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North Island robins are sexually dimorphic, males having darker plumage on their back and upper breast. However, males show delayed plumage maturation, and do not acquire the characteristic male plumage until after their first breeding season, 12-16 months after fledging. Therefore, sexing based on plumage alone will overestimate the proportion of females, and this may result in highly skewed sex ratios for translocations. Using measurements from robins of known sex on Tiritiri Matangi Island, I found tarsus length to be a useful indicator of sex. Of 82 robins measured, 80% of birds with tarsus length greater than 35.6 mm were male and 77% of other birds were female. If tarsus length is used in combination with plumage, it should allow sex ratios to be estimated reasonably accurately and without bias. However, additional data including wing chord measurements suggest that wing chord is superior to tarsus length for determining sex.