THE MIXED SPECIES FEEDING FLOCKS OF SRI LANKA

de Silva Wijeyeratne, G. (2007). The Mixed Species Feeding Flocks of Sri Lanka. January 2007. Alula: Finland. Volume 13. ISSN 1455-9439. Pages 20-31.
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Tropical rainforests are well known for the evolution of mixed species feeding flocks with coherent and stable social dynamics. A good example of these are the mixed species feeding flocks of birds in Sinharaja a lowland rainforest in the south-west of the ialsnd. These flocks are special for a number of reasons. They are one of the longest running field studies of mixed species flocks in the world. Field studies on them were begun by the late P.B. Karunaratne in 1981 under a Smithsonian funded project. The continuous data collected over two decades have shown them to be the largest mixed bird species flocks in the world with on average 41 individuals in attendance. The average number of species present is 12, although it is not unusual to encounter flocks with over twenty species of birds. The data collected shows that 21 species are ‘regular’in the sense they have been present in over a quarter of the feeding flocks on which data has been collected. As many as 59 species of birds have been observed to participate in feeding flocks. On many occasions, I have observed over half a dozen endemic birds can be seen in a flock including species such as the Ceylon Hanging-parrot (Loriculus beryllinus), Layard’s Parakeet (Psittacula calthropae), Green-billed Coucal (Centropus chlororhynchus), Red-faced Malkoha (Phaenicophaeus pyrrhocephalus), Ceylon Grey Hornbill (Ocyceros gingalensis), Yellow-fronted Barbet (Megalaima flavifrons), Black-capped Bulbul (Pycnonotus melanicterus), Ashy-headed Laughingthrush (Garrulax cinereifrons), Ceylon Rufous Babbler (Turdoides rufescens), Legge’s Flowerpecker (Dicaeum vincens), White-faced Starling (Sturnia albofrontatus) and Ceylon Crested Drongo (Dicrurus lophorhinus).
During the northern winter migrant birds also join the feeding flocks. Of these the only regular species is the Asian Paradise Flycatcher ((Terpsiphone paradisi paradisi). The sub-species seen in the wet is the migrant race paradisi and not the resident race ceylonenis. Over a period of three years the adult male of the migrant race turns white. Some second year birds have well developed tail streamers and show no hint of white leading to confusion with the resident race. Other migrant birds I have observed in the feeding flocks include the Chestnut-winged Cuckoo (Clamator coromandus).

The feeding flocks have a fascinating social structure with the key flock constituents behaving like a super organism with defined roles for its members. The flocks also appear to have a territory and a regular pattern of movement. At certain times, the guides in Sinharaja can almost predict at what time and where the ‘barrier flock’ will cross the road. Even though I have not had the luxury to engage in research, being a regular visitor to Sinharaja I have gathered a ‘feel’ of the home ranges of different flock and where their day time resting places are located. I have also learnt to hurry vack towards the entrance gate to catch a crossing of the ‘barrier flock’. Quantitative data on which credible conclusions may be drawn on the social dynamics of these flocks have been gathered over two decades by Professor Sarath Kotagama of the Field Ornithology Group of Sri Lanka (FOGLS). My interest in the feeding flocks were stimulated after listening to lectures by him, the late PB Karunaratne and others who have done field work with FOGSL.

Some species in the flocks play a pivotal role. The Ceylon Crested Drongo plays the role of sentinel responsible for security and warding off predators. It also summons the flock with a special call. The Ceylon Rufous Babbler, is another nucleus species. It keeps an acoustic signature on the flock’s perambulation through the forest. It also contributes the largest number of individuals to a feeding flock, at times as many as 50. Ashy-headed Laughingthrushes, Red-faced Malkohas and Malabar Trogons are other species which seem to belong to a particular flock. In fact, I have never seen Red-faced Malkohas other than in the presence of a flock. Equally, it is rare to encounter species such as the Ceylon Crested Drongo or Ceylon Rufous Babbler without another flock species being close to hand.

The mixed species feeding flocks traverse the forest like a giant vacuum cleaner, devouring edible plant and animal matter in its path. The presence of so many individuals together create competition for food. But the disturbance created by the flock also flushes insects which creates feeding opportunities. About half of the species observed in the feeding flocks are insectivores with the balance almost equally being frugivores and omnivores. Insectivores and omnivores would benefit from the flushing if insects as a result of a wave of disturbance.

Another benefit is enhanced security. With so many birds together there are more eyes looking out. Besides a flock can mob a predator with more confidence. Flocks members may also benefit from the dilution effect, of statistically reducing their chance of being the animal to be taken by a predator. On the whole, the benefits of foraging in a mixed species feeding flock seems to outweigh the costs and a stable structure seems to have evolved.
Territory holding flocks of Dark-fronted Babblers (Rhopocichla atriceps) join and leave the flock as its cuts through their territory. Black-naped Blue Monarch (Hypothymis azurea) is another species where different pairs will temporarily join a flock which is passing through. The species in a flock even have well defined patterns as to the order in which species will make a crossing, when they encounter a road or clearing. Crested Drongos and Ceylon Rufous Babblers are the first followed by Ashy-headed Laughingthrushes, Malabar Trogons (Harpactes fasciatus), Red-faced Malkohas, etc. The Lesser Yellownape (Picus chlorolophus) is often the last to cross.
Small mammals such as the Dusky-striped Squirrel (Funambulus sublineatus), Layard’s Squirrel (Funambulus layardi) and Giant Squirrel (Ratufa macroura) also join these feeding flocks. Quite often a feeding flock seems to have at least one or two species of squirrel with it. The squirrels like with the Dark-fronted Babblers have a smaller territory than the foraging home range of the feeding flock. Therefore different territory holders will join the flocks at different stages of the flocks’ passage.
Many species of birds will join flocks opportunistically. These include species such as the White-faced Starling, powerful fliers who can disperse widely in search of fruiting trees. Often, the only opportunity for birders to get a good view of them is when they are foraging with a mixed species feeding flock. Similarly, endemics such as the Yellow-fronted Barbet, Layard’s Parakeet and Ceylon Hanging-parrot will also join flocks, although they are not regular members who will traverse all day long with the flock.

In temperate countries, mixed species feeding flocks are observed more during the winter when food is scarce. In the tropics they are observed all year long. Given the relative richness of tropical forests, the scarcity of food may not be the determining factor for flock formation. It is probably the overall benefit of having access to food whilst enjoying the security of numbers. Even in home gardens in the busy capital of Colombo, I have noticed birds forming into mixed species feeding flocks. Purple-rumped Sunbird (Leptocoma zeylonica), Loten’s Sunbird (Cinnyris lotenius), Common Tailorbird (Orthotomus sutorius), Yellow-billed Babbler ((Turdoides affinis), White-bellied Drongo (Dicrurus caerulescens), Red-vented Bulbul ((Pycnonotus cafer) and Oriental Magpie-robin (Copsychus saularis) regularly form into mixed species feeding flocks in my garden. In much the same manner as the forest squirrels such as the Dusky-striped Squirrel and the Layard’s Squirrel, the Palm Squirrel (Funambulus palmarum) who have small territories join the flocks in their passage through home gardens. The flocking in towns and village may be a response to predation pressures from domestic cats, House Crows (Corvus splendens) whose numbers have multiplied to high densities and possibly the presence of accipiters such as the Shikra (Accipiter badius).
Bibliography
de Silva Wijeyeratne, G., Warakagoda, D. and de Zylva, Dr T.S.U. (2000) A Photographic Guide to the Birds of Sri Lanka. 144 pages. New Holland: London. ISBN 1–85974-511-3
de Silva Wijeyeratne, G. (2006). Gehan’s Photo Booklet: Birds of Sri Lanka and Southern India. 42 plates. Eco Holidays: Colombo. ISBN 955-1079-10-8
Kotagama, S. and Goodale, E. (2006). Bird Flocks of Sinharaja. Field Ornithology Group of Sri Lanka: Colombo. 24 pages. ISBN 955-8576-21-2.
Kotagama, S. and Goodale, E. (2004). The composition and spatial organisation of mixed-species flocks in a Sri Lankan rainforest. Forktail 20, Pages 73-70. Oriental Bird Club: UK.
Gehan de Silva Wijeyeratne (gehan@jetwing.lk) is the CEO of Jetwing Eco Holidays. He is one of the best known wildlife popularisers in Sri Lanka. Free downloads of his publications on birds, butterflies and dragonflies are available on stag2.mydemoview.com/jetwingeco.