The suggestion that sex of Adélie penguins, Pygoscelis adeliae, might be assigned by observing which member of the pair (the male) takes the first long incubation shift (Anon. 1991) was examined for each of the 1990–91, 1991–92 and 1992–93 breeding seasons. There was an 8- or 9-day period when more than 90% of the incubating birds were male and a 6- or 7-day period when more than 90% of the birds were female. The dates of these peak periods of male or female presence overlapped by only 2–5 days between the three seasons but were constant to within 2 days relative to the commencement of egg laying. Peak presence of males occurred 15–21 days after the appearance of the first egg in the colony and peak presence of females after 33–36 days from this date. In all three seasons male birds could be identified with 91∙8–98∙6% accuracy within 15–21 days after the first sighting of an egg. The method provides, therefore, a means of identifying the sex of Adélie penguins with an accuracy greater than 90% and is applicable to whole colonies containing several hundred pairs without recourse to continuous observations or capturing the birds.
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