d Ecologic Significance

The chief factors that determine the economic bearing of crows locally
are: the yearly diet, the time of year in which each food item is taken,
and fluctuation in the population density at different times of year. In
the study here reported upon, the yearly diet was computed by averaging
the percentages of each item determined for each biweekly period. Of the
twenty-one collecting periods shown in the tables, six are overlapping
pairs; that is to say, each includes one collection from eastern Harvey
County and one from the western part of the study area. The average of
these pairs was used in computing the yearly average. The yearly average
is therefore based upon eighteen separate samples.

The percentages are weighted toward the food items taken in summer and
autumn, since many biweekly periods in late winter and early spring are
not represented. Of the collecting periods represented, two were in
spring, six were in summer, seven were in autumn, and three were in
winter. Pellets collected at a number of different localities are
averaged together as a percentage; consequently the figures obtainable
do not necessarily represent the diet of any one group of crows.
Nevertheless the percentages obtained by this method are perhaps valid
as a general indication of the diet of the crows in this area.

In my samples, plant material amounted to 69 per cent of the
indigestible residues. Similar percentages have been found in other
studies, ranging from 57 per cent (Barrows and Schwarz, 1895:72) to
71.86 per cent (Kalmbach, 1918:43). The percentage of plant material was
highest in the winter. In one collection from a wintering crow roost it
amounted to 99.5 per cent. In December in eastern Harvey County it
averaged only 85.3 per cent. The lowest percentage (20) was found in the
first half of October in eastern Harvey County when grasshoppers
amounted to more than half the diet. At this same time pellets collected
from the wintering roosts contained 72.4 per cent plant material.

Percentages of the chief items in the total food residues,
and (in parentheses) number of sampling periods in which
each item was represented, are shown in the following list:
wheat 23.2 per cent (20), sorghum 15.2 (16), oat 7.8 (8),
sunflower 7.2 (8), corn 5.4 (12), brome 4.2 (5), other grass
2.4 (7), cherry 1.2 (2), beetle 13.3 (21), grasshopper 9.3
(19), ant .7 (3), miscellaneous insect .2