COMPOSITION, STRUCTURE AND ORGANIZATION OF MIXED-SPECIES BIRD FLOCKS IN WALAUWA TTA-WA TURANA SWAMP FOREST, KALUTARA DISTRICT, WESTERN PROVINCE OF SRI LANKA

Abstract

Mixed- species foraging bird flocks in Walauwatta-Waturan Swamp Forest (WWSF) were studiedfor a period of six months from February to July in 2001. Field data was collected from 07:00 to18:00 hrs and flocks were followed as long as possible to record their composition. Scan samplingmethod (Altman, 1975) was used to record their behavioural activity, height of feeding, feedingsite and the role of each individual within the flock. Two minute scan samples were done with fiveminute intervals and almost all the birds participating in the flock were scanned within the twominutes time period.

The flocking tendency of birds in WWSF was 28.7% and 27 flocks were studied. A total of 31 birdspecies were recorded to participate in mixed-species flocks in WWSF. These include 28 residentand 3 migrate species. Five species were endemic to Sri Lanka. The flock composition varied from3 to 15 species (5.89±2.61) and 4 to 31 individuals (I2.96±7.41). There was a positive correlationbetween the number of species and ~ock size (r= 0.796).

Crested Drongo (77.78%) and Black-naped Monarch (70.37%) were the most frequent birds foundin flocks while Sri Lanka Orange-billed Babbler was the most abundant (8.78±3.67 individuals perflock). Territorial species such as Purple-romped Sunbird joined the flock when it was passingthrough their territory. Birds in mixed-species flocks in WWSF were classified as nuclear species,lead species, regular species, and occasional species. 2 major nuclear species (Crested Drongo &Sri Lanka Orange-billed Babbler), 4 lead species (Above two with Malabar Trogon and AsianParadise Flycatcher), 12 regular species, and IS occasional species were identified. Associations ofbird species participating in mixed species flocks in WWSF were studied using cluster analysis.Two main clusters could be identified, one of which could be again divided into two subassociations.The first cluster comprised of Crested Drongo and Black-naped Monarch who showedthe highest level of association. Black Bulbul and Sri Lanka Orange-billed Babbler could not bepooled into any clusters.

Different species in mixed species flocks had different optimum height levels of movementthrough the forest and appeared to have characteristic feeding sites. The vertical distribution of theregular species in flocks was consistent. Sri Lanka Orange-billed Babbler was found at a widerange of foraging heights. But, Scarlet Minivet and Black Bulbul were observed mostly in thecanopy and the Crested Drongo was observed in the middle strata of the forest (10 to 20 ft). TheAsian Paradise Flycatcher was observed in the same range. However, other fly catching species(Black-naped Monarch and Tickell's Blue flycatcher) were observed mostly in the 10 to 15ftvertical range. There was an extensive overlap of height ranges between some species. But theywere observed to use different activity/feeding sites. Black-naped Monarch and Velvet-frontedBlue Nuthatch were found in the same height range, but feeding sites utilized by them wasdifferent (foliage and tree trunk respectively). Crested Drongo and Asian Paradise Flycatcher wereobserved to occupy the same vertical height range and feeding sites, but they were using differentfeeding techniques. Even being a small area of 12 ha (CEA, 1994) IIWTOUIIdedby Rubber plantations and homegardens, the Swamp Forest have a considerable flocking tendency with compared to Sinharaja Forest Reserve 30%)(Kotagama et. AI., 1986) and Knuckles Conservation Forest (30.S) (Sbriyani, 2000), thus worth conserve thehabitat