Download Diet of Jungle Crows in an Urban Landscape

193
Diet of Jungle Crows in an Urban Landscape
Reiko KUROSAWA1*, Ryo KONO2, Takahiko KONDO3 and Yutaka KANAI4
1
Biodiversity Lab, Institute of Low Temperature Science, Hokkaido University,
Nishi 8-chome, Kita 19-jo, Kitaku, Sapporo, Hoddaido 060‐0819, Japan
2
Ecosys 3-11-18-201, Higashi, Kunitachi, Tokyo 186-0002, Japan
3
Environmental Ecosystem Lab, Rakunogakuen University,
582-1 Midoricho, Bunkyodai, Ebetsu, Hokkaido 069-8501, Japan
4
Wild Bird Society of Japan, 2-35-2, Minamidaira, Hino, Tokyo 191-0041, Japan
* e-mail: [email protected]
Abstract
An animal species’ diet constitutes the basic ecology of that species and is indispensable for
understanding its population dynamics. As jungle crows have increased in the urban areas of Tokyo,
conflicts between humans and crows have increased. Since highly urbanized areas are poor in natural
food resources, it is possible that jungle crows depend heavily on human-related food in Tokyo. We
have, therefore, determined their diet and compared the results with those of other regions, taking steps
firstly, to assess the reliability of information from pellets, feces and direct observations of feeding
crows.
Jungle crows exploit a wide variety of food in response to temporal and seasonal availability in
urban as well as rural areas. Their winter diet consists primarily of plant matter, suggesting that they
retain their original food habit of eating fruits and berries of trees and shrubs, even at a heavily
urbanized site. The proportion of food scraps in their diet was similar to that of animal matter in our
study, suggesting an unusually ample availability of anthropogenic food sources in this city. If food
scraps in Tokyo could be reduced, a great proportion of the food currently available to the crows would
be lost, driving down the carrying capacity. We suggest ecological studies be conducted before
making some action plans to solve wildlife issues.
Key words: diet, food scrap, fruit eater, jungle crow (Corvus macrorhynchos), urban landscape
1. Introduction
Jungle crows (Corvus macrorhynchos) are large
crows (body length ca. 55 cm) distributed in the
forested regions of East Asia through the Far East Asia
（Madge & Burn, 1994; Iwasa et al., 2002）. They
inhabit a variety of forests, such as broad-leaved
deciduous, broad-leaved evergreen and mixed coniferous forests. Forest dwellers, as the English name
implies, they occur in farmlands with hedges and in
wooded areas of city parks as well, making them one
of the most familiar bird species to Japanese people
（Higuchi, 1979; Karasawa, 2003）.
Having a close relationship with behavior as well
as morphology, feeding habits are a basic ecological
parameter of animal species. Jungle crows are yearround residents and are reported to have a home range
of less than 8 km in Tokyo (Morishita et al., 2003).
Therefore, the diet of this species probably reflects the
ecosystems of its local habitats, which implies, on the
other hand, that the diet varies from region to region.
Jungle crows are known to have a wide range of food
preferences. They feed on insects, bird eggs and
chicks, while they scavenge dead animals. They eat
various fruits and seeds of trees as well, and thus their
role as a seed disperser has been pointed out (Ueda &
Fukui, 1992).
Although Jungle crows have a relationship, direct
or indirect, with a wide variety of wildlife in their
habitats, few studies on their feeding habits have been
carried out in predominantly urban areas except for
Osaka (Ueda & Fukui, 1992).
In Tokyo, a megalopolis with 12 million people,
approximately 36,000 crows were counted in the
winter of 2001-2002 (Ueta et al., 2003). Food scraps
in this city are disposed of in transparent plastic bags
and lie outdoors until garbage trucks come to collect
them. Since the plastic bags are flimsy, they are
vulnerable to scavenging by crows, cats and rats
(Kurosawa et al., 1999a). The availability of food
scraps in plastic bags on the streets is suggested to be
responsible for the marked increase in crows in Tokyo
(Kawachi & Matsuda, 1999; Kurosawa et al., 2001,
194
R. KUROSAWA et al.
Ueta et al., 2003). The conflict between humans and
crows has increased as the crow population increased
in urban areas. The major complaint by the citizens
was scattering of trash on streets by scavenging crows
(Ministry of the Environment, 2001; Kurosawa et al.,
2003). The citizens in areas with a higher frequency
of garbage scattering had strong negative feelings
towards crows. Since highly urbanized areas are
poor in natural resources, it is possible that jungle
crows depend heavily on human-related food in
Tokyo.
In this paper, we have attempted to determine the
ecological status of this crow in a highly urbanized
area by analyzing its diet, and comparing it with those
in other regions of Japan. In so doing, we would like
to find a new light to reflect on the state of coexistence of man and crows in Tokyo.
Jungle crows eat food, frequently tearing it into
small pieces, and they feed their young in their nests
directly from the gular pouch, making it hard to
identify the food items by video recording. They
gather at communal roosts in the evening, and
regularly regurgitate indigestible portions of food as
pellets (Yamagishi, 1982). Since the pellets are frequently found on the ground under the roost, they are
readily available for research workers. Food remains
in the pellets can shed some light on the feeding
environment of the crows that have discarded them.
Since the food remains in the pellets are indigestible
portions of the food the crows have taken in, however,
they do not necessarily represent their food completely.
Therefore, we have compared the results of the
analyses of pellets and feces with direct observations
of feeding crows (Kurosawa, 1999), in order to
determine the reliability of pellets as an indicator of
feeding habits.
2. Methods
The study sites were communal roosts of crows in
parks and a corporate institute with a large wooded
area in Tokyo; namely, Mizumoto Park, Rikugien Park,
Fig. 1 Map of study sites in Tokyo.
Tama Zoo and Hitachi Central Research Center
(Fig. 1). The roosts were located in canopy-closed
parts of broad-leaved deciduous or evergreen stands
mixed with clusters of conifers (Cryptomeria, and
Chamaecyparis). Since almost all the crows (97%)
in Tokyo in 1999 were jungle crows (Kurosawa et al.,
1999b), so we assumed that the pellets and feces
belonged to this species. Collecting pellets and feces
beneath the roosts between December 1999 and
March 2000, we preserved them in a 75% alcohol
solution for later analysis. Cleaning by water and
screening all the specimens (44 pellets and 15 feces),
we identified them using 8-50 power microscopes.
In 1998 we observed jungle crows feeding in
Inokashira Park (Kurosawa, 1999).
We used
observation data obtained during the winter months
(Dec. to Mar.) for comparison.
We classified the substances identified in the
pellets and feces into the following four categories:
1) Plant matter (fruits, berries, nuts, seeds, buds and
other plant parts)
2) Animal matter (bones, insects, bird feathers,
mammal hair, fish scales and others)
3) Artificial matter (cloth, fiber, paper, plastic, metal,
glass and others)
4) Unidentified substances including inorganic matter
(clay, pebbles and others).
We generally identified plants and animals at a
family level, and when possible, at a species level
(Asano, 1996; Yamatokeikokusha, 2000; Nakane et al.,
1992). We assorted all fishes into animal matter
without discriminating between marine and fresh
water fishes. We placed all mammal hairs in animal
matter. We excluded crow feathers from analysis as
an alien object taken in by chance.
We worked out the frequency of each category.
When a pellet contained parts of fruits, leaves, buds,
bones, insects, plastic and paper, for instance, the
frequency for plant matter was 3, animal matter 2,
artificial matter 2, and inorganic matter 0. After we
had compared the characteristics of the analyses of
pellets, feces and direct observations, we made a list
shows crow roost sites.
Diet of Jungle Crows in an Urban Landscape
of food items according to the most suitable method
and compared the results with former studies in
regions other than Tokyo.
3. Results
3.1 Reliabilities of pellets, feces and direct
observations as indicators of diet
The pellets of crows are reported to be shaped like
bullets (Fig. 2; Yamagishi, 1982). Most of the pellets
collected at the study sites, however, came in
indefinable shapes (Fig. 3), with a length up to 30 mm
and the width less than 30 mm.
Each pellet contained a variety of substances
(Table 1), which suggests that a crow ingests a wide
spectrum of food in a short period of time (Kondo,
2003). Most of the pellets consisted of fragile
substances which tend to crumble easily. The largest
hard object was a 22 mm by 3 mm piece of twig and
the bulkiest object was a spherical seed with a radius
Fig. 2 Bullet-shaped pellet of jungle crows reported as
typical in former studies (photo by R. Kurosawa).
Table 1
195
of 5.0 mm (family Myrsinaceae). Pieces of plastic
and mandarin orange endocarp were intact and folded
(Fig. 4). Substances such as seeds, pieces of bones
and the outer chitin of insects were easy to identify.
Most of the substances contained in the feces were,
however, very small fragments of leaves, tiny seeds
less than 1 mm in diameter and other objects which
were difficult to identify.
Plant matter was found in most of the pellets
analyzed (43/44, Table 1). Animal matter and artificial matter were contained in two thirds of the pellet
specimens (34/44). We compared the categories
based on the results of the analyses of the pellets and
feces and the result of the direct observation of
feeding crows (Fig. 5). The frequency of each
category statistically differed among the three sources
(χ2 = 52.7057, df = 6, Ｐ < 0.001), but the differences
were not significant between any of the pairs
（Fisher’s PLSD test: F = 0, df = 2, Ｐ > 0.05; F =
0.4569, df = 3, Ｐ > 0.05）.
Fig. 3 Pellet with an indefinable shape most frequently
observed in this study (photo by M. Matsuda).
Bar indicates 3 cm.
Food categories observed in the diet of jungle crows. Values show the cases with % occurrence observed in parentheses.
Food categories
Pellets
Feces
19
2
18
17
42
43 (97.7)
4
1
15
15 (100)
bones
12
1
insect parts
17
5
feathers, hair
scales
Total animal matter
18
1
34 (77.3)
3
0
8 (53.3)
cloth, paper
plastic, metal etc.
Artificial objects
18
23
34 (77.3)
3
3
6 (40.0)
plastic, styrofoam, aluminum foil, paper
glass, fiber (mainly cotton)
pebble, clay, unknown
Inorganic and/or unidentified matters
Total
25
25 (56.8)
44
8
8 (40.0)
15
stones, clay, shells
fruit
seeds
buds
leaves, stalks, roots etc.
Total plant matter
Major items (Genus/species & family)
Citrus (Rutaceae), Rubus (Rosaceae), Rhus (Anacardiaceae),
Diospyros kaki (Ebenaceae), Ardisia (Myrsinaceae), Quercus
myrsinaefolia (Fagaceae), Castanea crenata (Fagaceae)
Poaceae
Hierodula patellifera (egg case), Rhomborrhina japonica & other
Scarabaeidae
birds, mammals
fish, reptiles
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R. KUROSAWA et al.
Fig. 4
Orange endocarp and egg case of mantis observed in a
single pellet (photo by R. Kono).
Fig. 5
Proportion of food categories of jungle crows in each
sample source.
A balanced proportion of all categories was
represented in pellets, whereas a large proportion of
unidentified matter was seen in direct observations.
On the other hand, artificial matter tended to be
identified best in the direct observations, which
frequently revealed the origin of the food items.
Parts of insects, fruits and seeds were best
represented in pellet analysis, because the pellets
contained larger pieces of indigestible substances than
did feces. Direct observation had the advantage of
identifying large food items as crows handled them.
The analysis of pellets was, on the other hand,
effective at identifying small food items containing
indigestible substances that defied direct observation.
In addition, pellets were easy to collect. In contrast,
feces had the lowest rate of identification, indicating
that they are not suitable materials for determining
crows’ diets.
Therefore, we used pellet analysis to determine the
crows’ diets, complementing its short-comings with
the direct observation of feeding crows. The analysis
of pellets could allow for comparison between
different regions, if conducted under a standardized
method and categorization.
3.2 Details of jungle crow diets in Tokyo
Plant matter was found in almost all of the pellets
analyzed (97.7%; Table 2). Most nutritional among
this were fruits and berries of trees and shrubs. The
plant species found in the crows’ diets seemed to
reflect the regional flora, because they were common
in the Tokyo region both in natural and cultivated
landscapes. Mandarin oranges (Citrus deliciosa,
Rutaceae), chestnuts (Castanea crenata, Fagaceae)
and persimmons (Diospyros kaki, Ebenaceae) are
probably common food sources for crows. Seeds of
Poaceae were also contained in the pellets, indicating
that jungle crows fed on grass seeds in winter in
Tokyo as they do in more agricultural regions also
(Suzuki, 2002).
Second in frequency to plant matter in the pellets
were artificial and animal matters (77.3%). The
most common substances (17/44 cases) among the
artificial matter were small pieces of white or
transparent plastic bags usually used as garbage bags
in Tokyo. The pellets also contained aluminum foil,
Styrofoam and paper, all of which are used for food
wrappings, suggesting their association with food
scraps. Other inorganic matter, such as pebbles and
clay were also found in the pellets.
The most frequent animal matter was insects, such
as the ootheca (egg cases) of mantises (Hierodula
patellifera, Mantidae), and beetles (Sacarabedae).
The same groups of insects were reported in previous
studies in rural regions (Ikeda, 1957; Suzuki, 2002).
Most of the mammal hairs were human hair, which
has a probability of being taken in by chance as the
crows scavenged food scraps at garbage collection
sites.
Table 2 shows the results of direct observation of
feeding jungle crows (Kurosawa, 1999). Jungle
crows were observed eating a variety of fruits and
berries of trees, supporting the result of the pellet
analysis. They were also observed to carry fruits of
Rhus succedanea (Anacardiaceae) ， Zanthoxylum
ailanthoides (Rutaceae)，Diospyros kaki，and genus
Citrus species to their roosts (Reiko Kurosawa pers.
observ.). We found saplings of Rhus succedanea,
Zanthoxylum ailanthoides and Ardisia japonica
(Myrsinaceae) under the roost sites.
Jungle crows were often observed scavenging food
scraps in dustbins, in picnic areas, and at sites where
people feed birds and fish. On a few occasions, they
were observed snatching bags of snack food directly
away from stalls in a park (Kurosawa, 1999). Thus,
they readily use human food resources in Tokyo.
Animal matter identified in direct observations
included rats, cats, feral rock doves, frogs, snails and
earthworms. In addition, songbirds, such as fledgling Gray Starlings and Tree Sparrows were taken in
May and June (Table 2).
Diet of Jungle Crows in an Urban Landscape
197
Table 2 List of food items of jungle crows found by direct observation (Kurosawa, 1999; with permission
from WBSJ).
Plant matter
Fruits
(including berries, nuts and beans)
Animal matter
Mammal
Bird
Others
Eriobotrya japonica (Rosaceae)
Diospyros kaki (Ebenaceae)
Viburnum odoratissimum (Caprifoliaceae)
Rhus succedanea (Anacardiaceae)
Aphananthe aspera (Ulmaceae)
Sweda controversa (Cornaceae)
Magnolia praecocissima (Magnoliaceae)
Gleditsia japonica (Leguminosae)
Rats & mice (Muridae)
Cats (Felis catus) (young)
Rock Doves (Columba livia)
Grey Starlings (Sturnus cineraceus) (young & eggs)
Tree Sparrows (Passer montanus) (young & eggs)
Land snails (Gastropoda)
Earthworms (Oligocchaeta)
4. Discussion
The habit of procuring a wide variety of food in
response to temporal and seasonal availability was a
common feature among jungle crows in urbanized as
well as rural areas (Ikeda, 1957; Liu et al., 1986).
Among these, the major food substance was plant
matter such as fruits and berries of trees and shrubs.
The fruits of genus Rhus in particular were reported as
a food item of jungle crows in previous studies (Ikeda,
1957; Yamagishi, 1982; Ueda & Fukui, 1992; Suzuki
2002), suggesting that these fruits are a common and
important food resource for this species. Therefore,
jungle crows also retained their original food habit of
eating fruits and berries of trees in a heavily urbanized
site as Tokyo.
The second most important food substances for the
jungle crows in Tokyo were both artificial and animal
matters. The latter represents a natural food source
commonly observed in the previous studies throughout this crows’ range (Ikeda, 1957; Yamagishi, 1982;
Liu et al., 1986; Suzuki, 2002; Kondo, 2003).
However, the high frequency of artificial substances
would be a characteristic of crows’ diets in Tokyo.
There was a significant difference in the proportion
of artificial substances found in jungle crows’ diet
among regions: 77% in Tokyo, and 7.7% and 7.9% in
the agricultural areas of Tsuruoka City, Yamagata
Prefecture (Suzuki, 2002), and Ebetsu City, Hokkaido
(Kondo, 2003) respectively (χ2 = 103.42, df = 2, P <
0.001; Fig. 6). If we consider marine fish, mandarin
oranges and human hair as taken from food scraps, the
proportion of crows’ dependence on human-related
food source is even greater. The high frequency of
artificial substances in jungle crows’ diets suggests a
greater availability of anthropogenic food sources in
Tokyo. Therefore, it is highly probable that the
carrying capacity of jungle crows was unnaturally
raised, contributing to an increase of this species in
urban landscapes. If food scraps in Tokyo could be
reduced, a significant proportion of food currently
available to the crows would be lost. Since no life
can sustain itself far above the carrying capacity, the
population of crows would follow suit.
Jungle crows feed on insects such as Scarabedae
beetles and mantis in winter when insects are not
active. These insects are common in a wide range of
habitats and are probably food sources for jungle
crows in a natural state. The beetles are primary
consumers of plants, and mantis is a predator in the
food web. The predation of these insects has a
probability of increasing biodiversity at the levels of
producers and primary consumers of forest ecosystems （Kullberg & Ekman, 2000）. Control of one
organism will inevitably affect the other organisms in
the food web, and history warns us against excessive
hunting/controlling of crows because their removal
has resulted in a population explosion among insects,
such as beetles and grasshoppers, greatly damaging
the regional economies (Forbush, 1927; Inugai &
Haga, 1953).
During the breeding season, jungle crows are
known to prey on the nests and young of other bird
species, including larger birds of prey in Tokyo (Ueta,
Fig. 6
Different frequencies of artificial substances found in
pellets of jungle crows in urban (Tokyo) and rural
(Tsuruoka and Ebetsu; Suzuki, 2002; Kondo, 2003)
areas.
198
R. KUROSAWA et al.
1998). In winter, however, bird remains were not
found in the pellets in our study, suggesting that the
crows capture eggs and chicks of other birds primarily
during the breeding season, probably to meet the
demand of growing young. Thus, jungle crows were
not a major predator on small bird species in winter in
our study sites.
The natural inorganic matter such as pebbles and
clay found in crows’ pellets in our study supports
previous reports of their frequency in the pellets of
crows (Liu et al., 1986; Suzuki, 2002; Kondo, 2003).
Among the hypotheses is that crows deliberately take
in ashes for nutrition (Takenaka, 2003). An analysis
of the nutrient values and experiments on the
importance of inorganic matter will be required to
determine this issue.
Acknowledgements
Shigeharu Koshikawa helped us identify plant
seeds. Nanae Kato lent us a microscope. Michio
Matsuda as well as the assistant researchers of the
Wild Bird Society of Japan cooperated in collecting
pellet and fecal samples at crow roosts. Kazuji
Fukui translated an article into Japanese.
The
Hitachi Central Research Center, park offices of
Rikugien, Mizumoto and Tama Zoological parks gave
us permission to conduct research at these sites.
Takashi Kurosawa read and gave advice on the early
manuscript. We would like to express our sincere
gratitude to all these people.
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(Received 31 October 2003, Accepted 2 December 2003)