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moult timing chart
Gulls typically moult twice a year: a complete moult in the mid-summer to fall period, going from summer to winter plumage; and a partial moult in the spring, going from winter to summer plumage. All the feathers, including the primaries and retrices (tail feathers) are moulted in the complete moult. The partial moult is typically the head and body feathers, as well as some of the coverts (though much of this is unknown). This is why birds in the spring and summer may look very tattered and faded, as their flight feathers are quite worn. There is variability from bird to bird regarding both moult timing and the actual feathers moulted. Thus, moult timing is, at best, only a supporting clue to identification.

For decades, it was thought that young birds moult from juvenile to first winter (partial moult) to first summer (another partial moult) over the course of little more than half a year. As Howell et al. (The Condor 2003) point out, most large gulls follow a moult pattern whereby young birds begin in juvenal plumage, go through a partial pre-alternate moult (that may begin in fall, winter, or spring) into "first summer" plumage, and then do a complete pre-basic moult (beginning around their first birthday) of all feathers into "second winter" plumage. Thus, their juvenal plumage is essentially their "first winter" plumage, and what we used to call "first winter" feathers are actually "first summer".

my son, Caleb, holding up a tattered and recently moulted P6 from a nominate occidentalis Western Gull; July 18, Bodega Bay, CA

SPECIES FIRST PRE-ALTERNATE MOULT
out of juvenile plumage
DEFINITIVE PRE-BASIC MOULT
from adult summer to adult winter
hyperboreous Jan-Mar
glaucoides
kumlieni
thayeri
smithonianus west coast
smithonianus east coast
argenteus
argentatus
vegae
michahellis
cachinaans
graellsii, intermedius
marinus
dominicus
shistisagus
occidentalis, wymani Oct-Dec July-Oct
livens July-Sept June-Sept
glaucescens Nov-Mar
californicus
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Send comments to me at shampton at ospr.dfg.ca.gov