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.