Animal Behaviour- Assignment 2
Roosting Behaviour in House Crows
Group:
Rachana Bhave (20101084), Prarabdha Jagdhane (20101021), V Saudamini (20101010)
Species of Interest:
Corvus splendens (House Crow)
Corvus splendens, commonly known as the Indian House Crow is an extremely common bird
belonging to the crow family (Corvidae). It is found extensively throughout the Indian subcontinent
and has spread to other parts of the world where it is considered as an invasive species. It is a
moderately sized (40cm long) bird, with glossy black feathers all over its body, except its neck and
breast which is grey in colour.
There have been next to no studies on these species describing their behaviour and ecology. We
decided to study the roosting behaviour of these birds.
Roosting:
A location where birds rest or sleep is referred to as a roost. House crows (and many others from the
same family) are known to aggregate in large numbers, in orders of hundreds and thousands at the
end of the day. House crows roost communally with other birds, most often with mynahs. The roost
locations are usually surrounded by tall buildings in dense human habitation. Roosting has been
studied in various birds, and within the corvids in American crows. However, the exact reason why
birds collect in such large numbers is still a matter of conjecture.
We discovered a roost by following flocks of crows to a park, Chittaranjan Vaatika, Model Colony,
situated in the middle of a residential area. On questioning regular visitors, we learnt that the crows
and mynahs have been roosting in the park all year round. Crows start arriving by 1745 hrs and
leave the roost by 0630 hrs.
Method of data collection and Observations:
1. We carried out ad-libidum sampling at different times of the day, in order to characterise the
range of behaviours displayed by the House Crows (Table 1).
2. We observed that the birds preferred to perch on an open canopy when they first arrive,
slowly moving into the denser canopy as the sun sets. We chose an open canopy tree that
could be fully observed and performed counts on the number of crows perched on it. (Table
2).
3. We have time-dependent occupancy of crows for only one tree as this one was easier to fully
monitor. However, we have made qualitative observations for other trees for about 3
consecutive days. We walked around the park noting the peripheral trees that contained
around 10 birds or more. We repeated this again at around 1900 hrs to find these trees with
no or barely any birds, indicating that the crows do move into shaded canopy as it becomes
dark. Further we also walked in the lanes around the park, to see if the crows moved to some
other location after dusk. Since, we couldn’t see any crows fly out of the park to another
place it was safe to assume that, the crows take refuge in denser canopies within the park
itself.
Table 1: Ethogram describing behaviours of the House Crows
Behaviour
States
Fly
Perched
On ground
Motor Activities
Fly
Flap
Hover
Perched
Hop
Sidle
On ground
Walk
Hop
Interaction
Beak rub
Groom
Peck
Intimidate
Vocalisation
Caw
Other
Clean beak
Ruffle
Groom
Scratch
Open beak
Look around
Crouch
Hold
Peck
Bite
Pick
Swallow
Sleep
Description
Opening wings and lifting/staying up in the air
Sitting (both feet in contact) on a tree, rock, building
Sitting (both feet in contact) on road, path, ground
While flying, moving wings up and down rapidly
While flying, motion without movement of wings
Jump by lifting both feet up in the air and landing. Movement could be from
one branch to another, or to turn oneself in the same place.
Walk sideways along a branch
Put one foot ahead of another to move forward
Jump by lifting both feet up in the air and landing. Movement could result in
moving forward, or in turning oneself in the same place.
Rub beak against beak of another individual
Rub beak against back, wings and feathers of another individual
Tap beak against another individual repeatedly (aggressive)
Flap wings or move suddenly towards another individual resulting in retreat of
the other individual (aggressive)
Open beak and emit a croaky sound similar to caw
Move beak back and forth across the bark of a branch, rub the sides of the
beak alternatively against the bark of a branch
Shake feathers while perched
Rub beak against one's own chest, back, feathers and under wings
Rub claws under wings and against feathers
Keep beak open while emitting no sound
Move head in different directions
Bend forward such that beak can almost touch the feet
Use feet (talons) to immobilise object
Tap beak against object (branch, meat, etc.) repeatedly
Open and close beak repeatedly while beak contains or is in contact with food
Hold object within one's beak
Lifting head in order to ingest food/water
Contract in length and stay immobile after dark
4. We also noted the direction of the arrival of crows. Our focal tree is situated at the northwest periphery of the park. For the first 30 minutes, the birds arrived primarily from the
north and the west. However, the surge in the arrival of birds after this was primarily from
the south and the east, most probably from within the park itself.
5. Cawing of the crows within the park was at a peak at about 1845 hrs, coinciding with the
peak of the number of birds on the focal tree. Hence, from around to 1815 to 1845 hrs, birds
that had already collected within the park aggregated in the peripheral open canopied tree.
After this the birds receded back into the denser canopy within the next ten minutes.
Table 2: Time dependent occupancy
Time (hrs) No. of crows Remarks
1745
7
1750
1 All flew south
1755
8
1800
5
1805
11
1810
11
1815
30
1820
26
1824
3 All flew south
1825
60
1830
45
1835
45
1836
5 All flew south
1840
60
1845
65
1850
40
1855
5
1908
0
6. Most of the crows perched on the focal tree showed non-aggressive behaviours. Crows upon
arrival display beak cleaning and grooming behaviour (See Table 2). Other behaviours
shown subsequently include interactions such as grooming and beak rubbing. The grooming
interaction however was observed only for some pairs of crows indicating possible mate
pairs or related individuals. In the context of occupying a particular spot on the tree,
aggression towards another crow by pecking or intimidation was observed. However, this
behaviour was mainly directed towards conspecifics and not other bird species which
occupied the same tree. Further, we found that a particular tree (not the same as the focal
tree) over consecutive days had similar number of birds perched on it.
7. An interesting group behaviour observed was the occasional spontaneous spurts of flight
that the crows displayed. The crows fly out as flock, circle around the dense canopy and
return to the tree. The behaviour seems to be spontaneous as it is not necessarily correlated
with the intensity of calls, arrival of a new group of crows or a predator. This group
behaviour seems to be well coordinated with all the birds moving in the same direction at
the same time without any prior detectable signal that. We cannot confidently say if the birds
return to the same tree, since crows from other trees also follow suit, moving from denser
canopies towards the sparse open canopies. It is difficult to ascertain why the crows could be
showing this seemingly costly behaviour, but further observations will be needed.
Questions that can be asked based on these observations:
1. Since the crows eventually move into denser canopies it evades reason as to why they
compete for an open canopy initially?
2. Is there a functional significance to the occasional aerobatics that they display by flying
in flocks and returning to the same tree since it can also attract attention to predators?
3. Is there a social structure or hierarchy in occupancy of various branches of the trees? Are
these places fixed or random?
4. How is a site/tree for roosting selected? Is it correlated with the distance from the
foraging site or direction?
Further observations that we plan to take to answer questions 1 and 4:
1) Record no. of crows perched on other open canopied trees in the periphery of the park as
well as the denser canopies. Also estimates of how this number changes with time will be
recorded. This will give us an estimate of the no. of crows that inhabit the park, possible
foraging sites and quality of the roost site.
2) Instantaneous/Scan sampling of behaviours shown by the crows perched on different trees to
investigate if any real differences exist between the groups.
3) All occurrences sampling of one of the behaviours that can help us make some prediction
regarding the functionality of gathering on the open canopy trees and a time –budget
analysis.
Difficulties in designing a good experiment or observation table to test any of the hypotheses:
1) To detect the nature of social structure of these birds it is important to be able to identify
individuals and distinguish between them. In our case this may not be possible. Further it is
difficult to even distinguish between male and female crows.
2) The crows roost in very large numbers and a lot of seemingly unexplainable behaviours are
displayed making it difficult to
a) Track an individual for a meaningful period of time
b) Distinguish between behaviours shown by particular groups especially calling which will
be confounded by surrounding groups of crows
3) It is also difficult to observe behaviours shown by crows in the denser canopies unless an
aerial view can be obtained.
References:
1. Wikipedia.
2. Kelvin S.-H. Peh and Navjot S. Sodhi (2002). Characteristics of Nocturnal Roosts of House
Crows in Singapore. The Journal of Wildlife Management 66(4):1128-1133.
3. Crow FAQ, Kevin J McGowan, Senior Research Associate, Cornell Laboratory of
Ornithology, Ithaca, NY.
4. Nyari, A., Ryall, C. and Peterson, A. T. 2006. Global invasive potential of the house crow
Corvus splendens based on ecological niche modelling. J. Avian Biol. 37:306-311.
5. Eiserer, Leonard A., Communal Roosting in Birds,Bird Behavior, Volume 5, Numbers 2-3,
May,1984 , pp. 61-80(20).
6. Madhav Gadgil, Salim Ali. Communal roosting habits of Indian Birds. Journal of the
Bombay Natural History Society, 1975.