Ardea
Official journal of the Netherlands Ornithologists' Union

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Bartels E. (1962) Wat doen Boomeenden (Dendrocygna) in en voor zoet- en brakwaterplassen op Java? ARDEA 50 (3-4): 207-211
The ecological significance of the Whistling Ducks, Dendrocygna javanica and D. arcuata, in fresh and brackish water lakes in Western and Central Java is discussed. The author reports to have observed in Western Java Whistling Ducks feeding upside down along the borders of shallow fresh and brackish inland waters. As long as these ducks stayed there in sufficient numbers the patches of open water showed little if any change in extent. The author states a game warden's report about a prolific fishing water in Central Java which was surrounded by a vast swamp and where in 1933 the Whistling Duck population was estimated at about 75,000 birds. By that time local inhabitants started large scale collecting of duck's eggs, continuing this locally new habit for several subsequent years. As the ducks did not seem to nest outside these swamps, their numbers gradually diminished until in 1940 they had declined to no more than 5,000 birds. What was worse: the reeds had invaded the lake and the open water was about all gone; so were the fish. The author is inclined to correlate this striking change of the vegetation with the feeding habits of the ducks. He gives reasons for considering that these ducks, when in large numbers, destroy all seeds of reeds and other water plants from the bottom of the lake to a depth of at least 20 to 22 inch, in a zone mainly along the edge of the open water. Simultaneously the birds probably destroy the tender subaquatic shoots with which these plants multiply themselves vegetatively. The theory is substantiated by the fact that during the time that these ducks were numerous there was usually a gully of clear water, some 2 feet wide and 1 1/2 feet deep all along the front of the outer reeds. The whole ecological situation of course must be more complicated. But in view of the economic importance to keep fish-waters open, it seems appropriate to draw the attention to the above described situation. The author is convinced that it seems pretty well established that, when undisturbed, these ducks kept the reeds in check, somehow.


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