The niche hypothesis predicts that some introduced species establish and spread successfully because their new environment provides expanded niche opportunities compared with their native environments. By investigating nestling survival, growth and body condition in relation to diet composition and prey abundance, we tested the prediction that the success of Yellowhammers Emberiza citrinella in New Zealand could be explained by the availability of better quality food resources in its introduced range compared with its native range. We found that Yellowhammer nestlings in New Zealand were larger and heavier than those in Britain, but that nestling survival and body condition did not differ between countries. The preferred prey items, which had some influence on nestling development and survival, were more prevalent in the diet of nestlings in New Zealand, suggesting that nestlings in the introduced range had a superior quality diet. However, there was little evidence to support the prediction that the preferred prey were more abundant in the introduced range, implying that some other factor such as prey accessibility or adult fitness may account for the differences in diet between countries.