Sinharaja is home to one of the world’s finest examples of a “mixed-species feeding flock”, as it’s technically known, or bird wave, as it’s popularly described: a memorably colourful and noisy rainforest spectacle during which myriad different species can be seen flying and foraging together, “scouring the forest from top to bottom like a giant vacuum cleaner, devouring animals and plant matter in their path” (as Sri Lankan wildlife expert Gehan de Silva Wijeyeratne puts it). From an evolutionary point of view, such collective feeding has clear practical advantages. Safety-in-numbers is one benefit, with members of the bird wave sometimes flocking together to beat off larger predators, while increased feeding efficiency is another.
Different species fly at different levels (ground, rainforest under-storey, canopy, and so on) with mutual benefits – a species feeding in the under-storey, for instance, may disturb insects which fly up and become easy prey for birds in the canopy above, while fruit and seeds dislodged by birds in the canopy may fall to species foraging on the ground.
Bird waves in Sinharaja may consist of over a hundred birds from dozens of different species. The crested drongo is the accepted leader of the pack, calling to other species to begin flocking and also taking responsibility for collective security, sounding an alarm call when danger threatens – at which point the whole flock will suddenly, silently freeze until the drongos give the all-clear. Other species also commonly join in with the wave as it passes through their territory, sometimes offering the remarkable sight of half-a-dozen rare endemics flocking together, while other animals including giant squirrels and mouse deer are also often seen following along.