Group size affects spontaneous quantity discrimination performance in wild Western Australian magpies (Gymnorhina tibicen dorsalis)

Animals may benefit from the ability to discriminate between quantities in their environment; for example, when choosing between foraging patches differing in food availability or assessing the size of rival groups. Numerous studies utilising spontaneous quantity discrimination tasks have found that a wide range of species possess the ability to discriminate between quantities, with large interspecific differences being found in these capabilities. However, the causes of variation in quantity discrimination have received less attention, particularly when considering intraspecific variation. Here, we use a spontaneous quantity discrimination task to (i) investigate if Western Australian magpies possess quantity discrimination abilities, and (ii) determine the factors that underlie individual variation in this ability. We found that magpies were able to discriminate between two discrete quantities of a food reward and chose the larger quantity of food more often than expected by chance, with their accuracy increasing as the difference between the two quantities of food items increased (i.e. as the ratio decreased). Individual performance on the assay was significantly affected by group size, with individuals from smaller groups choosing the larger quantity of food more often than individuals from larger groups when presented with the 2 vs. 5 combination. This group size difference may arise because individuals from smaller groups benefit more from enhanced quantity discrimination abilities compared to individuals from larger groups due to the greater risk of competition and loss of resources from intergroup conflict with larger groups. Our study is the first to investigate and identify group size as a source of intraspecific variation in spontaneous quantity discrimination abilities and highlights the importance of considering the causes of individual variation in cognitive performance.