A preliminary
report on the development of young Indian Eagle Owl Bubo bengalensis(Franklin, 1831) in and around Puducherry, southern
India
M. Eric Ramanujam1 & T. Murugavel ²
1 Principal Investigator (Faunistics),Pitchandikulam BioresourceCentre / Pitchandikulam Forest Consultants, Auroville,
Tamil Nadu 605101, India
2 Core Project Co-ordinator, Trust for
Environmental Monitoring and Action Initiating, 101, 4thStreet, A I Block, Shanti Colony, Anna Nagar,
Chennai, Tamil Nadu 600040, India
Email: 1 ericramanujam@yahoo.co.in; 2 hawkcuckoo@yahoo.com
Date of
publication (online): 26 October 2009
Date of
publication (print): 26 October 2009
ISSN 0974-7907
(online) | 0974-7893 (print)
Editor: Rajah Jayapal
Manuscript
details:
Ms # o1762
Received 16
April 2007
Final received
15 October 2009
Finally accepted
15 October 2009
Citation: Ramanujam, M.E. & T. Murugavel(2009). A preliminary report on the development of young
Indian Eagle Owl Bubo bengalensis (Franklin,
1831) in and around Puducherry, southern India. Journal of Threatened Taxa 1(10): 519-524.
Copyright: © M. Eric Ramanujam & T. Murugavel2009. Creative Commons Attribution 3.0 UnportedLicense. JoTT allows unrestricted use of this article
in any medium for non-profit purposes, reproduction and distribution by
providing adequate credit to the authors and the source of publication.
Author Details: M. Eric Ramanujamcurrently involved in biodiversity surveys of vertebrates on the Adyar Poonga Project (wetland
restoration) in Chennai. Has previously conducted surveys of
the Kaliveli region under the European Commission and
UBS projects. Has been researching imagery as a tool to enhance
environmental sensitization and has illustrated posters, booklets, journal
articles, etc. Has been conducting autecologicaland ethological investigations on Bubo bengalensissince 1997.
T. Murugavel has been involved in
environmental activism in addition to being an amateur bird watcher and
photographer. Currently employed as Professor (English), Sri Venkateswara College of Engineering, Pennalur,Sriperumbudur, Tamil Nadu.
Acknowledgement:We
are greatly indebted to photographers A. Lakshmikantanand R.T. Sriram for their excellent work in recording
the natural history of the Indian Eagle Owl.
Abstract:Morphometric, weight and photographic
data on the growth patterns of young Bubo bengalensiswas obtained in three areas in and around Puducherry.
We continuously monitored and observed the growth patterns of nine chicks in
total between 10 and 60 days after hatching. Among the morphometric variables, bill and
wing chord length showed a mild yet steady increase throughout the observation
period, while tarsus length displayed a steep increase till the 20th day, fluctuated till the 40thday and then increased marginally till the 60thday. Body mass showed a steady increase
before reaching a plateau between 35 and 45 days, after which only a marginal
increase was observed. Chicks were born with whitish natal down. The prejuvenile moult occurred around the 14th or
15th day. Full
juvenile plumage was attained by the 35th day and
persisted even after 60 days. Morphometric data showed a marked variability, and in this
context, ageing of nestlings based on measurements alone may not be accurate
and must be complemented with feather moult patterns.
Keywords:Bubo bengalensis, morphometry, nidification
For Figures, Images &
Tables – click here
Introduction
Growth rates
represent a fundamental aspect that can provide information for ageing
nestlings in birds. Unfortunately, in spite of their importance in bird
population monitoring for environmental degradation, little information on the
development of young Strigiformes is available in
India. To the best of our knowledge, detailed literature is available only for
two species: nidification of Bubo bengalensis (Eates 1937) and morphometry and nidification of Athene brama (Kumar
1985). This is in stark contrast to
studies in other parts of the world where Strigiformesare among the best studied taxa for nesting and nidification among birds and many species have been studied
in detail – for example, Tyto alba(Sumner 1929; Pickwell 1948; Howell 1964), Otus asio (Sumner
1928, 1929), Otus flammeolus(McCallum et al. 1995), Bubo bubo (Penteriani et al. 2005), Bubo virginianus(Sumner 1929; Hoffmeister & Setzer1947), Nyctea scandiaca(Watson 1949), Asio flammeus(Clark 1975; Holt 1992), Strix occidentalis (Forsman 1981),
and Ciccaba virgataand Ciccaba nigrolineata(Gerhardt et al. 1994).
This report
concerns development of young in the Indian Eagle Owl Bubo bengalensis and presents primary data on morphometry and body weight along with photographic
documentation. Though this report could
be used as a guide to age young Bubo bengalensisin the region, it must be remembered that this investigation is still
preliminary in nature and therefore any generalization of relationships should
await corroboration from further studies.
Methods
Study area: A number of
ravines extend from Kalapet (12001’N & 79051’E)
near the seashore, southwestwards through the Aurovilleplateau to the Hermitage - Aranya - Merveille area (11055’N & 79046’E) adjacent to Ousteri lake - all in a radius of 10 to 15 km from Puducherry (formerly Pondicherry) city. Though most of these areas are environmental
disasters, a few still contain a wealth of biota - including resident breeding Bubobengalensis.
Field studies
centered around four breeding pairs and their young from February 1998 to March
2006. For further details refer Table 1.
Methods: Morphometric measurements were recorded when the young were
10 days old until the young were fully able to fly – i.e., the 60th day. In fact it
was difficult after the 35th day to catch the young
birds and take measurements as they could glide and flap away and the terrain
made access very difficult. Only nests with chicks at least 10 days old were
monitored at close range as new-born owl chicks are known to be very sensitive
to human handling. Penterianiet al. (2005) caution against approaching nests of Bubo bubo with chicks less than 15 days old and McCallum et al. (1995) suggest a
week’s period of habituation for sampling a small cavity nesting owl like Otus flammeolus.
Nevertheless, detailed photographic records were made and maintained from the
time of egg laying to fledging of chicks.
The following
measurements were taken following Ali & Ripley (1987); only the right tarsus
and wing were measured as a matter of convenience.
Wing length
(cm): From the carpal joint to the longest feather along the upper side of the
wing. In case of very young birds the
wing was pressed out on a ruler, while wing chord was directly measured for
older chicks.
Bill length
(mm): From skull (lacrimal) to tip of premaxilla.
Tarsus length
(mm): From the tibiotarsal joint to the base of the
middle toe.
Body weight (g):
Measured using a portable digital balance accurate to 0.01g.
Differences in
growth patterns between sexes were not examined because of the small sample
size and difficulty in sexing the chicks.
Plumage
descriptions are after Humphrey & Parkes (1959).
To distinguish between chicks non-toxic colouredsprays were used on the birds’ under parts near the vent as these were least
likely to show in photographs. We do not
recommend banding of chicks as parents were observed tugging at plastic bands
causing extreme discomfort to chicks.
Results and
Discussion
Young grew
rapidly during the first 20 days. Though
there was steady monotonic growth throughout the observation period, each morphometric character differed significantly in its growth
rate. For example, while bill length
showed a steady, yet mild increase in length, the tarsus length recorded a
steep increase till the 20th day after which it
flattened until the 40th day before it picked up
again (Fig 1). The rate of increase in
wing chord length was uniform till the 60th day (Fig
2). But body weight showed a steady
increase till the 35th day before reaching a plateau
between the 35th and 45thday after which there was a marginal spurt in its growth rate (Fig 3). Our extended observations showed that the
young completed their growth much later - i.e., over 90 days (when the maximum
weight was recorded as 1,373.78g). Such
alternate spurts and regression phases in growth factors of chicks make it
unreliable to base ageing of nestlings solely on morphometricdata. What could be the factors for such
wide individual variations? Is it food
availability and supply or predation threat or disturbance or is it a normal
phenomenon? This would also require a
large sample size to standardize ageing criteria. It is further compounded by the fact that thetaxon exhibits decreasing clinalvariations in body size from north to south (Ali & Ripley 1987).
A more reliable
way to age young Bubo bengalensis are
differences in feather morphology and this is evident from the photographic
record:
1. Clutch of
five eggs. Three to four is the usual
clutch size, but five is not uncommon (Image 1).
2. Chick using
egg tooth to break out of egg. The use
of this feature is obvious - viz, to chip away the
shell between the upper mandible and egg tooth (Image 2).
3. 0-5 day old
nestlings. Eggs hatch asynchronously on
most occasions. The nidicolousand semi-altrical young are covered in whitish natal
down and their eyes are closed. They are
semi-ptilopaedic as some parts (back, shoulders and
belly) are bare, resulting in pink patches. Nestlings lie prostrate, the belly in contact with the substrate (Image
3).
4.
Seven day old nestling. Pre-juvenilemoult has begun and sheaths (pin feathers) are
visible on the shoulders and wings. The eyes begin to open at 4 days and can be
completely opened by this stage. The entire eye is grayish-black in colour (Image 4).
5. Five and 10
day old siblings. The difference in size
is because of asynchronous hatching (5 days apart). At 10 days the belly begins to be covered in
down and there is no sign of the egg tooth (Image 5).
6.
15 day old nestling. The woolly prejuvenile moult is in progress,
though parts of the belly are still bare of feathers. A yellow iris is obvious around the
grayish-blue pupil. Ear tufts begin
their development. It can adopt the intimidatory posture if threatened, though in a weak manner (Image 6).
7.
20 day old nestling. The entire body is covered in juvenile
plumage. The iris is larger and darker
(darkness increases with age). Feet and
claws start to resemble an adult’s. They start to emit a harsh growling hiss
like call (Image
7).
8. 25 and 30 day
old siblings. At 25 days (subject on the
right) the streaked aspect is noticeable. Vibrissa begins to appear. At 30 days (subject on the left) the first
streaked appearance is more pronounced (Image 8). The black mask rim appears. Although the
elder will continue to be larger, there is not the vast difference in size as
was in evidence in Image 5.
9.
35 day old brancher. The aspect is more like that of an adult due
to the eruption of brownish juvenile plumage due to the first prebasic moult. The facial mask is well defined and ear tufts
pronounced. Remigesand retrices are well in evidence and form a
horizontal bar along the wings. At this
stage they move out of the nest and roost away from it. The primaries are well developed and if
disturbed they throw themselves off the rock face and glide away making
collecting morphometric data difficult, if not
impossible (Image 9).
10. 40
days. Tail feathers are pronounced. Feathers are emerging on the nape, head and
facial mask. From this stage changes are
subtle and ageing from feather pattern becomes difficult (Image 10).
11. 45
days. The first prebasic moult is advanced and the aspect is more like an
adult, especially around the nape, facial mask and wings (Image 11).
12. 50 days.
First basic plumage is visible on the breast. Wings appear separated in clearly
defined bands. A pale downy band
contrasts markedly between dark secondaries and upper
coverts (Image 12).
13. 55
days. The contrasting downy area on the
wings has nearly been replaced (Image 13).
14. 60
days. The undersides are still covered
with juvenile down and first basic plumage feathers erupt on the neck and
back. The downy underpartscontrast markedly with adults, the ear tufts are smaller and iris lighter
(Image 14).
15. An adult for
comparison (Image 15).
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