Preferential Rating of 
Duck Food Plants 
PARAUNUIS (Cl, Te BAA BART Oey Begs bse 
URING the past decade, the 
[exe of research on the food 
plants used by migratory water- 
fowl has increased notably. The basis 
for much of this research has been data 
obtained by the U. S. Biological Survey 
(succeeded by the U. S. Fish and Wild- 
life Service) through analyses of the con- 
tents of several thousand duck stom- 
achs, collection of which was begun in 
1901. These data have recently been 
summarized by Martin & Uhler (1939), 
who based their study on 7,998 stom- 
achs, and by Cottam (1939). 
Analyses of duck stomachs by the 
Biological Survey, other organizations 
and individuals has prompted wildlife 
investigators to attempt to ascertain the 
important duck food plants and their 
relative values in many regions of the 
United States. In all these studies, each 
plant species has been judged solely on 
its use, as determined by laboratory 
analyses of stomachs. While this method 
ascertains the important duck food 
plants, it does not reflect the relative 
values of these plants, for no consider- 
ation is given to the abundance of the 
plant species in the areas in which the 
stomachs have been collected. If in a 
certain area a plant species covering 
only 10 acres shows the same volume of 
seed use by ducks as another plant spe- 
cies covering 100 acres, the two plants 
should not be considered of equal value 
as duck food sources; indications are 
that the plant species occupying 10 
acres 1s potentially 10 times as valuable 
as the species occupying 100 acres. 
The shortcoming in the method of de- 
termining the value of various plants as 
food for waterfowl has been evident to 
several investigators. For instance, 
Martin & Uhler (1939) in commenting 
ERACRERS Yor Go AINE) ER SON 
on the percentage and ranking of duck 
food items in several tables state that 
“it will be desirable to have more ex- 
tensive, careful field observations to 
supplement present conclusions, which 
are founded primarily on laboratory 
analyses.” Concerning the heavy use 
made by ducks of pondweed and sedge 
seeds, Pirnie (1935) says that this may 
reflect abundance of the seeds rather 
than preference of ducks for these seeds. 
McAtee (1918) states that “superior 
availability after all is the guiding prin- 
ciple in the choice of food by birds.” 
The writers,* 1938-1940, had the 
opportunity to make an intensive study 
of the occurrence and use of duck food 
plants in the Illinois River valley. A 
preliminary report covering the results 
of the 1938 investigation has been issued 
(Bellrose & Anderson 1940). Because 
altered water levels in 1939 and 1940 
greatly changed the food resources of 
many bottomland lakes through the de- 
velopment of extensive communities of 
moist-soil plants, it is thought desirable 
to summarize at this time the data for 
allo Vears: 
Method of Rating Food Plants 
In the present paper, the authors have 
attempted to obtain a numerical rating 
of the value of certain waterfowl plants 
by dividing the per cent of use made by 
ducks of the plant parts by the per cent 
of abundance of the plants. Per cent of 
use data are based on the volumetric 
*FRield work relating to plant species was done by 
Bellrose, then as now Assistant Game Technician of the 
Illinois Natural History Survey. Analyses of stomach 
contents was the work of Anderson, at the time employed 
by the Survey as Junior Biologist on Project 2-R of the 
Federal Aid in Wildlife Restoration Program, carried 
on in cooperation with the State Department of Conser- 
vation and the U. S. Fish and Wildlife Service. 
[ 417 ] 
