The digestive tract of many animals is morphologically flexible and can adjust over time to make the most efficient use of the foods available. Differences between captive and wild diets often cause large differences in the gut morphology of captive and wild birds. This is potentially an issue when captive-bred birds are used to establish or supplement populations in the wild, such as in the Brown Teal Anas chlorotis, an endangered duck endemic to New Zealand. We compared the size and mass of the digestive organs (proventriculus, gizzard, small intestine, caeca, rectum and liver) of 57 wild, eight captive and four captive-bred released Brown Teal. Captive Brown Teal had much shorter and lighter small intestines and caeca than wild Brown Teal. These differences could reduce the ability of captive-bred teal to efficiently digest a wild diet in the weeks following release, and are likely to contribute to the number of released teal found dead in extremely poor nutritional condition. Increased fibre and diversity in the captive diet together with supplementary feeding post-release are recommended to improve the survival of captive-bred Brown Teal released to the wild.