ying call also has a signicant difference among months (Mar-
Jun) (H =10.453, df =3, p=0.01). The ying call had developed completely when the young birds were 55 days old in May, when
the daily call rate was 0.45 times/day; this increased to 1.67 times/day in June. Only one audio recordings were parent–offspring
contact calls. This type of call was only recorded when the juvenile had reached 70 days old.
4. Discussion
4.1. Call types
Call description for Mountain Hawk-Eagles has often focused on ying calls (Ferguson-Lee and Christie, 2001; Severinghaus et al.,
2017). This type of call is probably the most common vocalization of the species and in the present study, ying calls were the only type
of vocalization documented throughout the entire year. Our results also suggested that adult and young Hawk-Eagles in Taiwan also
produce at least ve other types of vocalization during the breeding season and nonbreeding seasons. A captive Hawk-Eagle also
produced one other type of vocalization that was never heard from wild individuals. When a bird’s call is heard from a long distance,
the high frequencies are absent due to the susceptibility of high frequencies to degradation in the forest environment (Mathevon et al.,
2008). The high-pitched call had very high frequency and short duration, which only been recorded from the captive bird. Conse-
quently, it may not have been transmittable over a long distance. Because most of the vocalizations collected from wild individuals in
this study were obtained from at least 30 m away, the observer may not have been able to record such high-pitched calls even if they
were produced by wild Mountain Hawk-Eagles.
Most birds of prey produce ying calls during ight. A study on the Ornate Hawk-Eagle (Spizeatus ornatus) suggested that adult
males tend to vocalize when they y close to their nest (Klein et al., 1988). Our study showed that although most ying calls are made
while Mountain Hawk-Eagles is in the air, they can also be produced by perching birds. From the perspective of behavior and context,
ying calls may have several functions, including advertising the caller’s existence (territory advertisement call), calling the mate and
enabling it to perceive the distance from the caller and its relative position (especially when hunting with a breeding mate; contact
call), and informing the spouse of delivery of the prey (food delivery call). Whether the function of the ying call was to defend
territory or communicate with a group member was unclear. The precise function of ying calls must be further investigated.
Flying calls were often produced along with other types of vocalization such as trills and alarm calls. Why the Mountain Hawk-
Eagles produced these call combinations is unclear because those calls were actually produced alone on many occasions. Whether
such combinations have varying functions from when the calls are produced on their own is also vague. Many studies have reported
0.00
1.00
2.00
3.00
4.00
5.00
6.00
Mar Apr May Jun
Daily call rate
Month
flying call
female begging call
parent-offspring contac
t
call
trill
Fig. 7. Variation in daily rate of several call types for Mountain Hawk-Eagle adults observed and recorded at their nest in each month during
breeding season (March–June) (n =56).
0.00
1.00
2.00
3.00
4.00
5.00
6.00
7.00
8.00
Mar Apr May Jun
Daily call rate
Month
flying call
juvenile begging call
Fig. 8. Variation in daily rate of several call types for Mountain Hawk-Eagle chicks observed and recorded at their nest in each month during
breeding season (March–June) (n =56).
Y.-K. Huang et al.
Global Ecology and Conservation 28 (2021) e01655
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that passerines’ call combinations have specic meaning (Engesser et al., 2016; Suzuki et al., 2016). Suzuki et al. (2016) studies
conrmed that Japanese Great Tits (Parus minor) has the ability to recognize the combination of different meaningful units as
compositional calls and in line with the criteria for compositional syntax. A cooperatively breeding passerine, Southern Pied Babbler
(Turdoides bicolor) receiver can extract elementary composition information from aurally different, context-specic calls: alert and
recruitment calls. Moreover, the caller exibly combines recruitment calls with at least two other call types with different functions
according to different contexts (Engesser et al., 2016). However, reports on the call combination of diurnal raptors are quite lacking.
Penteriani (2001) reported that 63.2% of the chattering calls made by Northern Goshawks (Accipiter gentilis) precede and/or follow one
or a sequence of calls at an interval of ≤1 min, but whether the meaning of the combination of these calls is greater than that of the
separate calls was not discussed. Our study could not clarify whether the call combinations observed in this study were the same as for
any cases of the aforementioned species. Further research is required to determine whether Mountain Hawk-Eagles can organize their
calls to express different meanings or enhance meaning, as has been found for primates and some bird species.
The female begging calls observed in this study may have been produced by only adult females; more observation is required to
conrm this. If this is true, the vocalization could be employed to monitor the number of breeding pairs in a breeding season. Trill calls
and other call combinations were often observed when Mountain Hawk-Eagles were in a highly emotional state, such as when per-
forming the mating display or hunting with a pair member. While the researcher was trying to nd a nest on the forestry road, an adult
was observed uttering trill calls. We are unsure whether the caller’s vocalization was evoked by the researcher’s appearance. Assuming
the two were related and there was a nest nearby, the call event and related behavior may have been a displacement-type distraction
display (Humphreys and Ruxton, 2020), in which the caller was aiming to attract the attention of a potential threat to protect the eaglet
in its nest. When we visited the hunters of the Paiwan ethnic group in southern Taiwan, they also revealed traditional ecological
knowledge describing similar distraction display behavior. Interviewees from the Paiwan ethnic group mentioned that Mountain
Hawk-Eagle parents would deliberately call in the opposite direction of the nest to mislead hunters’ approaching to the nest. Further
experiments are needed to determine the exact function of these calls.
We recorded parent–offspring contact calls that may have been made by the parent to the juvenile, and the juvenile responded with
a begging or ying call in most cases. (Carlson, 2020) collected parent–offspring vocalizations for various birds, some of which were
one-way and others two-way. Whether the communication between parent and juvenile Mountain Hawk-Eagles in Taiwan can be used
to identify individuals and whether the communication is two-way or one-way must be claried by subsequent experiments.
Parent–offspring calls were frequently recorded in the preedgling period (May) and postedging dependence period (after June).
Therefore, we conclude that the main function of this call type is to connect parents with offspring who have just acquired the ability to
y, identify offspring, deliver food, and obtain information about the offspring’s location. (O’Toole et al., 1999) mentioned that the
call rate of the juvenile Golden Eagle (Aquila chrysaetos) after edging is higher when the juvenile is accompanied by its parents than
that when the parents are absent. Increased calling may help the Golden Eagle locate its offspring or inform the parent of the chick’s
nutritional status. How long this call persists between parent and juvenile Mountain Hawk-Eagles lasts during the postedgling
dependence period also needs to be conrmed by follow-up studies.
Juvenile begging calls all had identical structure in the rst few weeks after hatching. Later, the call structure began to vary and
contain multiple notes within a single call. Flying calls were recorded one or two weeks before the chick left the nest. Such devel-
opment suggests that the vocal repertoire of the Mountain Hawk-Eagle may start to develop before the juvenile leaves the nest.
Previous research has not described the process of vocal development in young Mountain Hawk-Eagles, but this type of development
has been discovered in other nonpasserine species including Cranes (Grus spp.; Klenova et al., 2007, 2014) and Falcons (Falco spar-
verius; Smallwood et al., 2003). The question remains whether all vocalizations are genetically determined or acquired from later social
context. Further playback experiments may help answer this question.
All alarm calls recorded in this study were almost immediately followed by ying calls. This type of call was mainly recorded when
a Mountain Hawk-Eagle approached the blind tent in which the researcher was located. The population of Mountain Hawk-Eagles in
southern Taiwan has been suggested to have experienced a high level of hunting pressure (Sun, 2007, 2010). This is attributable to the
demand for the feathers of Mountain Hawk-Eagles in Indigenous cultures and that for chicks for Taiwan’s falconry market (Sun, 2007).
Consequently, during the breeding season, adult Mountain Hawk-Eagles expressed alert behavior or even attack behavior against
humans who approached the nest (Sun, 2007, 2010). Thus, alarm calls were frequently made during the breeding season.
The alarm call or nest defense call of many raptors is closely related to the breeding season (Klein et al., 1988; Lyon and Kuhnigk,
1985; McClaren et al., 2003; Penteriani, 2001). Among the seven types of Mountain Hawk-Eagle call recorded in Japan, ve were
performed when a potential predator approached (Morimoto and Iida, 1992). The “Gutsugutsu” mentioned by Morimoto and Iida
(1992) is similar to the alarm call recorded in the present research in terms of sound narration and context. Morimoto and Iida (1992)
reported that the sounds were related to different levels or states of alertness, perhaps showing that the Japanese subspecies of the
Mountain Hawk-Eagle makes ne distinctions between different threats to the nest or individuals. The Mountain Hawk-Eagle in
Taiwan may make sounds and display corresponding behaviors in varying contexts that were not recorded in this study. Identication
of call types by observers who in this research and past research after observing and recording these vocal behaviors may also have
been subjective. When Lyon and Kuhnigk (1985) described the breeding behavior of the Ornate Hawk-Eagle, they mentioned that
parents exhibited nest defense behavior in several contexts, including calling, and that the strength of nest defense depended on the
distance of the intruder.
One study reported that Mountain Hawk-Eagles in Taiwan utter calls when they encounter Formosan rock macaques, Taiwan Blue-
Magpies (Urocissa caerulea), and Himalayan Tree-Magpies (Dendrocitta formosae; Tsai, 2007). However, no details were provided
regarding the types of call. Female Hawk-Eagles were observed in the present study to attempt to expel the Yellow-Throated Marten
(Martes avigula chrysospila), Eurasian Jay, and Crested Serpent-Eagle (Spilornis cheela). In Japan, mobbing of Mountain Hawk-Eagles
Y.-K. Huang et al.
Global Ecology and Conservation 28 (2021) e01655
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by Corvidae has also been recorded; during such mobbing, the Mountain Hawk-Eagles made a warning or screaming “Pyiyo” call
(Morimoto and Iida, 1992). In the present study, mobbing by the Bronzed Drongo (Dicrurus aeneus braunianus), Eurasian Jay, and Large
Cuckooshrike (Coracina macei) was observed, but the female Mountain Hawk-Eagle made a female begging or ying call only when the
mobbing was by a Eurasian Jay.
4.2. Daily and monthly variation in vocalizations
4.2.1. Intermonth variation in daily call rate
The overall call rate was discovered to be higher during the period of November–April in our road transect survey; this timing
conforms to the preterm to midterm breeding period of Mountain Hawk-Eagles (Sun, 2007, 2010; Tsai, 2007). The rate of ying calls
and trill peaked in November–December in the road transect survey, and these calls were accompanied by the wavelike ight or
porpoising courtship display. Tsai (2007) reported that Mountain Hawk-Eagles perform courtship displays—such as the mated pair
gliding together, porpoising, and aerial talon-grappling—from December to January, and this result approximately conforms with the
monthly call rate variation in our study. Penteriani (2001) mentioned that the annual voice pattern of the adult Northern Goshawk
primarily has the functions of territorial defense and internal communication. The prominent peak in the Goshawk’s vocalization
during the year is consistent with its courtship period; thus, Penteriani assumed that the Goshawk also uses calls to ensure the paternity
of offspring. Generally, the call rate was lower from May to September, which exactly corresponds to the edgling stage. In addition,
there might be many subtle calls produced between individuals which cannot be recorded by the researcher in transect surveys. Among
the call types we have included, there are several call types may be classied as subtle calls that may not be spread far away due to
weaker call intensity or higher frequencies. For example, juvenile begging call, and high-pitch call.
As we discussed earlier, due to high-pitch call had very high frequency and short duration, which are absent due to the suscep-
tibility of high frequencies to degradation in the forest environment. The subtle juvenile begging call is only recorded near the nest
during the breeding period, which also has considerable temporal and spatial limitations for survey records.
Observations made in the breeding season (March–June) near the nest site revealed that adults make calls of various types during
this period. The daily call rate was highest in April and May. This was because these two months are the most critical time for brooding.
We observed many vocal behaviors related to this, including food delivery and the female calling to the male. Morimoto and Iida
(1992) reported similar vocalization of the Mountain Hawk-Eagle during the breeding season in Japan. Results from the observation
during breeding season revealed an increase in the call rate of juveniles as they age, with these calls being not only the juvenile begging
call, which is uttered from when the chick hatches, but also the ying call, which develops later. This trend is similar to that in juvenile
Northern Goshawks in France (Penteriani, 2001). The increasing volume of chicks’ vocalizations as they grow older makes it easier for
researchers to detect its vocal behavior, as does the increasing parent–chick communication as the chicks age. The parent–offspring
contact call developed when the juvenile was close to becoming a edgling. Our only record of a parent–offspring contact call from a
juvenile was obtained when the juvenile was responding to the same call type from the parent on the other side of the valley.
4.2.2. Variation in hourly call rate among different hours of the day
The results of the road transect survey demonstrated that the call rate of Mountain Hawk-Eagles peaked at 11:00 and was lower at
twilight, with the representative graph having a single-hump shape. The peak hourly call rate of Mountain Hawk-Eagles during a day
was similar to results obtained through observation and radio telemetry. The hunting behavior of the Mountain Hawk-Eagle in Japan is
most common at 10:00–11:00 and 13:00–14:00 (Morimoto and Iida, 1992). In Taiwan, the activity patterns of three adults tracked
using radio telemetry peaked at 09:00–11:00 and then decreased gradually. Sometimes, a small peak in activity was discovered at
16:00–17:00, but ight activity ceased by 18:00 (Sun, 2007, 2010). These patterns are different from what was observed at nests
during the breeding season. The breeding period observation uncovered that the hourly call rate of adults and juveniles peaked at
08:00 and 07:00, respectively, and then gradually decreased until a small peak occurred at 15:00 and 17:00, respectively.
The breeding pairs observed in the road transect survey and at nearby nest sites in the breeding season were different individuals;
nonetheless, more records were made of vocal behavior near the nest site than that at the road transect during the breeding period,
regardless of the call rate, basic call type, and call combination. This may imply that vocal behavior is more frequent around the core
area near the nest site than on the edge of the home range. Morimoto and Iida (1992) also mentioned that the vocal behavior of the
Japanese Mountain Hawk-Eagle during the breeding season is mostly concentrated within a 1-km radius of the nest. However, this
must be veried by more intensive vocal behavior investigations at different distances from nests in the home range of a given breeding
pair.
5. Conclusions
This study recorded seven call types and vocal backgrounds: the ying call, female begging call, juvenile begging call, Paren-
t–offspring contact call, trill, alarm call, and high-pitched call. Among them, the ying call was the most frequently recorded and could
be heard year round. The female begging call, juvenile begging call, and Parent–offspring contact call were mainly recorded during the
breeding season from March to June.
The road transect survey conducted throughout the year revealed variation in the daily call rate among months, with the call rate
being higher during the courtship display period (November–December). The hourly call rate peaked 1–2 h before noon during the
day. Observation at nests during the breeding season (March–June) showed that adults performed up to ve call types during this
period. The daily call rate of adults was higher in April–May. The daily call rate (juvenile begging call) of juveniles increased with their
Y.-K. Huang et al.
Global Ecology and Conservation 28 (2021) e01655
12
growth and peaked in June before the juveniles became edglings. As the juveniles grew, other call types gradually developed (the
ying call and parent–offspring contact call). The hourly call rate of adults peaked 4 h before noon (08:00) during the day, and there
was also a small peak in the third hour after noon (15:00). The pattern of the juveniles was slightly different from that of the adults;
their hourly call rate peaked 5 h before noon in the day (07:00), and a small peak at 17:00 before nightfall was found. At present, the
function of each call type of the Mountain Hawk-Eagle is unknown, and it could be claried by operational experiments in the future.
This study systematically recorded the call types of the Mountain Hawk-Eagle and is expected to serve as a foundation for further
research on this bird’s behavior, population monitoring, or clarifying the status of classication.
Declaration of Competing Interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to
inuence the work reported in this paper.
Acknowledgment
This research was supported by the Taiwan Forestry Bureau (95-00-8-02) and Pingtung Forest District Ofce (98R23). Rong-De
Zhu, Zhen-Cheng You, Zhen-Qi Wu, Fang-Ru Li, Zheng-Xiong Yang, Shiao-Yu Hong, Yan-Ting Lai, Chih-Yao Chang, Hong-Chang
Chen, Ming-Shu Jian, Ming-De Jian, and Hua Jian assisted us in collecting data in the wild. We also thank Yu-Sheng Cheng and Yi-
Wen Yang for their considerable help. We are very grateful to the Pingtung Rescue Center for all their assistance during our
recording the vocalizations of the bird in captivity. We are indebted to Zen-Ci Wu and Fu-Chian Yang for their information about the
vocalization of the Mountain Hawk-Eagle. This manuscript was edited by Wallace Academic Editing.
Appendix A. Supporting information
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.gecco.2021.e01655.
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