CHAPTER-3 ROOSTING ECOLOGY

Transcription

1 CHAPTER-3 ROOSTING ECOLOGY 3.1 INTRODUCTION The word roost is derived from the German language meaning a sleeping house for fowls. The roosting behaviour of various avian species has been studied all over the world. Some of the important investigations about the roosting include the study on Starling s roosting (Brown 1946); roosting of Swallow by (Rudebeak 1985, Gurr 1968) studied roosting of Australian Harrier. Wynne-Edwards (1962) worked on the behaviors of birds prior to their occupation of communal roosts and hypothesized that roosting behavior to have a function of regulation of population size in relation to local food supply. Zahavi (1970) described pre-roosting gathering and communal roosting in details. Ward and Zahavi (1973) interpreted the roosting of birds as the information centre for food fiding, whereas Erwin (1983) suggested that food location might not enhance through any information exchanged at the colony but it takes place via local enhancement. Anderson (1981a) discussed the information centre hypothesis with reference to gulls. Draulans et al. (1986) and Brown (1946) also carried out research on information centre. Gadgil and Ali (1976) studied communal roosting habits of some passerines. Burger et al. (1977) has described intra and interspecific interactions at a mixed species roost of ciconiiformes in Mexico. Sykes (1985) studied the roosting behaviour and its relationship to the number of roosts with the Snail Kite (Rostrhamus sociabillis) in Florida, USA. Very little information is available on the factors afecting the roosting site selection, arrival and departure patterns, Pre-roosting and roosting behaviors, roosting periods, association with other avian species roosting communally and the occupation of roost in relation to the population of myna in India. 28

2 3.2 MATERIAL AND METHODS The present this study was carried out from August 2006 to December 2008 at Junagadh. The roosting sites were located by following the mynas leaving their foraging ground in the evenings; the roosting sites identified were Sakkarbaug (SBZ), Lalbaug (LB), Junagadh Agricultural University Campus (JAU) and Civil Hospital (CVLH). Usually the observations were recorded during late evenings and the following morning. The flock size and its relationship with the microhabitat were determined by recording departing and arriving birds at the interval of every five minutes. It was important to make a note on total number of individuals, arrived and departed at the study site to avoid a bias in doing comparison of time required to enter and to leave the roost by the same number of birds. Sunset and sunrise timings were obtained from the Aerodrome Authority, Junagadh, to determine the relationship between periods of light with the peak arrival as well as departure timings of the mynas. 3.3 RESULTS Four roosting sites were identified in study area from August 2006 to December These are Sakkarbaug Zoo (SBZ), Lalbaug (LB), Junagadh Agricultural University campus (JAU) and Civil Hospital (CVLH). These roosting sites were utilized throughout the year i.e; during the breeding and non breeding seasons of the mynas considered for the study. Selection of the roosting site Selection of the roosting site by myna is affected by physical characteristics of the sites and behavioral aspects, such as anti-predation tactics, low anthropogenic disturbance, availability of large trees, and distance to feeding sites. It was observed that from roosting sites mynas disperse and leave for foraging to far and near sites in different group sizes (Table 3.3) during the early hours depending upon the distances of the feeding sites (Table 3.4). Table 3.5 shows tree species used by the mynas in the present study along with their scientific names, tree height, Canopy and Girth of Trunk at Breast Height (GBH). The trees included Indian fir, Bullet wood, Neem, Coconut palm, Elephant fig and Bottle palm, which have been commonly used trees by myna for roosting. 29

3 Out of the four roosting sites, maximum number of mynas was observed in JAU, followed by SBZ, CVLH and LB respectively (Plate-2). Selection of roosting sites differed from species to species. Common Myna was absent in CVLH site whereas Brahminy Myna was found only at SBZ. Bank Myna prefered CVLH site but also found in good numbers at JAU and SBZ but were completely absent LB. Common Myna was relatively higher in number at JAU but was also observed in good numbers at SBZ and LB (Table 3.7 and Fig. 3.1). Arrival and Departure The total arrival period (min) for all three species studied was the highest in April (upto 92 min; Table 3.1). The reason might be scarcity of food, the birds needed more time to search their food. However, the total number of mynas that arrived at roosting site was highest during December (3885). This might be due to the addition of new fledglings during postbreeding period. The departure period was the highest in August (63 min), probably due to cloudy weather during rainy season. In accordance with this, the total number of myna departed was the lowest in August (3098). The rate of myna arrival and departure per min is as shown in Table 3.1. The average and Standard Deviation of total arrival period (min) were and 22.2 respectively, for total number of myna arrived it was and respectively, total departure period (min) it was 48 and respectively and total number of myna departed it was and respectively. During arrival, flock size was smaller than 5 birds for most of the cases (53.93%) and flock size larger than 40 birds were least occurred (0.06%; Table 3.3). Similarly the percentage frequency of flock size of departing myna was the highest for less than 5 birds (38.97%) and least for greater than 40 (1.27%; Table 3.3). Their flock size does not remain constant during the arrival and departure from the roost site. Mynas started movement before sunrise. Normally preening and scratching is observed in the early morning. Departure from the roost started just after sunrise in small or large flocks. Departure took 39 to 63 minutes ( =48, SD ± 10.92, n=6), which was shorter than that of arrival. The data revealed that the mean arrival time of the myna was significantly longer than the mean departure time. An average flock size of arriving and departing birds was statistically different (Table 3.1). 30

4 It may be observed from Table 3.2 that the arrival and departure durations of myna on the roost site were different for the species under consideration. The number of arrival and departure for Common and Bank Mynas were much higher than Brahminy Myna. The number of Common and Bank Mynasn were almost the same. But in December both of them came in more number, whereas Common and Bank Myna came in lesser numbers during August and April respectively. Figure 3.2 shows that the percentage frequency of flock size of the myna arriving from the roost. It was observed that the percentage frequency of Brahminy Myna was the highest in number for the flock of less than five. For flock size of 5 10, Common and Bank Mynas were equal in number. No Brahminy Mynas were found in flock size. The numbers of Common Myna in a flock size of 5 were higher than Bank Myna. For the flock size of 5 10 and the number was less than Bank Myna but greater than that of Brahminy Myna. For all the flock sizes from and more, the number of Common and Bank Mynas arrived was equal. The flock size of Common Mynas was nevere recorded more than 35 birds. Brahminy Myna arrived less in number than Common and Bank Mynas. Figuer 3.2 revealed that the frequency of flock size among three species of mynas the Bank Myna found in highest numners compared to other two species of mynas in the flock size 5-10, 10-15, 15-20, and Figure 3.3 indicates the percentage frequency of flock size of the myna departing from the roost sites. It was observed that Common Myna was the highest in number with respect to flock size less than five. But for all the other, flock sizes greater than 5, it was less than Bank Myna but greater than that of Brahminy Mynas. The Bank Mynas departed less in number than Common Myna and higher than Brahminy Myna for the flock size less than five. For all the other flock sizes great than 5, the number of Bank Myna departing was higher than that of the Common and Brahminy Mynas. The flock size of Brahminy Myna was always less than five. They did not form in any larger flock of more than 5 individual and therefore, remained lower in number than Common and Bank Mynas for all the flock sizes. This was probably because the density of Brahminy Myna was less than that of other two species. Figure 3.5 shows that there was no much difference between arrival start (hr) and arrival end (hr) in roosting i.e., =18:39 and = 18:53 respectively in all seasons. The same 31

5 trend was found in loud calls start (hr) and loud call end (hr) in all seasons. The result also revealed that there was no significant difference between summer and monsoon, but significant difference was found in summer and post monsoon. This indicated that all the roosting parameters i.e., arrival start and arrival end, loud call start and end and last loud call values were higher in monsoon and summer as compared to post monsoon and winter. The total number of loud calls was highest in summer and followed by monsoon, post monsoon and winter season i.e., 28, 24, 21, and 12 respectively. Figure 3.6 reveals that the roosting departure parameters were different in all seasons. The departure first loud call started in post monsoon, monsoon, summer and winter at 5:31, 5:42, 5:49 and 6:19 hrs respectively. The same trend found in the last loud call before departure (hr). Departure started in post monsoon, monsoon, summer and winter at 6:29, 6:20, 6:11, and 7:10hrs respectively. Departure end (hr) in post monsoon, monsoon, summer and winter were at 6:41, 6:35, 6:19 and 6:16 hrs respectively. Departure start and departure end timings indicated that there was very short time between departure start and departure end. The result also revealed that the departure start was early in winter than in other seasons. At the departure total number of loud calls was higher in summer which was lowerd by monsoon, post monsoon and winter i.e., 24, 23, 12 and 8 respectively (Fig.3.7). The arrival departure pattern for all the species of mynas had been interesting. All the mynas at all the locations arrieved from the same direction and departed also in the same direction. For example at SBZ, the mynas were arriving from North and departing towards North. This means that the mynas were arriving from their feeding site and departing to the same place. Pre-roosting and roosting behavior Figure 3.4 shows that in monsoon was no much difference between arrival start (hr) and arrival end i.e., =18:13 and = 18:29 respectively. The same trend found in summer also i.e., arrival start (hr) and arrival end (hr) was =18:11 and = 18:40 respectively. 32

6 Whereas during post monsoon the arrival start (hr) and arrival end (hr) was =17:26 and = 17:43 respectively. The same trend found in winter demonstrating the arrival start (hr) and arrival end (hr) was =17:32 and = 17:57 respectively. This indicated that there was much difference in arrival start (hr) and arrival end (hr) in summer and post monsoon. In monsoon and summer myna arrival start (hr) and arrival end (hr) were late as compared to that during post monsoon and winter. calls. During arrivals to the roosts, all the species of birds including the myna produced loud During early roosting stage, mynas were also involved in maintenance behavior like preening, scratching and loud calls at intervals during the night. Mynas started awaking before sunrise. Preening, scratching of the feathers was normally observed mainly in the early morning. It was observed that the roosting birds did not change their position during night. Communal roosting The present study confirms that mynas are heterogeneous communal rooster 11 other species of birds were recorded roosting along with the myna at the study site (Table 3.6). The roosting sites discovered, were utilized by mynas for the entire study period. 3.4 DISCUSSION Selection of the roosting site Roost-site preferences of mynas and other species birds of have been studied with the aim of manipulating roost habitats. Structural characteristics of roost trees (e.g., tree canopy density and girth of the trunk). Other factors including proximity to food sources, drinking water, and human structures, combined with the structural characteristics of the habitat influence the selection of roosting sites by birds such as starlings (Sturnus spp.; Kuroda 1973), and mynas (Kang and Yeo 1993). The proximity of food sources to roots makes intuitive sence since the sorter distance travelled daily by the birds could mean longer feeding time and less energy expended during foraging flights (Charlotte et al 2002). 33

7 Various natural and a biotic constraint such as of vegetation, tree height and density, microclimate, safety from predators, anthropogenic disturbance and distance to feeding sites affect the roost site selection by mynas in study area. Selection of roosting sites was different from species to species. Common and Brahminy Mynas were absent in CVLH site, which is highly disturbed. Brahminy Mynas were found at SBZ, which is the least disturbed area. Thus Common Myna and Brahminy Myna avoided places of disturbance while selecting the roost. However, Bank Mynas were found at CVLH, JAU and SBZ but completely abandononed less disturbed site like LB (Figure 3.1). Thus it is inferred that the disturbances had a negative effect in selecting a roost in the case of Brahminy Myna and Common Myna, whereas this was not observed in the case of Bank Myna. Other factors too influenced the phenomenon, but major factor was disturbance. The main reason for selecting highly disturbed site by Bank Myna is the availability of food resource and distance to feeding sites in urban areas or localities. This is a very important observation which indicated the differentiation among species in the same area constantly under the pressure of interspecies competition. Tree canopy (Table 3.5) were scattered in CVLH and increased from SBZ, to LB to JAU, which revealed a positive correlation with the number of roosting individuals of Common Myna, this phenomenon was less applicable to the other species of mynas studied. Arrival and departure Light duration (day length) most certainly influenced the roosting time (Davies and Lussenshop 1970). Arrival and departure timings varied seasonally depending upon photoperiod. Siegfried s (1971) found no seasonal differences in the Cattle Egrets time taken to arrive or depart. While our study suggested that the arrival and departure timing were apparently related to the geological location and the difference was almost half an hour but seasonally the difference in the day length was as high as two hours between summer and winter. Morning departure of the myna from the roost was mainly based upon factors related to exploiting the foraging ground, which resulted in a clump of birds leaving the roost site in a 34

8 particular direction in a shorter period of time. However, during evenings, the arrival of the myna at intervals took a longer period of time to fill the roost. Similar observatios were made in Black Ibis by Vyas (1996) & Chavda (1997) and in white Ibis by Gadhvi (2001). Patchy distribution of food resulted in the dispersion of the birds in clumps. The occurrence of socially induced flights i.e., when one flies from the colony the others are induced to do likewise were observed (Bayer 1981; Erwin 1983). According to Gadgil and Ali (1975), the proportion of flock feeder is markedly greater among communal rooster. The result of short dispersion is apparently relevant to the flock feeding habit of the myna. This tends to reinforce that they remain in a flock on the feeding ground and triggers them to leave the site as a group. Though duration of departure time was shorter, the flock size of myna arriving at or departing from the roost site was not similar. This confirms that the threshold to leave the roosting site due to empty digestive tracts may result in quicker dispersion. Pre-roosting and roosting behavior Pre-roosting gathering has been seen particularly among the species that feed together and roost communally (Wards and Zahavi 1973), which serves as a roosting advertisement and acts as an information center related to the potential food sources. Salimkumar (1982) and Chavda (1988) observed that several of Black Ibis in flocks gathered either at the feeding ground or near their roost. The present study also supports their finding. They exhibited behaviors such as preening, fluffing of feather, shaking of body, etc. during pre-roosting. Occurrence of mating during pre-roosting exposes one of the roles of flock gathering in pair formation as well as influencing strength of bonding for next breeding season (Vyas 1996, Chavda 1997). The communal roosting habit of myna suggests that the aggregation of large number of birds may be helpful to minimize the potential danger against predators. Again selection of the same spot suggests experience of a safe location and preference towards the center of a flock helps in hiding lessens exposures to the wind. Preening and scratching behavior were found to be essential for the body maintenance. Preening is the basic and more significant signal act that a bird performs to take care of its 35

9 feathers (Simmons 1964). The body shakes or fluffing of feathers during standing quietly was performed to remove the parasites (Mckinney 1983) and scales. Sengupta (1973) concluded that communal roosting in Common Myna had evolved primarily as an antipredator adaptation. Communal roosting was advantageous with respect to food location in this species because the birds he studied fed either individually (in towns), or in small parties (in country areas), rather than in large flocks. Ward and Zahavi (1973) stressed that large communal roosts may exist even when all members of the roost are adequately fed, but could act as an insurance against any unpredictable food shortage affecting part of the population. Solitary feeding in birds that roost communally, and which will feed in flocks if suitable food is available, need not therefore negate the hypothesis that communal roosting has evolved as a method of disseminating information about food distribution. Assemblages of birds could attract predators, and therefore required protected positions for the inactive period and anti-predator behavior while the birds were assembling (Ward and Zahavi 1973; Feare et al., 1974). Mixed roosting strongly supports the notion of avoidance of predation being an important function of communal roosting (Gadgil 1975). Mynas were introduced to the Seychelles probably in the early nineteenth century (Gaymer et al. 1969) and have become successful colonizers. Mynas are common except in the highest forest. In lowland areas they feed singly and in small parties in plantations and other fairly open habitats, but assembled in larger flocks. Gadgil and Ali (1975) reported that passerine birds normally aggregate in as small an area as possible and do not leave trees unoccupied. After the arrival on the roost site it was observed that some myna try to take position in the middle of the flock but this shifting may increase an intraspecific inter actions. Compromising with the competition, increase in flock size may be energetically adaptive by presuming profit from others to escape directly from danger without continuous scanning or remaining alert. This would allow an individual to spend more time in self care such as preening and scratching (Drulans et al. 1986). Mynas prefer mixed communal roost. 36

10 The species feeding in flock on an unevenly distributed food supply tend to roost communally (Gadgil and Ali 1975). The adaptive significance of communal roosting was sighted here involve both peace and safety of birds as well as social feeding (Ward 1969, Ward and Zahavi 1973) and some social attraction (Gadgil and Ali 1975). This was observed in case of myna at Junagadh also. 3.5 CONCLUSIONS We have studied roosting behaviour of three species of myna at four different sites in Junagadh during The data revealed that gathering and flocking tendencies of the mynas within its own species and with other species of birds during day time and at communal roosts have been evolved through various factors like food location, antipredator adaption, pair formation, avoidance of disturbed location, climate conditions, tree height and density, vegetation, drinking water, building of different types, season and geological conditions. It also evolved to get certain benefits out of the social system particularly the synchronization of various activities. It was found that the Brahminy and Common Myna did not prefer disturbed locations whereas Bank Mynas were found at highly disturbed site. Seasonal variation is found in all three species studied here. Almost 2 hour difference in arrival and departure is found between different seasons. Brahminy Myna and Common Myna were absent from CVLH where predators were abundant in numbers. But even though Bank Mynas were found there, which means that antipredator adaptation in roost site selection was more in Brahminy and Common Myna compared to Bank Myna.. 37

11 Table 3.1 Roosting population of three species of myna. Observation Total Total Total no. Total no. Myna Myna Months arrival Departure of Myna of Myna Arrival / departure & Year period Period arrived departed min / min (min) (min) 1 Dec Apr Aug Dec Apr Aug Average SD ± Table 3.2 Arrival and departure number of myna on the roost site. Roosting population Observation Month and Year Common Myna Bank Myna Brahminy Myna 1 Dec Apr Aug Dec Apr Aug TOTAL

12 Table 3.3 Percentage frequency of flock size of the myna arriving and departing from the roost site. Flock size % Frequency Arrival Departure < to to to to to to to > Table 3.4 Distance between roosting and feeding sites. Sr. no. Feeding site Distance (km) from roost site 1 LB 1 to 5 2 JAU 1 to 6 3 GGS 1 to 8 4 TV 2 to 7 5 PTC 4 to 10 Abbreviation LB JAU GGS TV PTC CVLH GBH Lalbaugh Junagadh Agricultural university Gandhigram society Timbavadi Police training center Civil Hospital Girth at breast height 39

13 Table 3.5 Use of different tree species in myna roosting. Tree species Canopy Common Tree height GBH (m) Scientific name diameter (m) name (m) ( ± SD) ( ± SD) ( ± SD) Asopalav Polyalthaea longifolia Bth. & Hook ± ± ± 0.11 Borsali Mimusops elengi Lnn ± ± ± 0.06 Neem Azadirachta indica Juss ± ± ± 0.14 Naliyeri Cocos nucifera Linn ± ± ± 0.05 Peeper Ficus tsiela Roxb ± ± ± 0.03 Rajtad Roystonea regia H.B. & K ± ± ± 0.03 Table 3.6 Members of Communal roosters with myna. Bird species Common name Scientific name Black ibis Pseudibis papillosa Black crowned Night Heron Nycticorax nycticorax Cattle Egret Bubulcus ibis Grey Heron Ardea cinerea House Crow Corvus splendens Indian pond Heron Ardeola grayii Indian Peafowl Pavo cristatus Little Egret Egretta garzetta Large billed Crow Corvus macrorhyncuos Rosy Starling Sturnus roseus Table 3.7 Number of individuals of three species of myna roosting at different site in study area. Species SBZ LB JAU CVLH Common Myna Bank Myna Brahminy Myna Total

14 Number of individuals Common Myna Bank Myna Brahminy 0 SBZ LB JAU CVLH R oos ting sites Fig.3.1 Number of individuals of three species of myna roosting at different sites in the study area. Fig.3.2 Percentage frequency of flock size of the mynas arriving to the roost site. 41

15 Fig.3.3 Percentage frequency of flock size of the myna departing from the roost site. Fig: 3.4 Pre roosting in different seassons. 42

16 Fig.3.5 Arrival at Roosting site. Fig.3.6 Departure from roosting site. 43

17 Fig.3.7 Seasonal arrival and departure loud call. 44

18 I II III IV Plate: 2 Roosting of mynas at different sites 45