COMPARATIVE BEHAVIOURAL ECOLOGY OF THE MYNAS, ACRIDOTHERES TRISTIS (LINNAEUS) AND A. JAVANICUS (CABANIS) IN SINGAPORE

Abstract

The mynas, Acridotheres tristis (Linnaeus) and A. javanicus ( Cabanis), have become two of the most common species of birds in Singapore because deforestation and urbanisation provide numerous suitable habitats for them. However, A. javanicus (the White-vented Myna) is now more numerous than A. tristis (the Common Myna) by two to three times, and in most habitats in Singapore. The aims of this project were twofold. Firstly to gather basic information on general aspects of the behaviour of mynas that might indicate niche separation between these two ecologically similar species. The second aim was to gather data relevant to the planning of an integrated management program for these birds which are considered pest species by some sections of the public and government bodies. The ecological relationships between A. tristis and A. javanicus were investigated by comparing differences in their behaviour and utilisation of resources, such as type of habitat, food sources and nest holes available on mainland Singapore. Possible niche differentiation and A. tristis and A. javanicus was identified in two areas : firstly choice of habitats and secondly use of available food sources. In its selection of habitats, A. javanicus is more of a generalist as it shows a relatively even pattern of dispersion across different types of habitat. A. tristis is more of a specialist as its numbers are chiefly concentrated in areas whose surroundings are less urbanised. The foraging strategies adopted by A. tristis and A. javanicus are also different. A. javanicus is more successful and opportunistic than A. tristis in supplementing its regular sources of food by exploiting ephemeral concentrations of food. A. tristis does not use ephemeral food patches as frequently as A. javanicus, but relies predominantly on regular or predictable supplies of food at familiar locations on its daily range. Both species display differences in other aspects of their behaviour such as sizes of total range and home range, roosting and flocking behaviour and in their use of sites for different categories of behaviour which may be adaptive to each species' foraging strategy and mode of life. It is suggested that the different foraging strategies adopted by A. tristis and A. javanicus in Singapore have evolved as a result of interspecific competition. By exploiting a wider range of available food sources to include ephemeral concentrations of food, A. javanicus avoids or reduces interference from the larger and more aggressive A. tristis for other common sources of food. A second possible explanation is that in its indigenous range in Java, one of the most densely populated countries in the world, A. iavanicus has adapted by exploiting ephemeral concentrations of food created by human activity (e.g. food scraps, clumps of fruiting trees). This behaviour may be advantageous in urban Singapore where similar ephemeral sources of food are also present, e.g. food scraps, flowering and fruiting ornamental trees. And even though A. tristis is more aggressive in one-to-one disputes with A. javanicus, the energetic and physical costs of aggression probably limit its overt antagonism to those situations where the benefits gained from aggression at least balance or exceed the costs of the encounter; for example, rich and regular supplies of food or where A. javanicus does not grossly outnumber A. tristis. Human activity such as the deliberate planting of flowering and fruit trees has probably favoured the foraging strategy and success of A. javanicus by providing additional sources of ephemeral foods. And by being more of a generalist in its choice of habitats, A· javanicus may be less affected than A. tristis by certain alterations to the habitat, for instance, a reduction in the proportion of rural areas. Thus effective exploitation of available sources of food and habitats, enhanced by human activity, have probably contributed to A. javanicus becoming the predominant species of myna in most habitats in Singapore. No supportive evidence was obtained that either species of myna has, or could have, affected the diversity and population size of the other avifauna in Singapore. They are considered pests primarily because of their habit of roosting communally at night in residential areas, sometimes in large numbers. This study indicated that a program of removal of resources (e.g. food, nest holes, roosting sites) would be impractical in the management of mynas because of their ability to exploit a diversity of resources. It is suggested that current methods of roost dispersal should be integrated with simultaneous efforts to provide alternative roosting sites for the mynas in areas where they would not create problems. The public should also be educated to understand that the complete eradication of mynas would be ecologically undesirable, and to tolerate reasonable levels of inconvenience from a problem that is primarily caused by man-made changes to the natural vegetation of Singapore.