Babblers in Borneo
It took a trip to Borneo for me to learn about cooperative breeding in birds, a system of reproduction used by only about 3% (300 species) of bird species worldwide.
As guests of close friends from the San Francisco Bay Area, my husband and I were most fortunate to visit Sabah, Malaysia in northeastern Borneo in April to see some of the most biologically diverse habitat in the world. From the Kinabatangan River floodplain with its abundant wildlife, to the spectacular Dipterocarp forests of Danum Valley, the tallest of all tropical rainforests, we focused on birding. Nature itself, and very competent guides, provided thrilling opportunities to see many other orders of wildlife as well, including many endemic to Borneo.
You may recall in my last post in March I discussed the lekking behavior of Orange-collared Manakins, a regional specialty here in southwestern Costa Rica where I live. Males gather, select a forest area that females are known to visit, and prepare an arena on the forest floor, where they perform impressive mating rituals. The female selects the most attractive or able performer and off they go to breed. It may be a one time fling.
In the case of Stachyris erythroptera bicolor, the Chestnut-winged Babbler, a flock of which we encountered in the forests near the Sukau Rainforest Lodge, we were able to observe activity that was completely unfamiliar to us: up to six birds were engaged in what appeared to be a complex activity. Impossible to know for sure which was male, and which female, as I later learned the sexes look very similar. Birds seemed to display and then hide blue skin patches around the throat. In each of three pairs, one individual–probably the male– shook its body vigorously and suddenly darted away, closely followed by the observer to another nearby branch. Activity was going on concurrently by the different pairs within a two square meter area, and all under observation from above by a small gallery of an adult and two juveniles. Those birds were very vocal, as were the performers.
Fascinated by the degree to which the birds were so intensely engaged in their activity, I photographed from close by without seeming to perturb the birds. When one performer suddenly flew away, all the other birds dispersed as well.
Back at home I learned that breeding by these Babblers occurs in Borneo between March and July. Group activity of 4 to 26 birds is common for insect foraging in middle and understorey plants in rain forests up to 1200 M. Nests are built by both sexes, but there have been observations of up to four birds building a single nest. Some species of well-studied other Babblers practice cooperative breeding, where offspring help parents raise more offspring without breeding on their own. The non-parental adults helping the breeding pair are called “extra-pair helpers”, “auxiliaries” or “supernumeraries”. The Handbook of the Birds of the World could only say that Stachyris erythroptera bicolor is “probably multi brooded” and are “partly social breeders”. Clearly more research is needed. In some systems of cooperative breeding Stanford researchers found that dominant females succeed in monopolizing reproduction in a group. Female offspring stay near to help as subordinates, but their reproduction is rare. All new to me.
It would be very interesting to know exactly what was going on during those charming early afternoon performances, but in any case it was a magical experience for us all.
Note: The Chestnut-winged Babbler’s range is from extreme southern Burma and Thailand to peninsular Malaysia, Sumatra and Borneo.
del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.) (2016). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from http://www.hbw.com/ on 16 May 2016).
Cooperative Breeding by Paul Ehrlich, et al, 1988, Stanford University
The Sibley Guide to Bird Life & Behavior, pp. 566-567.