In human psychometric testing, individual performances across a range of cognitive processes are positively correlated, providing evidence for a "general intelligence" factor known as ‘g’. The structure of intelligence and individual cognitive variation have been extensively studied in humans, yet they have received far less attention in non-human animals. In particular, the development of a test battery suitable for quantifying individual cognitive performance in birds remains in its infancy. Additionally, implementing this approach in the wild, where the ecological significance of cognition can also be explored, presents considerable logistical challenges for most species. We developed a cognitive test battery for wild New Zealand North Island robins (Petroica longipes). Our battery was comprised of six tasks based on established measures of avian cognitive performance: a motor task, colour and shape discrimination, reversal learning, spatial memory and inhibitory control. Robins (N = 20) varied greatly in their ability to solve these tasks and we found weakly positive, non-significant correlations between most tasks. A principal components analysis (PCA) of task performances yielded two factors with eigenvalues >1. The first component extracted explained over 34% of the variance in cognitive performance and all six tasks loaded positively on this first component. We show that these results are robust using randomisation tests. Our results thus suggest that a general cognitive factor, analogous to human g, underpins cognitive performance in wild North Island robins tested in their natural habitat. We monitored the reproductive behaviour and output of robins during the 2014-2015 breeding season. General cognitive ability was not predictive of any measures of life-history variation for male robins (N = 13). However, male spatial memory performance influenced the number of eggs that a pair produced, suggesting a potential link between cognition and fitness in this species.