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What Made the Crow Lay Stones on the Railroad Tracks?
Hiroyoshi HIGUCHI* and Emiko MORISHITA
Laboratory of Biodiversity Science, School of Agriculture and Life Sciences,
The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan*e-mail: higuchi@es.a.u-tokyo.ac.jp

Abstract
In Yokohama, central Japan, a carrion crow laid stones on railroad tracks when hoarding bread or
retrieving it to eat. The crow picked up stones on either occasion. When the stones were heavy or
awkward for the crow to hold with its beak, the crow laid the stones on the tracks, perhaps intending
then to take a more comfortable grip on the stone. The crow also put stones on the tracks to remove
breadcrumbs from its beak. The suitable height and flat surface of the tracks probably induced thecrow to lay the stones on the tracks. When the crow laid stones on the tracks, it often picked them up
again and carried them away. On rare occasions, however, the crow left stones on the tracks, causing
the trains to make emergency stops.
Key words: crows, food caching, stone-laying behavior, Yokohama

1. Introduction
In June 1996, trains made several emergency stops
due to stones laid on railroad tracks in Yokohama,
Kanagawa Prefecture, central Japan. Staking out thescene of the sabotage, the police discovered who haddone it. Surprisingly, the culprit was a crow. It was
one of a pair of carrion crows (Corvus corone) thatheld a territory in the neighborhood. The carrion
crow was video-taped while placing stones on the
tracks, and the pictures were broadcast on television.
The news of the stone-placing crow was carried innewspapers and magazines as well. It was a mystery,
however, what made the crow do it. Thus we set
about unraveling the mystery. On-site observation
and analysis of the videotapes recorded by the media
revealed the details of the crow’s behavior. This
crow affair also provided a good opportunity to reflect
on how to associate with wildlife in humanlandscapes.
In this article, we report on the background of theincident, details of the crow behavior, and the mechanism of the incident. This is basically a translationof our Japanese article (Higuchi & Morishita, 1997),
but we added here some further information on similar
incidents in other areas of Japan.

2. A Brief Sketch of the Incident
Stones were laid on railroad tracks during June andJuly 1996. A Carrion Crow was observed or video-

taped putting stones on the tracks on seven occasions
during 23 days between June 8 and June 30 by police
or TV crews. On the other hand, during a total of 97
hours and 2 minutes between July 13 and July 31, theauthors and cooperators observed the stone- placing
behavior only twice. During June and July 1996,
therefore, the carrion crow was sighted placing stoneson the railroad tracks on nine occasions in total (Table
1).
Stones were laid on the tracks primarily on rainy or
cloudy days, especially early in the morning or
evening. The locations of the stones were concentrated along the tracks within about 100 m of the Iijima
Overpass in the Sakae district of Yokohama. The
Iijima Overpass is located between Totsuka and Ofuna
Stations on the Japan Railroad (JR) Yokosuka Line,the JR Tokaido Line and a freight train line. No
apparent preference for particular tracks of these lines
was observed.
A pair of carrion crows held a territory in thevicinity of the Iijima Overpass (we call this the “Iijimapair” hereafter). Within about 2 km of this overpass
were four other pairs of crows holding territories.
Adjacent to the Iijima pair, there were a territorial pairof jungle crows (Corvus macrorhynchos) to the north,to the side of Totsuka Station, and a pair of carrion
crows to the south, toward Ofuna Station (Fig. 1).
Neither pair adjacent to the Iijima pair put any stoneson the tracks. Within the Iijima pair, it was only one
individual that laid stones on the tracks. This
individual had a characteristic white patch on its wing,
and was easy to identify.

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Table 1

Date of occurrence and weather at the Iijima Overpass, Yokohama in 1996.

Date
Jun.8
Jun.19Jun.22

WeatherCloudy with a rain shower
Fair with temporary cloudiness
Cloudy with a rain shower

Time
5:40
18:00
5:17

Jun.24
Jun.26
Jun.30Jul.21
Jul.24

Occasional rain after cloudiness
Rainy
Cloudy with temporary sunshine
Cloudy after rain
Cloudy with a rain shower

13:45
12:30
5:30
16:37
6:45

Remarks
Reported by Kanagawa Police
Reported by Kanagawa Police
Video-taped by Kanagawa Police
Another case was observed
Train made an emergency stop
Video-taped by NTNC
Video-taped by NHK
Observed by Higuchi
Observed by Higuchi & Morishita

Fig. 1 Distribution of territorial crows in the vicinity of the Iijima Overpass, Yokohama, Kanagawa Prefecture.
M1 : a territory of jungle crows. C1 and C2 : territories of carrion crows.
●: locations on tracks where stones were laid. ？: exact spots of stones unknown.
Stones were only laid on the tracks inside the territory of the C1 pair.

3. Why Did the Crow Pick Up and Put
Stones on the Tracks?
The process of placing a stone on a track is
composed of two steps. First the crow picked up astone with its beak (first step, Fig. 2), and then put iton the track (second step).

Fig. 2

A carrion crow holding a stone in its beak.
Yoshiaki Iijima.)(Photo by

As for the first step, crows are rarely observed
picking up stones with their beaks, probably because
they are inedible. Soon after we began to observe
crows in the neighborhood of the Iijima Overpass,
however, we discovered that they picked up stones
when they cached food underneath the railroad tracks.
When the Iijima pair secured extra supplies of food,
one member of the pair frequently cached them below
the tracks. The pieces of bread that the crow hoarded
were originally provided by people for the carp in the
Kashio River nearby and feral pigeons gatheredaround them.
Filling up its gular pouch with pieces of bread
picked from the riverside, the crow flew to therailroad tracks. It took out the pieces of bread fromits pouch at a spot along the tracks. Then the crowpicked up a piece of bread and pushed it between
stones on the gravel bed of the tracks. After that, it
picked up a stone nearby and placed it carefully on topof the hiding spot. The bird added another couple ofstones to the spot. It repeated these actions at five toeight spots, to hide every piece of the bread it had
carried to the railroad tracks (Fig. 3).
When retrieving the cached bread, the crow simply
pushed aside the stones hiding the bread. At thisWhat Made the Crow Lay Stones on the Railroad Tracks?

time the crow would pick up the stones again.
Apparently the crow had no difficulty in locating thehoarded bread, regardless of whether stones wereplaced on the track or not. It is unknown if the crowdepended on some marks to re-locate the hiding
places. At some spots, however, the crow removed
stones and found no cached food.
It rained for many days in June and July–the rainyseason in Japan. The wet pieces of bread stuck tostones in the cache sites. We experimentally dampened pieces of bread in the laboratory, and foundlater that they stuck firmly to a stone placed immediately adjacent to it. When we tore the wet piece ofbread from the stone, a part of it remained stuck there.
Considering a light drizzle as well, we lightly
moistened pieces of bread with water spray. We
obtained the same result. These observations indicated that wet pieces of bread stuck to stones irrespective of the amount of precipitation. After eatingsome of the breadcrumbs sticking to a stone, the crowoccasionally carried the stone with remaining breadcrumbs on to another place to hide it again. This wasanother condition under which the crow picked upstones.
What then made the crow put the stones on the
tracks (the second step)? The crow picked up stones
when it hoarded the food. After picking up stones,
however, the crow rarely put them on the railroad
tracks. We observed the crow hiding its food along
the tracks on 50 occasions, but the crow laid a stone
on the track only once. Therefore, some other conditions or factors, other than the sole act of caching,were responsible for the crow placing stones on the
tracks. Of the 9 occasions that the crow was observed putting stones on the tracks, the details are
known for only 4 occasions.

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Case 1: June 26. 12:30. Video-recorded by Nippon
Television Network Corporation (including a part thatwas not broadcast on TV).
After having retrieved and eaten the cached bread,the carrion crow perched on a track and rubbed its
beak against it. It moved down between the tracks
and picked up a thick stone beside a cross tie. It
walked to one track with the stone in its beak and laid
it down beside the track. Then the crow picked up
the stone again and tried to lay it on the track, but thestone fell. Picking up the stone again, this time, thecrow itself stepped onto the track with stone in beakand placed it there. After pecking the stone, the bird
picked it up again and came down on the other side of
the track. Then it laid down the stone beside a crosstie, and placed two stones at a short distance from thecross tie, as it did when caching bread.
Case 2: July 24. 06:45. Observed by the authors(Higuchi & Morishita).
After the carrion crow carried some pieces ofbread to the railroad tracks to hoard them, it ate all but
one piece, which the crow pushed between gravel by across tie. Then the crow laid a stone on the spot.
The crow pulled another nearby stone and picked it up
to add it to the hiding place. It laid the stone on topof a track nearby, however, probably because the stone
was heavy, or awkward to hold with its beak.
Picking up the stone again immediately, the crow laid
it on top of the spot where the bread was hidden.
Case 3: July 21. 16:37. Observed by Higuchi.
Having retrieved and eaten the cached bread, thecarrion crow picked up the stone with somebreadcrumbs still stuck to it, and began to walk acrossthe railroad over three tracks. On the fourth track,
however, the crow stopped and laid the stone on top
on the track. Then the crow ate some of the bread

Fig. 3 Caching behavior of crows on the railroad track. (Illustration by Michiko Shigehara.)(1) A crow picks up some bread.
(2) The crow carries it to the railroad.
(3) The crow hides it between gravel and puts a stone on the hiding spot.
(4) The crow picks up and removes a stone from the hiding spot.
(5) The crow eats the cached bread.

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H. HIGUCHI and E. MORISHITA

still sticking to the stone, picked up the stone again tocarry it to still another place, and hid it there. Somebread still stuck to that stone.
Case 4: June 30. 05:30. Video-recorded by NHK.
Having retrieved and eaten the cached bread, thecarrion crow picked up a stone and carried it somedistance. The crow stepped onto a track with thestone in its beak. It rubbed its beak against the track
to remove some breadcrumbs from its beak and atethem. Having eaten the breadcrumbs, the crow flew
away, leaving the stone on the track.
To summarize, in three of four cases (cases 1, 3
and 4), the carrion crow laid stones on the railroad
tracks when it retrieved and ate its cached bread. In
one case (case 2) the crow placed a stone on a track as
it hid the bread. Except for case 4, the stones thecrow laid on the tracks looked slightly too large or
thick for the crow to hold with its beak.
In cases 1, 2 and 3, it is assumed that the crow laidstones on the tracks to hold them more comfortably
because they appeared awkward to continue holding
with the beak. In addition, the crow in case 3 ate
some of the bread sticking to a stone that was laid on a
track. The crow in this case used the track as a table.
In these cases, placing the stone on the track was astep in the process of hoarding and eating the hoarded
bread. At the end of the procedure, the crow carried
the stone again, instead of leaving it on the tracks.
In case 4, the crow probably noticed somebreadcrumbs sticking to its beak, when it picked up
the stone. In order to remove or eat the breadcrumbs
on its beak, the crow had to put the stone down
somewhere nearby. The series of actions the croworiginally started to conduct was interrupted as thecrow noticed the bread on its beak. After removing
the breadcrumbs from its beak and eating them, thecrow forgot about the stone, leaving it on the track.

It is assumed that the stones that stopped the trains
were left behind on the tracks in this context.
Why then was it on the railroad tracks that thecarrion crow chose to place stones? We cannot thinkof any other reason than because the tracks were thereat that site. Within a right-of-way for a railroad,tracks are very convenient for crows to put things on
because the surface of the tracks is flat and theirheight is suitable as well. The crow laid stones on
the railroad tracks casually, as if for no special reason.

4. The Mechanism of the Incident
As seen in earlier sections, the incident with stones
laid on tracks is associated with food caching behavior
that is well known in crows (Vander Wall, 1990;
Cramp & Perrins, 1994; Bugnyarm & Kotrschal,
2002). The mechanism of the incident is shown in
Fig. 4. Some people threw pieces of bread for carp
in the river and for feral pigeons gathered at thosesites. If carp and feral pigeons alone had eaten thebreadcrumbs, this incident would not have occurred.
Crows were also attracted by the scattered breadcrumbs. The people also gave some bread to thecrows gathered around them. One of the crows carried some bread to the railroad and cached it there.
The crow laid stones on the railroad tracks when it
hoarded this bread or retrieved it to eat. The crowpicked up stones on either occasion. When thestones were heavy or awkward for the crow to holdwith its beak, the crow laid the stones on the tracks,
perhaps intending then to take a more comfortablegrip on the stone. The crow also put stones on the
tracks to remove some breadcrumbs from its beak.
The suitable height and flat surface of the tracks
probably induced the crow to lay stones on the tracks.
When the crow laid stones on the tracks, it often

Fig. 4 Mechanism of the stone-placing incident by crows.

What Made the Crow Lay Stones on the Railroad Tracks?

picked up the stones again and carried them away.
On rare occasions, however, the crow left stones on
the tracks, causing the trains to make emergency
stops.
What measures would then be effective to prevent
crows from putting stones on the tracks? Since thecrow put stones on the tracks while caching food, it
will cease to place stones on the tracks if it loses theopportunity to cache. To achieve this, it is requiredthat people stop feeding the crows.
Since it became clear how the crow was involvedin the incident, the people have voluntarily stopped
scattering about pieces of bread profusely, and they
started throwing breadcrumbs only into the river. As
a result, the crow has stopped carrying pieces of breadto the railroad and putting stones on the tracks as well.

5. Similar Incidents in Other Areas
Similar incidents with stones laid on tracks also
have occurred in other areas of Japan, such as Akita,
Aichi, Saga and Kumamoto Prefectures. However,they were reported by mass media and the details are
not known.
In October 2003, we observed crows in thenorthwestern part of Kumamoto, Kyushu, and foundthat at least some carrion crows really did lay stoneson the railroad tracks. The crows were among thethousands of carrion and jungle crows inhabiting thearea around hog pens located along the JR Kagoshima
Lines. Both crow species often carried pieces ofbean-curd from the pens to the surrounding sites, andhoarded them.
The carrion crows in question
hoarded bean-curd between gravel on the railroad, andrecovered it later. On a couple of such occasions, theindividual crows laid a stone on top of the track,
although they removed the stone immediately after.
It is not known how many carrion crows were involved in these events.

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In the Kumamoto and Saga areas of Kyushu,
dozens of railroad incidents with stones laid on tracks
have occurred in the last few years, in some of which
crows may have been involved (JR West personalcommunication). JR West is nervous about thisproblem, because their Super Express service will get
started in the spring of 2004 and stones laid on tracks
may cause serious incidents with the trains running
through at speeds of 250 km/hour.

Acknowledgements
Naohiro Yasukawa of NHK and Toru Shimizu of
Nippon Television Network Corporation cooperated
with us when we examined in detail the caching
behavior of the carrion crow that laid stones on therailroad tracks. Yoshiaki Iijima and some students of
Keio and Hosei Universities helped us to observe
crows at the scene of the train-stop incident. Some
staffs of JR West helped us to observe crows inKyushu. Takashi Kurosawa was very helpful in
preparing the earlier draft, and Jason Minton
improved some parts of the draft. We would like toexpress our sincere gratitude for their cooperation.
This study was partly funded by the Ministry of
Education and Culture (No. 10490011).
References
Bugnyarm, T. and K. Kotrschal (2002) Observational learningand the raiding of food caches in ravens, Corvus corax: is it‘tactical’ deception? Animal Behaviour, 64:185-195.
Cramp, S. and C.M. Perrins (1994) Handbook of the Birds ofEurope, the Middle East and North Africa. The Birds of theWestern Palearctic. Volume VIII Crows to Finches. Oxford
University Press, New York.
Higuchi, H. and E. Morishita (1997) Crows placing stones on
railway lines. Kagaku, 67:173-178. (in Japanese)
Vander Wall, S. B. (1990) Food Hoarding in Animals.
Chicago University Press, Chicago.

(Received 7 November 2003, Accepted 2 December 2003)