An Experimental Investigation of Filter-Feeding on Zooplankton by Some Specialized Waterfowl.

Abstract

A study was made of the filtering ability and anatomy of the mouthparts of 4 species of waterfowl: pink-eared duck, Australasian shoveler, freckled duck and grey teal. The first 3 are highly specialized for filter-feeding on zooplankton whereas the last is a more generalized anatid. The pink-eared duck and Australasian shoveler have the filtering lamellae on the bills variously elaborated, whereas the grey teal is similar to published descriptions of the mallard. The freckled duck has bill features more characteristic of flamingoes. On the basis of anatomy it was predicted that pink-eared ducks should be able to filter the smallest particles, followed by shovelers, freckled ducks and grey teal. In feeding experiments the proportions of plankton of various sizes removed by the species was compared. For the 3 specialists the distributions of proportion removed compared with plankton size was, in general, best fitted by a threshold model as follows: y = 1.0[x > P]; y = aebx[60 mu m < x < P], where P is a threshold size above which all plankton can be filtered, y is the proportion filtered out and x is the size class of the plankton (in mu m). Grey teal were inefficient feeders on plankton and gave results only poorly fitted by a simple linear model. The 3 specialists had significantly different threshold sizes, and the results indicated that pink-eared ducks were the most able filterers, followed by freckled ducks, then Australasian shovelers. The failure of a simple mechanistic analysis of anatomy to predict filtering ability is discussed. On the basis of the abundance of these specialized filter-feeders possibly being related to plankton abundance, counts of pink-eared ducks and Australasian shovelers were regressed against plankton counts for a semi-permanent lake in western New South Wales. No simple relationship was found, and the significance of planktivory in these species is discussed.