Pellets from roosts of resident crows were collected on a year round basis in eastern Harvey County near Newton (see tables 1 and 2). The data from these pellets were interpreted separately from data on collections made in the western part of the study area from under roosts of wintering crows (see tables 3 and 4).
In studies of the food of owls analysis of materials in regurgitated pellets has been widely used, but with crows this method has been little used because the nature of their food makes identification of material more difficult. Analysis of pellets has certain merits, however, and, if closely correlated with field studies, can give valuable information concerning food habits. The availability of pellets and the ease of collecting them are obvious advantages. Under large roosts in winter the number that can be collected is almost unlimited. At other seasons, pellets are scarcer, but even so they usually are more available than stomachs.
The technique of pellet analysis is more easily applied to a study of the yearly diet than is the technique of stomach analysis. The crow is euryphagous and, as shown by this study, the diets of crows a few miles apart may differ. Therefore a study made on a limited area within one biotic community, on a year round basis, and correlated with changes in the habitat should be of greatest value. For such a study, collection of stomachs is not practical unless individuals are abundant so that many can be sacrificed, but collection of pellets is practical and profitable.
One limitation of data based on material from pellets is the impossibility of closely correlating the volume of indigestible residues with the proportion of food items actually eaten. Such correlation is prevented not only by the different percentages of indigestible residues in different food items but also by irregularities in regurgitation and in the efficiency of the crow's digestive system. Barrows and Schwarz (1895:24-25) cite several instances of such irregularities in captive crows. In certain pellets that I studied, part of the wheat or other grain was undigested or partly digested, whereas in other pellets the only residue was finely divided chaff. Certain foods that lack hard parts may leave no recognizable residues in pellets. A captive crow that I raised did not form pellets when fed soft food. Nevertheless, data from analysis of pellets when supplemented by field observations, should serve as a sound basis for valid conclusions concerning the relative proportions of various foods eaten. The following field observations of habitat factors aid in interpreting the information obtained from pellet analysis.
Field Observations and Correlations
Resident Crows in Eastern Harvey County.—Although no field observations were made on feeding behavior in April, the large percentage of oat hulls found in the pellets suggests that newly sown fields of oats must have been one of the major feeding grounds in that month. Oats were planted between February 15 and March 20.
The pellets collected in June were all from the roost of one family group of crows. This group spent much time in a cherry orchard and in the shelterbelt near it. Residues of cherry and wheat constituted the only plant foods found in the pellets. In both frequency and percentage, scarabaeid beetles constituted the other important food source. The wheat harvest started on June 17.
The cherry harvest was over by June 29. Grain harvest was over and the fields were being plowed by July 2. Alfalfa was being cut for hay in early July and crows were then feeding on plowed fields and the newly mown alfalfa fields. Much time in the middle of the day was spent along the creeks where crayfish could be obtained. During most of the summer, pellets were difficult to find because the roosts were small, shifting, and scattered and because few pellets were produced. For weeks at a time there were no usable pellets under roosts occupied by hundreds of birds, although droppings and feathers were present. At other times large collections of pellets could be gathered from small roosts. Plowing was a major farm operation at the season when pellets were most scarce. Larvae of insects (especially beetles), and earthworms might have provided a major source of food which lacked sufficient indigestible material to form pellets. A few feces were collected and analyzed in an attempt to find the residue of such soft-bodied foods. Indigestible materials were found in the feces, but these were of the same types as those found in the pellets. Only a few fragments were found which might have been the mouthparts of grubs.
After mid-July pellets were common under one small roost. In late July they were scarce, even at a roost with several hundred crows. The principal feeding grounds of crows were stubble fields and plowed fields. All grain picked up at this time was waste. Plowing was interrupted by rain from July 11-18 but was the major farm operation again after July 19.
From late July into early September crows fed in plowed fields, stubble fields, pastures, and newly mown hay fields. Pellets were scarce, considering that hundreds of crows used the roost where pellets were collected. Plowing was almost over by July 31. Brome grass was in full head during the early part of this period. Corn was in the milk stage during the early part of August but did not show up in any pellets. Although Sudan grass was in head during the early part of this period, other sorghum did not head out until September.