While it is generally considered that males and females have different body measurements, other factors may be important in affecting the uniformity of morphometric parameters within populations. In this 4-year longitudinal study I examined the influence of sex, age and time of year on five body measurements in the Stitchbird (or Hihi (Notiomystis cincta)): (1) tarsal length, (2) total head-length, (3) wing-length, (4) length of ear-tuft, and (5) weight. As with previous analyses of Stitchbird morphometrics, males were significantly larger and heavier than females. However, the age of birds and the time of year significantly affected several body measures. Tarsus was the most stable measure, remaining unchanged after 21 days of age. Wings were the least stable, being significantly affected by sex (females smaller than males), age (1-year-olds smaller than 2-year-olds, which were smaller than 3-year-olds) and season (with wing-length pre-moult smaller than wing-length post-moult). Differences between age-classes within the population reflected changes in individuals over time, with individuals having progressively longer wings and ear-tufts as they aged. Male and female Stitchbirds showed significant seasonal variation in body-weight, with weight reaching a maximum in winter and a minimum during the spring–summer breeding season. While the starvation–predation hypothesis commonly explains seasonal fluctuations in weight of birds, Stitchbirds lose weight during the coldest part of the year, immediately before the breeding season. This change in weight potentially maximises their agility when competing for mates in a mating system characterised by intense sperm competition.