Heinrich Frötscher



Mixed-species bird flocks across an elevational gradient in West Java (Indonesia)

A mixed-species bird flock or bird wave is defined as an aggregation of birds of different species foraging together. Contrary to feeding aggregations of birds on spots of high food availability e.g. in fruiting trees or on mud flats, these birds join to move through the forest while foraging. Studies of mixed-species flocks in Southeast Asia were rare until recent years and the abundance and structure of mixed-species flocks along an elevation gradient was never comprehensively studied in Southeast Asia. In this proposed study the following questions will be addressed: (1) The decrease of bird diversity towards higher altitudes most likely also causes a declining richness of birds joining mixed species flocks. (2) It can be assumed that the shift in structure and composition of mixed species flocks across the elevation gradient is related to the
altitudinally restricted vertical distribution of core species. (3) Declining species richness at higher altitudes may result in a less developed vertical stratification and a decreased resource partitioning of forest birds due to a competitive release. (4) Harsher climate at higher altitudes may have a negative effect on foraging efficiency of insectivorous birds due to declining insect abundance and a temporarily limited access to prey. (5) Changing vegetation structure across the elevational gradient may affect species richness of individual mixed species flocks more significantly than bird species richness in general.
The survey will be conducted from August to October 2009 in close collaboration with Prof. Dr. Damayanti Buchori (Department of Plant Protection, Institut Pertanian Bogor) at elevations between about 700 and 3,000 m in Mount Gede–Pangrango National Park and Mount Halimun–Salak National Park in West Java.