Seasonal changes in the diet, food availability and food preference of the greater bilby (Macrotis lagotis) in south-western Queensland

Abstract

Diet and food availability of the greater bilby (Macrotis lagotis), was examined at two sites and over two summer and two winter seasons in Astrebla Downs National Park in south-western Queensland. The presence of both invertebrate and plant material in almost all faecal pellets (99.6% and 98.5%, respectively) indicated that bilbies in south-western Queensland are omnivorous. The plant component of the bilby diet consisted predominantly of Dactyloctenium radulans seeds (frequency of occurrence: 94.4%). The major invertebrate prey items were Isop-tera (termites, 73.1%) and Formicidae (ants, 77.6%). The frequency of occurrence of invertebrates in faecal pellets was higher during summer than winter, but the frequency of occurrence of plants was relatively constant regardless of season. Within the invertebrate categories, Isoptera and Acrididae (grasshoppers) displayed the greatest seasonal difference, with a significantly higher proportion of both categories in the diet during summer than winter at both study sites. The availability of invertebrate prey items also varied with season, and for most taxa, frequency of oc-currence was significantly higher in summer than winter. In contrast, the frequency of occurrence of seeds and bulbs (Cyperus bulbosus) varied only slightly with season. Seasonal changes in food availability were reflected by variation in dietary composition; however, relative proportions of some dietary items differed from their proportional availability. Therefore, bilbies were considered to be qualitatively opportunistic; selecting more invertebrates when their apparent availability increased, but continually exploiting the available seeds and bulbs. Additionally, although bilbies appeared to have some preferences for particular food items, several food items were selected almost randomly, and therefore bilbies were also considered to be dietary generalists. Such an opportunistic feeding strategy is advantageous to an arid-zone species as it permits the continuous exploitation of food resources that are unpredictable in their availability both spatially and temporally.