A White-necked Stork carrying nest material to a large tree.

A few days ago we went for a two or three hour hike in the Periyar Tiger Reserve and saw a multitude of avian species that make Kerala a great place for both the amateur and ardent birder. I was also able to see very tangible examples of two related concepts that I’d learnt in my ecology and ornithology classes at Cornell: mixed-species foraging flocks and the ecological niche.

The American ecologist Robert MacArthur, in his seminal dissertation on five insectivorous species of warbler, noticed that Yellow-rumped, Bay-breasted, Blackburnian, Cape May, and Black-throated Green Warblers all mostly occupied different portions of the same tree (most often spruce, pine, or fir). Not only did they largely keep to their own branch and needle types, but they also tended toward different prey and had their preferred method of foraging, whether it was sallying out to catch flying insects like the Cape May Warbler or hovering more often like the Black-throated Green. MacArthur also noted that the species’ breeding cycles are slightly out of sync, making demand for resources slightly less clustered around peak times in the spring. MacArthur’s work showed how the five species of wood warbler weren’t competing against each other toward extinction (it is generally thought that if two or more species are pursuing the same resources then eventually — on the evolutionary scale — one will perform better enough to push the other one out of existence).

Left to right: Cape May, Yellow-rumped, Black-throated Green, Blackburnian, and Bay-breasted Warblers. Black areas in stylized conifers show where feeding is concentrated. Image and caption © Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye; 1988.

In the Western Ghats of India where the Periyar Tiger Reserve is situated, I saw this type of niche foraging in action through the tropical mixed-species foraging flocks made up of a Jungle Babbler or two on the ground foraging for insects in leaf litter, a Velvet-fronted Nuthatch or Common Flameback checking tree trunks for wood-dwelling insects (at different depths into the bark), a pair of Greater Racket-tailed Drongos vocally joining the babbler’s noisy calls from higher tree branches and going for flying insects scared up by other birds, and the occasional Treepie (White-bellied or Rufous) and Barbet (in our case the Malabar Barbet) up in the canopy as well. In other cases we also saw some Rosy and Scarlet Minivets, plenty of Common Hill Mynas, Red-whiskered Bulbuls, and an Asian Fairy-bluebird join the crowd.

The most exciting sightings of the day were a quite colorful Malabar Trogon and slightly frightening pair of Great Hornbills, with their easily audible and leathery wingbeats and seemingly prehistoric calls, casques, and silhouette. Some other species we saw were the Brown-cheeked Fulvetta, Malabar Grey Hornbill, Malabar Parakeet, Grey Jungle Fowl, Grey Tit, Bronzed Drongo, Red-wattled Lapwing, White-necked Stork, and Grey Wagtail.