Size matters: predation risk and juvenile growth in North Island brown kiwi (Apteryx mantelli)

This study investigated how predation risk in North Island brown kiwi changes as the birds grow and develop. Over a 10-year period, 53 adult and 126 young kiwi were radio-tagged at Lake Waikaremoana and studied to determine survival rates, causes of death, and rates of growth. Predation loss amongst adults was low (2.49% year^-1) and caused mainly by ferrets. Young kiwi suffered intense predation from stoats during their first four months of life, but thereafter became too large (> 800 g) for stoats to kill. Juveniles took at least 880 days to attain adult size, about four times longer than expected for a 2–3 kg bird. Growth rates peaked at about the point of hatch, rather than later on in development as in other birds. We suggest that a long evolutionary history dominated by resource limitation rather than predation may account for slow rates of development in kiwi, and that differences among kiwi species in their ability to persist in the presence of stoats are explained by differences in the time that they take to reach safe-size.