Skua Primary Moult Scores
This page is provisional and may change. In particular, some of the data for adults in the quoted references may involve some younger birds.
These charts have been derived from data in Olsen and Larsson, Hanzab Vol. 3 and BWP Vol. 3. The shaded areas indicate the zones of primary moult scores for juvenile birds (in brown) and adults (in orange). Primary moult starts with a score of 0 and ends with 10 new feathers and a score of 50.
Olsen and Larsson states that South Polar Skua takes between 45 and 60 days to complete the primary moult and between 150 and 180 days in the case of Brown and Great Skuas. This is not consistent with other data about start and end dates and possibly the zones should be enlarged slightly to accommodate this constraint.
Because of the paucity of data for juvenile Brown Skua, I have taken the Great Skua zone and shifted it 6 months. This seems reasonable because the moult zones for adults are similar apart from a 6 month phase shift.
Data for South Polar Skuas in the southern hemisphere is shown, but not used in the moult zones, as these are probably older birds that stay in the south and might not be consistent with what happens in the northern hemisphere.
Moult zones of ages other than juvenile and adult are not shown, but Olsen and Larsson states that 3rd year Great Skua [and presumably also Brown Skua] moults 1 month earlier than adults. Thus, the interval between annual moults of Great Skua must be longer than 12 months for them to get in synch with the adult cycle, whereas immature South Polar Skua requires an interval of less than 12 months to “catch up” with the adult cycle. The gaps between October and May for Brown Skua and between April and November for Great Skua can be occupied by birds of other ages.
We have not used our own observations in Senegal in deriving these charts, otherwise using them to identify Senegal birds would be a self-fulfilling prophesy. It is noteworthy that some birds do not accord, so if our identifications are right, then the charts need changing.
If these charts represent the truth, then this may help identify any bird actively moulting its primaries
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