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CENSUS OF ROOSTING INDIAN HOUSE CROWS
(Corvus solendens) ON MOMBASA ISLAND.
Paul L. A. Erftemeijer1 and Jan Seys2'*
l. Kenya Wildlife Service_(Wetlands Conservation and Training
Programme), P.O.Box 82144, Mombasa, Kenya
2. Kenya Marine and Fisheries Research Institute (Kenya-Belgium
*
Projecten Marine Science) , P.O. Box 81651, Mombasa, Kenya
Present address: Toekomststraat 7, 8310 Brugge, Belgium.
INTRODUCTION
The Indian House Crow (Corvus splendens) is an asiatic bird
species, that was introduced to the coast of East Af rica in the
1890s (Lewis & Pomeroy, 1989) . From Zanzibar, where colonial
authorities introduced the species to act as an urban scavenger,
the species has spread into coastal areas of Tanzania,
Mozambique, Soufch Af rica, Kenya, Somalia and Ethiopia. Af ter
being recorded for the first time in Kenya in 1947"in Mombasa,
this omnivorous scavenger is now very common in and around
Mombasa city and island, and has spread along the north and south
coast, and also expanded its distribution inland, largely
following the main Mombasa-Nairobi raad and rail links (Lewis &
Pomeroy, 1989). Perhaps the best example that demonstrates the
remarkable adaptive behaviour of this bird species to human
environments, is that of the preferred use by the crows of
metallic wire for nest building, instead of plant twigs and
&
branches, as reported f rom large cities in India (Altevogt
Davis, 1979). In and around Mombasa, the breeding season is welldefined, f rom September to January, with a marked peak in October
(Brown & Britton, 1980).
The success of the Indian House Crow is considered to be the
cause of the decline of some other bird species in the coastal
area around Mombasa, such as Pied Crow (Corvus albus), Speckled
Mousebird (Colius striatus), Mornincr Thrush (Cichladusa arauata)
and Golden PalmWeaver (Ploceus bojeri) (Lewis and Pomeroy,
1993). The species is also causing harassment to people at
s ome
of the hotels along the Kenyan coast, and reportedly may cause
economie losses by inflicting damage to crops and poultry
(Yousuf, 1982; Dhindsa et al., 1991). In addition, the species
has been reported to be a possible carrier and transmitter of
diseases such as cholera and dysenteria (Munguti, 1984;
Anonymous, 1995). In and around Mombasa, this crow has become a
pest species, and attempts for its eradication and/or control
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have been_on-going since 1984. Control efforts have focussed on
the use of traps, poisoned bait, shooting of adult birds'and
destroylng.of_nests/ eggs and^chicks. Thè_problems caused'by the
crows at the coast have even lead to the formation of a House
Crow Control Committee to coordinate the control and eradication
programme.
It is virtually impossible to monitor the success of control or
eradication programmes without any basic data on the distnbution
and numbers of the crows. At present, no reliable data-are
available on the_number of crows around Mombasa. As recoinmended
Ï?-^e^rïli?^^s^?_f t:he _last House Crow Control Committee meeting,
there is need for more research into the Indian House Crow. The
aims of_the present study we re= [l] to count the total number"of
crows that roost on Mombasa island; [2] to assess which areas
^^fe"?r?ws^m?,inly come from; t3] to study the group size and
behaviour at their arrival.
METHODS
On 28_August 1995, from 4.30 p.m. to 7.00 p.m., counts were made
?f..?^"Sr<?!/Ï.-?,rrivin? at Mombasa island f rom the surrounding
mainland (north, west and south). Counts were made by observers
stationed strategically at four observation points around the
"landjsee FÏ5-1>,:.KMFRI ^ooft°P. Nyali_Bridge, .Makupa Causeway
(Kibarani),_ and Likoni Ferry (on-top"of Cement silos)'"Birds'that
flew away from the island (l.e. in the opposite directionFwere
recorded as negative (substracted f rom the counting totals)."The
crows were counted per intervals of 15 minutes. Since most'birds
arrived in small to larger_groups, the number of birds present'in
each group was recorded (although sometimes it was difficult~to
tel^what^was one group) . Counting was sometimes aided by'the'use
of binoculars but most counts were performed by naked eye:~The
position of the different counting stations was chosen as to
mmimize the risk of double counts. Some small overlap might have
occurred of counfcs at KMFRI and Nyali Bridge, but this might be
compensated by the gap (although narrow) between the reach of
Nyali Bridge and that of Makupa Causeway. We therefore'believe,
that the counts are reasonably accurate, and the effect of
possible double counts is considered small and insignificant.
RESULTS
Table l summarizes the results of the counts. In total, we
counted 19440 Indian House Crows crossing the water towards
Mombasa island between 16.30 and 19.00 pïm. Besides Indian House
Crows, only 2 Pied Crows were observed during the counts. The
majority of the crows (12918 birds or 66.4%)"arrived f rom the
mainland north (combined counts f rom Nyali bridge and KMFRI),
whereas 3376 birds (17.4%) arrived froin the west (Makupa) and
3146 birds (16.2%) from the south (Likoni).
Figure 2 shows the distribution of the arrival of the birds over
time. The peak of arrival was between 17.45 and 18.15 p.m. Data
for each individual counting station yielded similar temporal
patterns. From this figure it becomes clear that the counting
period covered nearly all the arrivals, and that the number of
birds which arrived prior to 16.30 p.m. or after 19.00 p.m.
it became too dark to observe anything) is likely to be
(when
insignificant.
??i?^^°^?5?ssing^the ^at?r'. the.birds-often aPPeared to gather
in tree tops, roof s and electricity poles & wires at the water
edge. After their aggregations had" reached a certain size, thev
started crossing the water as a large group. The size of such
groups varied considerably, but usually ranged from 25 to 85
birds. This phenomenon was most noticeable during the peak period
of arrival, that is between 17.45 and 18.30 p.m.'lt is'noted,
however, that at least half of all our observations concerned
groups of less than 20 individuals. Generally, our observations
indicafce that the birds preferred to cross the water towards
Mombasa island at sites where the water body was narrowest. Such
crossing sites^occur at Makupa and Kipevu causeways, around Nyali
Bridge and at Likoni, and to a lesser extent also around KMFRÏ
(opposite Fort Jesus). All these observations suggest that the
crows fear to cross over large water bodies (unlèss in large
aggregations).
DISCUSSION
& CONCLUSIONS
This study provides the first detailed record of numbers of
Indian House Crows around Mombasa. The total number of crows that
was counted (19440 birds) does not include the resident crow
population of the Mombasa island itself, but only concerns birds
that arrive here f rom elsewhere for gregarious roosting. The
resident crow population of Mombasa (those that do not~leave the
island during the day) remains yet to be studied.
Preliminary observations indicate that the crows arrive from
areas up to as far as Kikambala (20 km from Mombasa) in the north
and f rom areas up to as f ar as Tiwi (c. 20 km f rom Mombasa) in
the south. If this is extrapolated similarly 20 km to the west
(that is up to as f ar as Mazeras and Rabai), than it means that
the total surf ace area f rom which the crows arrive for gregarious
roosting in Mombasa totals up to around 800 km2. This mèans that
the average density of Indian House Crows is estimated to be
around 24 birds per m2 for the entire area around Mombasa (or
between 32 and 35 birds per m2 in the area north of Mombasa, and
about 16 birds per m2 in the area south and west of Mombasa).
These estimates are very rough but give a good indication of the
extent of the crow problem. In Zanzibar, which has a surface area
of about 2400 km2, Tony Archer reported a total number of
approximately 60,000_crows (Anonymous, 1995), which is equivalent
to a density of 25 birds per m2, and compares well to our
estimated average density of 24 birds per m2 around Mombasa.
It has not yet been established where exactly the crows gather in
Mombasa affcer arrival on the island, but preliminary obsèrvations
indicate a rather scattered distribution with a strong preference
for large old trees, such as can be found near the railway
station, around Treasury Square and in parts of Old Town.
The f act that the majority of the crows arrive f rom the mainland
north is especially noteworthy, since this is the area where the
majority of coastal tourist hotels are located. The species
appears to be especially attracted to (uncovered) garbage sites
where food rests_are dumped. Distribution of the species further
north includes disjunct populations at Malindi and Watamu, which
appear to have established between 1977 and 1980, and were
probably derived from birds released locally (Britton, 1980).
Unlike_in Zanzibar, where the majority of the crow population has
been killed using DRC 1339 poison and traps (Anonymous, 1995),
the current control programme in the coastal area around Mombasa
does not seem very successful. In order to effectively monitor
the success of a control programme, it is suggested to perform a
regular census_of the kind described in this paper. The changes
in numbers arriving from the north, west or south of Mombasa will
indicate the rate of success of the eradication programmes of the
species in these respective areas.
Apart f rom monitoring the success of control programmes, future
studies on the Indian House Crow may focuss their attention on:
(l) the identification and census of other roosting sites along
the coast; (2) comparison of census results between counts in the
evening (such as in the present study) with those obtained in the
early morning (to which direction do they fly?
(3) study of
pellets to be gathered at their roosting sites to study their
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food composition; (4) inventory of the resident population of
crows on the Mombasa island; (5) identification of their breeding
areas (also of the Pied Crows); (6) monitoring of the further
distribution_and spreading of the Indian House Crow in Kenya
(and/or the increase in their numbers); (7) interaction with
other bird species (decline). In addition, other possible ways to
control the crow pest should be investigated, including the use
of repellents and reflecting tape, as proven successful in the
control of other bird pest species (DWRS, 1995).
ACKNOWLEDGEMENTS. We would like to thank the following people,
who_participated in the census:_Mike Mannaart, Caroline"Ochieng,
Doris, Ali Daya Wako, Swalew Ali Abdulrahman and Kassim. Mr.
Benjamin Kavu (warden Mombasa Marine Park) is acknowledged for
allowing 4 KWS rangers to participate in the census. The
management of Bamburi Cement factory is thanked for allowing us
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CITED REFERENCES
Altevogt^ R^ and T.A. Davis, 1979. Urbanization in nest building
of Indian House Crows (Corvus SElendens). J. Bombay Nat.
Hist. Soc. 76(2): 283-290.
Anonymous, 1995. Minutes of the House Crow Meeting held on May
19th, 1995. (Forest Action Netwerk, Nairobi);
5 pp.
Britton, P.L. (Ed.), 1980. Birds of East Africa: their habitat,
status and distribution. East Af rica Natural History
Society, Nairobi, 271 pp.
Brown, L.H. and P.L. Britton, 1980. The breeding seasons of East
African birds. East Af rica Natural History"Society, Nairobi,
164 pp.
Dhindsa, M.S., P.S. Sandhu, H.K. Saini and H.S. Toor, 1991. House
Crow damage to sprouting sunflower. Trop. Pest Manage.
37(2): 179-181.
DWRS, 1995. Denver Wildlife Research Center - Highlights Report,
Fiscal Year 1994. United States Department of Agriculture,
Animal and Plant Health Inspection Service. Miscellaneous
Publication No. 1530.
Lewis, A., and D. Pomeroy, 1989. A Bird Atlas of Kenya. Balkema,
Rotterdam, pp: 328-329.
Munguti, JA.M., 1984. The Indian House Crow at the coast. Ministry
of Tourism and Wildlife, Wildlife Conservation and
Management Department; research report;
3 pp.
Yousuf, M., 1982. Indian House Crow as a serious pest of wheat
erop in Faisalabad Pakistan. Pak. J. Zool. 14 (2): 238-239.
FIGURE l.
Map of the Mombasa area, showing the location of the counting
stations mentioned in the text.
FIGURE 2.
Total number of Indian House Crows counted (combined for the four
stations) for each time interval.
Table l.
5!sïlts of.counts of Indian House crows crossing towards
Mombasa-ÏB^nd at.the ?°"r.°bservation points-d-Sring-the
different time intervals (date: 28 August-Ï995)
Time
interval:
KMFRI
Nyali
Makupa/
bridge Kibarani
16.30-16.45
16.45-17.00
18.45-19.00
58
105
254
532
806
1061
863
607
146
311
408
631
872
1733
1489
1726
1218
98
Total:
4432
8486
17.00-17.15
17.15-17.30
17.30-17.45
17.45-18.00
18.00-18.15
18.15-18.30
18.30-18.45
62
12
116
379
730
834
800
443
3376
Likoni
Total:
170
185
307
415
464
538
728
329
10
62
551
814
1192
2198
3733
3922
4117
2597
254
3146
19440
Thousands
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