EPOW - Ecology Picture of the Week

Explanation:   One law of ecology states that each species defines its own unique niche.  But here, along the banks of the Kosi River in Corbett National Park in northern India, coexist, side by side, two species of water-redstarts with nearly identical behaviors and habitat selection habits.  How can this be?

Water-redstarts are small members of the large family Muscicapidae which includes chats and Old World flycatchers.  Shown in these photos is a male White-capped Water-redstart (above) and a female Plumbeous Water-redstart (right).  The male Plumbeous looks similar to the White-capped but, guess what, it lacks the white cap.

In India, both of these water-redstart species breed along the same rocky rushing streams and streamside forests in the lower to middle Himalayas during summer.  Both then move to lower elevations and south, to winter in peninsular India and Bangladesh and adjacent areas.  Both species are also more widely distributed throughout south and southeast Asia.

White-capped Water-redstarts feed on aquatic and flying insects and sometimes berries.  They build large deep cup nests of moss, leaves, and grass, lined with wood and hair, in holes or crevices and sometimes in houses and under roofs.  Plumbeous Water-redstarts also feed on aquatic insects and sometimes worms.  Their nests have not been described.

Along the same stretch of the Kosi River in India, I observed both bird species occupying their respective nests in shrubs and root tangles hanging low over the river bank.  

Their territorial signaling behavior is very interesting.  Along the streams and on perches, both species flick their tails and make short jerky movements.  White-capped Water-redstarts bob their bodies upon alighting on stream banks and rocks, and extend and wag their tail jerkily up and down sometimes with wings drooping at the sides.  Plumbeous Water-redstarts tend to wag their tail up and down with it fanned open, as shown here in the photo to the right.  

A number of other bird species associated with rushing-water habitats have the behavior of tail-flicking and body-bobbing, such as dippers, wagtails, and some sandpipers, sparrows, and New World flycatchers.  This behavior seems to have evolved separately in many different taxonomic lines because it may be an efficient way to visually display territoriality in a noisy environment where vocalizations may be masked by stream noise. 

For these two water-redstart species, their subtle differences in tail-wagging territorial behavior may help keep them separated in what otherwise seems to us like the same niche.

Information:
     Ali, S. 1987. Indian hill birds. Oxford University Press, Bombay. 188 pp.
     Ali, S., and S. D. Ripley. 1988. Concise handbook of the birds of India, Pakistan, Burma, and Sri Lanka. Oxford University Press, Bombay. 168 pp.
     Ali, S. 1989. Field guide to the birds of the eastern Himalayas. Oxford University Press, Bombay. 265 pp.
     Buckton, S. T., and S. J. Ormerod. 2002. Global patterns of diversity among the specialist birds of riverine landscapes. Freshwater Biology 47:695-709.