. Bulletin. Natural history; Natural history. 428 Illinois Natural History Survey Bulletin Vol. 22, Art. 5 other areas substantiates the Crane Lake observations. Seed production of buttonbush is not known to vary greatly from year to year; therefore, we must conckule that ducks do not relish the seeds and that these seeds are suffi- ciently palatable to be taken extensive- ly only when other foods are lacking. Spike rushes, /sA-or/wm spp., include mainly the Eleocharis palustris group. These short, round-stemmed species, growing on moist soil or in shallow water may be excellent duck foods in
. Bulletin. Natural history; Natural history. 428 Illinois Natural History Survey Bulletin Vol. 22, Art. 5 other areas substantiates the Crane Lake observations. Seed production of buttonbush is not known to vary greatly from year to year; therefore, we must conckule that ducks do not relish the seeds and that these seeds are suffi- ciently palatable to be taken extensive- ly only when other foods are lacking. Spike rushes, /sA-or/wm spp., include mainly the Eleocharis palustris group. These short, round-stemmed species, growing on moist soil or in shallow water may be excellent duck foods in some vears and poor foods in others, as illus- trated by the index values in tables 4, 5 and 6. Tables 5 and 6 show that, in 1939 and 1940, spike rushes formed about the same per cent of vegetation at the lakes studied. Yet in the first year the seed of these species consumed by ducks was three times as great as in the. Fig. 12.—Water hemp (Acnida luberciilata) better known as pigweed, is generally a large, coarse moist-soil plant. It is of lower value as a source of duck food than are most other species growing on the Illinois River mud flats. second year. We are at a loss to ac- count for this variance. The 3-year in- dex rating for these species places their duck food value slightly below that of the longleaf pondweed, table 3. Water hemp, Acnida lubcrculata, fig. 12, a large, coarse herb growing on drier soil sites than other moist-soil plants, has small seeds, pin-headed in shape, that may not always be as available as seeds of the other species of this group. Its index value for 1938-40 is , table 3. The data include seeds pro- duced prior to 1938, a year in which only a very few plants of this species grew in the areas studied. The 1940 rating of is probably the best indicator of its true value, table 6. Such moist-soil plants as smartweeds, millets and rice cut-grass are apparently about 10 to 40 times as good sources of duck food as is water hemp. Marsh cord
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