THE INDIAN EAGLE OWL

BUBO BENGALENSIS (FRANKLIN, 1831)

SCOPE OF OUR STUDIES AND EFFECTS AFFECTING ITS BREEDING POTENTIAL DUE TO

UNSEASONAL CLIMATIC CHANGE IN THE MONTH OF MAY 2004 IN THE AUROVILLE

INTERNATIONAL TOWNSHIP REGION

PITCHANDIKULAM FOREST, AUROVILLE INTERNATIONAL TOWNSHIP

BRIEF INTRODUCTION TO THE SPECIES

BUBO BENGALENSIS – TAXONOMIC REVIEW

Most popular field guides in circulation find this species mentioned as the Eurasian Eagle Owl Bubo bubo. For many years it was considered to be the sub-species Bubo bubo bengalensis. It is in fact a taxonomically valid

species in its own right based on molecular analysis (Konjetal, 2000)

Superficial characteristics that differentiate it from the nominate taxon include a black facial rim and the last digit of the phalanges and underside

of the feet bereft of feathering

DISTINGUISHING CALL

The call is distinct from the Eurasian Eagle Owl Bubo bubo. It has been rendered as “a deep, resonant, hollow bu – bo (accent on second

syllable much prolonged” (Ali & Ripley, 1969). The call of Bubo bubo is much the same, but the accent is on the first syllable

STUDY AREAS

The study areas were Aranya, Hermitage and Merveille near Ousteri Lake (11°95’N & 79°73’E) and Success, Forecomer’s Canyon and Ravena in the

Auroville plateau (11°98’N & 79°81’E)

OUR INVESTIGATIONS

ETHOLOGY – AUDITORY COMMUNICATION

Owls being nocturnal species use a variety of calls to communicate. We have recorded 9 distinct auditory traits in this species:

1) Long call – this is the distinguishing call

2) Intermediate call

3) Short call

4) Begging call

5) Alarm call

6) Anxiety call

7) Chick-like vocalizations

8) Hissing

9) Bill clicking or Bill clapping

ETHOLOGY - VISUAL COMMUNICATION

This species being crepuscular, it also uses a variety of visual cues to advertise its moods and intentions

Some are shown herewith

Positions of ‘horns’

Bobbing and bowing behaviour

When approached by an intruder (either human or any other animal it is interested in) it seems to take stock of it by looking at the object of interest from all angles. This may look curious,

but makes perfect sense

Spread-winged displays

Perhaps the most spectacular of all displays are the spread winged displays which most authors termed the “threat display” earlier

Spread-winged displays

Our investigations showed that there were at least three distinct types of spread winged displays – viz. Intimidatory display, Intermediate or

transition display and threat display

TROPHIC CONNECTIONS

OTHER OWLS

Four species of owls occur in the Auroville region – viz. Collared Scops Owl Otus bakkamoena, Indian Eagle Owl Bubo bengalensis, Spotted Owlet Athene brama and Mottled Wood Owl Strix ocellata. We have

managed to study pellets of Otus bakkamoena and Athene brama and have analysed food and niche overlap between these and Bubo

bengalensis

PREY ANALYSIS OF BUBO BENGALENSIS

Prey analysis was a combination of both pellet and prey remains to give an accurate estimate of the number of animals predated on by a pair /

family of owls within their territory

Rodents, the prime prey base

Though hares, bats, shrews, birds, monitor lizards, snakes, batrachians and invertebrates (including the venomous Rock Scorpion Heterometrus swammerdami and Tiger Centipede

Scolopendra hardwickii) were preyed on, rodents were the prime prey base accounting for 64.91%. Eight species of rodents were recorded from owl pellets and carcasses. We have worked out a simplified key to identify rodents based on their lower mandibles

alone

Rodent population estimates

The standard trapping method (Barnett & Dutton, 1995) was adopted. Three trapping stations, each 100 sq m in extent and forming a grid of 6 x 6 trap lines were maintained from 2002 to 2004 in three types of habitat (viz. ravine, grassland on plateau and forest plantation). Two trapping nights every month was the practice adopted and monthly trap success was the sum of traps entered on these two nights. The

percentage trap success and was calculated as follows:

% trap success = (traps entered / no.of traps set) x 100

BREEDING SEASON AND GROWTH RATES

Courtship usually began after the N.E. Monsoon in October / November. Eggs were laid between December - February. Incubation (by the female alone) was

approximately 30 days. They attained the juvenile stage in approximately 60 days. The fact remains that, whatever the time of reproduction, the young

require enormous quantities of basic food at this time, especially the months of April, May and June, if they were to survive to the ‘floater’

stage which precedes adulthood since they are not skilled enough until that time to hunt by themselves. The young stayed with their parents until

the abatement of the S.E. Monsoonal showers (roughly, end of August / beginning of September)

CO-RELATING RODENT ABUNDANCE TO FRUITING PHENOLOGYWe co-related rodent trap success to fruiting phenology of species whose

seeds / seed fragments were found in gut analysis of rodents. The species and values concerning a normal time (2002 – 2003) are given below

F ruiting phenolog y & trap s uc c es s for the year 2002 & 2003

0

10

20

30

40

50

60

70

80

J an F eb Mar A pr May J un J ul A ug S ep Oc t Nov Dec

F ruiting

T.Indic a

R .rattus

Mus s pp.

Fruiting% T.Indica R.rattus Mus spp.

Jan 41.43 9.25 2.31 9.72

Feb 47.69 8.79 2.77 8.79

Mar 56.92 8.33 3.7 8.33

Apr 66.15 8.79 4.16 8.33

May 75.38 9.72 6.48 9.72

Jun 75.38 10.64 6.94 11.11

Jul 73.84 8.79 5.55 11.57

Aug 72.3 8.33 5.55 9.25

Sep 61.53 8.79 4.62 8.33

Oct 56.92 8.79 4.16 8.33

Nov 44.61 9.25 4.16 8.79

Dec 40 8.79 2.77 9.25

Eggs

Three to four eggs is the usual complement of eggs laid, but five is not uncommon

Five and 10 day old chicks

The chicks hatch asynchronously which leads to great disparity in size. The nidicolous and altrical young are covered in white natal down and their eyes are

closed

15 day old nestling

The prejuvenile moult is in progress. A yellow iris is obvious. It can adopt the intimidatory posture if threatened

25 and 30 day old nestlings

These are the same two chicks seen the the slide before the previous one

Although the elder will continue to be larger, there is not the vast disparity of size

45 day old young

The first prebasic moult is evident and the general aspect is more like an adult. It has moved away from the nest site – a process called “branching”

60 day old young

Though looking like an adult and able to fly, the aigrettes are not fully developed and the underside is still covered with juvenile down. This is in strong contrast to adults – and adult is shown on the right for comparison

UNSEASONAL CLIMATE CHANGE

The Coromandel Coast has a disymmetric climatic regime. Though it receives some showers from the S.W. Monsoon, the bulk of the rains fall during the N.E.

Monsoon (October to December) due to depressions formed in the Bay of Bengal. Up to 1,200 mm per year has been recorded

In the month of May in 2004 the areas had unprecedented rains – over 200 mm in the hottest month

P rec ipitation for the years 2002-2004

0

100

200

300

400

500

600

J an F eb Mar A pr May J un J ul A ug S ep Oct Nov Dec

2002

2003

2004

2002 2003 2004

Jan 31 0 43

Feb 154.4 0 0

Mar 0 0 0

Apr 0 0 0

May 19.6 40 212.2

Jun 35.4 20 50

Jul 36.5 71.8 45

Aug 37.6 204.1 67

Sep 77 52 202

Oct 247.7 156.3 411.6

Nov 305.5 510.4 253

Dec 141 12 12

EFFECTS OF CLIMATE CHANGE

This unusual rainfall led to a drop in fruiting and flowering. This in turn led to a drop in rodent populations. This in turn affected the breeding of the owls that

depend on the rodent prey base. All the chicks of the three pairs of owls that bred that year died

F ruiting phenolog y & trap s uc c es s for the year 2004

0

10

20

30

40

50

60

70

J an F eb Mar A pr May J un J ul A ug S ep Oc t Nov Dec

Axi

s T

itle

F ruiting%

T.Indic a

R .rattus

Mus s pp.

Fruiting% T.Indica R.rattus Mus spp.

Jan 41.5 7.3 2.5 8.9

Feb 47.6 7.9 2.7 9.1

Mar 56.9 8.1 3.9 8.4

Apr 66.1 8.4 3.9 8.6

May 10.7 0 0 3.2

Jun 13.8 0 0 4.1

Jul 32.3 1.8 0 5

Aug 40 2.7 0 7.4

Sep 61.5 6 0 9.7

Oct 59.9 5 0 11.1

Nov 44.6 8.7 0.4 17.1

Dec 40 9.7 0.4 17.5

FOR FURTHER DETAILS

Journal of Threatened Taxa

www.threatenedtaxa.org

Zoos’ Print Journal

www.zoosprint.org

Author bar: Ramanujam

THANK YOU