Title: Shrewd alliances: mixed foraging associations between treeshrews, greater racket-tailed drongos and sparrowhawks on Great Nicobar Island
Authors: Meera Anna Oommen and Kartik Shanker
Electronic Supplementary Material: Sampling design, methods and study animals
Sampling design and methods As the first ever field study on the endangered Tupaia nicobarica, the overall field study design addressed basic status and population level issues as well as the ecology and behaviour of this little known island endemic. The study was carried out on Great Nicobar Island (6º 45’to 7º 15’ N, 93º 38’ to 93º 55’ E, 995 km²) in the Andaman Sea. A low-lying mixed-species littoral woodland of 5 km² at South Bay in Galathea National Park in the southern portion of the island was selected for intensive study as this location was relatively more accessible than the rest of the island. Following the study, in 2004, Galathea Bay (South Bay) was among the first locations to be submerged by the Indian Ocean tsunami and the area was flooded and largely destroyed.
The mixed foraging association between treeshrews, drongos and sparrowhawks was observed in all survey locations (the southern, eastern and western parts of Great Nicobar which were visited). To explore this intriguing mixed foraging association, we developed a sampling design that incorporated the assessment of interactions between all members of the group. Instantaneous scans and focal animal/group observations were the two main sources of data. The data were collected over a period of three and a half months.
Instantaneous scans Instantaneous scans were employed to understand group composition, distances between individuals of the group and to record primary activities of members of the group. Treeshrews were typically active from sunrise to sunset. This period of 12 hours was divided into 4 time categories of 3 hours each during which we tried to collect a minimum of 60 scans each during the study period. We recorded a total of 360 scans during the study. Once a group was located, we noted the activities of group members at 3 or 5 minute intervals till the group was lost. The interval for each successive scan was picked randomly. Activity data was collected for a period of 25 hours.
Within the mixed foraging association between treeshrews, drongos and sparrowhawks, we observed different combinations of individuals. These combinations were treated as groups ranging from 1-15 (for details, see legend to Figure 2). For example, GROUP 1 (consisted of 1 treeshrew and 1 drongo), GROUP 2 (1 treeshrew and 2 drongos), GROUP 3 (1 treeshrew and 3 drongos), GROUP 4 (1 treeshrew, 1 drongo and 1 sparrowhawk), etc. The differences between these groups in terms of foraging rates and distances were analysed to gain insights into foraging benefits and predator avoidance.
Focal group sampling In addition to instantaneous scans, we also employed focal animal/ group sampling where a group once located was followed till it disintegrated or was lost in the canopy. Focal animal sampling was used to assess foraging rates (the interval between 2 feeding events for all species). Additional data on distances between between individuals was also obtained from this form of sampling. Focal animal/ group sampling data amounted to 13 hours and 35 minutes and is additional to the instantaneous scan data. Treeshrews are in general secretive and difficult to observe and most reported sightings and observations of these animals are for a few seconds or at the most a few minutes.
We have observations of drongos and sparrowhawks foraging on their own for very short time periods. Despite active searches, drongo foraging outside interspecific associations could only be observed for a period of 63 minutes. One probable reason for this could be the high canopy cover on the island with very few treefall gaps or other clearings where drongos and other insectivorous birds can perch and sally for insects. A solitary sparrowhawk was observed for 30 minutes and during this period, feeding was not observed.
Data analysis and interpretation To examine the significance of the presence and absence of species in different group combinations, occurrence data pertaining to the association between treeshrews, sparrowhawks and drongos was explored by fitting a generalized linear model on the count data for a 2 × 2 × 2 contingency table with two factors for each species: tree shrew (solitary and pair), drongo (present and absent) and sparrowhawk (present and absent) (Crawley 2007). First, a saturated model was fitted to the data including all interactions. The second model removed the effect of the three-way interaction between treeshrews, drongos and sparrowhawks. The significance of the deviation between models is tested using a chi square test. The central question that was asked in context of this analysis was if sparrowhawk presence was contingent on the presence of drongos in the group.
The different combinations of individuals (treated as groups 1-15, see figure 2) were clubbed into groups with a species of interest present vs. absent (e.g. foraging success of groups with drongos vs. groups without drongos, distance from treeshrews to sparrowhawks in the presence of drongos vs. absence of drongos, etc.) As the data were not normally distrubuted, comparisons between foraging rates as well as foraging distances between species were carried out using the Wilcoxon rank-sum test on independent samples (also called the Mann Whitney U test).
All analyses was carried out using R software (R 2008, http://www.r-project.org/). Box and whiskers displays were plotted using S-PLUS statistical software (S-PLUS 2000).
The mixed foraging flock Treeshrews Treeshrews (Order Scandentia, Family Tupaiidae) are a group of tropical small mammals found in South and Southeast Asia. As is the case for many rainforest species occurring at low densities, field observations on treeshrews have been limited to a few studies mostly in Southeast Asia. Treeshrew behaviour has been studied in detail in some captivity, the most intriguing of which reports the prevalence of an ‘absentee parental care system’ where the mother visits the young only once in one or two days, for a couple of minutes (Martin 1968). During this period, the young are known to ingest about a third of their body weight in milk (which has very high fat content), lie motionless in the nest to conserve energy, and autogroom from time to time. Field studies by Emmons (2000) confirmed this practice for wild Tupaia in Borneo, and it is widely believed that this strategy points to predator avoidance.
The Nicobar treeshrew, T. nicobarica is a small tupaiid with a restricted range which is found only on two islands (Little and Great Nicobar islands with an area of 150 km2 and 995 km2 respectively) in the Andaman Sea. In addition to being one of the smallest among all Tupaids, the species is characterised by its extreme arboreality in comparison to other Tupaia, and a high degree of insectivory making this species one among a handful of non-volant foliage gleaning insectivorous small mammals worldwide. Predators on Great Nicobar Island are the reticulated python (Python molurus), house cats and a number of raptor species that could potentially prey on treeshrews. As one of the most narrowly distributed endemics among all Tupaiids, T. nicobarica is currently listed as Endangered (B1) by the IUCN (IUCN 1995, 2007).
Average size of the treeshrew – A single adult individual obtained so far from Great Nicobar weighed 75g (measurement by authors). Average reported weights of other small species of treeshrews range from 46g for the pen-tailed treeshrew (Ptilocercus lowii), 52g for the lesser treeshrew (T. minor) and 70g for the slender treeshrew (T. gracilis) (Emmons 2000).
Greater racket-tailed drongos Drongos are medium-sized insectivorous birds belonging to the family Dicruridae which is distributed in the tropical and subtropical regions of Africa, Asia and Australia. The majority of drongo species have been recorded to form associations with a wide range of species ranging from ants to mammals. For five species, there have been specific observations on their following other taxa to gain access to arthropods and small vertebrates that are flushed (Styring and Ickes 2001). Greater racket-tailed drongos are resident in many parts of South and Southeast Asia. These birds play various roles in foraging associations ranging from being mutualists that benefit co-occuring species, commensal feeders that attend on beaters taking advantage of flushed insects to occasional kleptoparasites that distract insectivorous birds (with alarm calls) at the moment of prey detection and stealing food (Satishchandra et al. 2007). A wide range of research output on this species is also available from work on mixed-feeding associations in Sri Lanka. Drongos are also known to mimic the songs, contact calls and alarm calls of other flock members and flock members often benefit from their vigilance especially in attacks by Accipiter hawks (Goodale and Kotagama 2005a,b). In our study, up to three drongos were observed with a single foraging treeshrew and we also suspect that the birds were mimicking treeshrew contact calls. However, mimicry was not observed directly and may be employed only occasionally as treeshrews themselves make contact calls on a regular basis. Additionally the birds also seem well aware of the routes taken by these small mammals. The drongos directly proceeded to frequently used patches and trees whereas the treeshrews used circuitous routes to use overhangs to reach the same site. On occasion it was also suspected that the drongos followed the researcher (MAO).
Average size of the drongo Dicrurus paradiseus – 74 -85g has been reported for the southern race by Ali and Ripley and 100 – 124 g for the northern race. No details for the species on islands is given, we feel that the species on the island were marginally bigger that those seen in southern peninsular India.
Sparrowhawks Accipiter species that have been reported from Great Nicobar Island include the endemic Nicobar sparrowhawk (Accipiter butleri, this species is generally considered to be an allospecies of the Chinese sparrowhawk/goshawk), Besra (Accipiter virgatus), the Chinese sparrowhawk (Accipiter soloensis), and the Japanese sparrowhawk/goshawk (Accipiter gularis). The latter two species are migrants that arrive in winter. The behavior, hunting mode and prey species of some Asian Accipiter species has been explored by Gamauf et al. (1998). Almost all these sparrowhawks are known to feed on large insects, small reptiles and amphibians, birds and occasionally small mammals. In species such as the Chinese sparrowhawk, there is a greater preference for frogs and lizards, however even this species is known to attack birds and small mammals occasionally (Gamauf et al. 1998). Species such as the Besra are more frequent in their attacks on small mammals and birds. At least some of the individuals that we observed were likely to be the Chinese sparrowhawk, however, there is confusion regarding plumage characteristics and records of occurrence of some of these species (see Rasmussen 2000). During the field study, a sparrowhawk was observed alone (without the foraging group) only for a period of 30 minutes. Within the mixed foraging group, there was never more than one individual of the species and its role was mostly to wait at a distance for geckos that were flushed out infrequently. However, we recorded aggressive interactions between the raptor and other species that were being observed (both treeshrews as well as drongos).
Average size of the sparrowhawk – B/w 300-350g. (Gamauf et al. 1998)
References Ali, S. and S.D. Ripley. 1987. Compact Handbook of the Birds of India and Pakistan: Together with those of Bangladesh, Nepal, Bhutan and Sri Lanka. Oxford University Press. USA.
Crawley, M.J. 2007. The R Book . John Wiley and Sons, Chichester, England.
Emmons, L.H. 2000. Tupai: A Field Study of Bornean Treeshrews. University of California Press, Berkeley.
Gamauf, A., M. Preleuthner, and H. Winkler. 1998. Philippine birds of prey: Interrelations among habitat, morphology and prey. Auk 115, 713-726.
Goodale, E. and S.W. Kotagama. 2005a. Testing the roles of species in mixed-species bird flocks of a Sri Lankan rain forest. Journal of Tropical Ecology21:666-676.
Goodale, E. and S.W. Kotagama. 2005b. Alarm calling in Sri Lankan mixed-species bird flocks. The Auk122(1),108-120.
IUCN (1995) Eurasian Insectivores and Tree Shrews – Status Survey and Conservation Action Plan(Compiled by Stone, R.D, IUCN / SSC Insectivore, Tree Shrew and Elephant Shrew Specialist Group). IUCN. Gland, Switzerland, vii + 108 pp.
IUCN 2007. 2007 IUCN Red List of Threatened Species. .
Martin, R.D. 1968. Reproduction and ontogeny in tree-shrews Tupaia belangeri with reference to their general behaviour and taxonomic relationships. Z. Tierpsychol. 25, 409-532.
R Development Core Team. 2008. A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. ISBN: 3-900051-07-0, URL:http//www.R-project.org.
Rasmussen, P. 2000. On the status of the Nicobar sparrowhawk Accipiter butleri on Great Nicobar Island, India. Forktail 16, 185-186.
Satishchandra, H.S.K., E.P. Kudavidanage, S.W. Kotagama, and E. Goodale. 2007. The benefits of joining mixed-species flocks for greater racket-tailed drongos Dicrurus paradiseus. Forktail 23, 143-148.
S-PLUS. 2000. Professional Release 1. Version 4.5. Copyright 1988-1999. MathSoft. Seattle, Washington, USA.
Styring, A.R. and K. Ickes. 2001. Interactions between the greater racket-tailed drongo (Dicrurus paradiseus) and woodpeckers in a lowland Malaysian rainforest. Forktail17,119-120.