There has been a recent upsurge of interest in the evolutionary ecology of cognition, in particular the causes and consequences of individual variation in cognitive performance. However, recent debate has highlighted the potential challenges and pitfalls associated with this endeavour. We focus on one of these issues: the importance of measuring repeatability when attempting to quantify individual differences in cognitive traits, and the interplay of individual variation with developmental processes. The repeatability of results must be considered when quantifying cognitive abilities in order to ensure that among-individual variation results from cognitive factors rather than non-cognitive variables, such as motivation or persistence. We presented wild Australian magpies from 16 different groups with tasks designed to measure inhibitory control and spatial memory. Tasks were presented to individuals at 100 and 200 days post-fledging. In order to determine the effect of age, we compared results between 100 and 200 days, and found no significant difference suggesting no clear developmental changes in cognitive ability over this period. Subsequent testing at older ages will examine this relationship further. Secondly, we compared cognitive performance within tasks and between tasks (carried out at the same age) to investigate repeatability. We found no significant differences in individual performance between and within tasks, indicating repeatability in performance in a cognitive task. Our results provide evidence of consistent within-individual differences in cognitive traits, suggesting our population harbours sufficient variation for selection to act.