California Gull californicus

(last update: 22-12-2016 )

Coordinators:
Alex Abela
John Cant
Mars Muusse

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1st > 2nd cycle (2CY): June

Living under sunny and marine conditions, the juvenile plumage is ragged and abraded in spring.
Quite commonly, CAGU molt some wing-coverts and/or upper tertials in the post-juvenile molt. They grow these feathers in autumn. By spring, even these replaced coverts may be very worn as well, despite of being one generation later.

PB2 molt in California Gull

By May/June, birds start PB2 molt, which is a complete molt taking the whole of summer to replace all feathers, including the large flight feathers. This PB2 molt commences by dropping P1, after which molt waves progress throughout the feather tracts in the wings in the next couple of months:
- a linear wave from P1 to P10 in the primaries (P10 fully grown as late as by October);
- two molt waves in the secondaries, both starting at the outermost feathers and progessing inwards to meet close to midway (S1 dropped when P6 is growing, to prevent large molt gaps in the wings);
- a transfugal molt in the rectrices, replacing the central tail-feathers first, then working its way outwards to both sides (R1 dropped at about the same time as S1; commonly R5 the last to be replaced);
- two molt waves in each of the wing-covert row (GC, MC, LLC), starting the outermost and innermost feathers and progessing inwards to meet close to midway;
- a quite random molt in the Lesser Coverts (LC).

In the very early stage of PB2 molt, when P1 is dropped, this is often far from obvious in resting birds. Still, it is an important moment, as technically speaking birds enter their "2nd cycle" now. In the field it takes a little time to witness the PB2 process going on in any particuar resting bird. Easiest check is on missingcoverts or new inner wing-coverts and the upper tertial. PB2 molt is in progress when very fresh wing-coverts start to appear, easily distinguished from the worn adjacent feathers. Especially in taxa like CAGU, fresh wing-coverts in early summer stand out, as post-juvenile wing-covert molt was finished by autumn and halted throughout winter and early-spring. Therefore, in May, both old juvenile and replaced 2nd generation wing-coverts are far from 'fresh feathers'.

above: California Gull californicus PB2 molt, June 27 2015, San Leandro, CA (John Cant).
Bird in complete molt: P1/P4.
Early stage of PB2 molt, with P1 growing but still not at full length. P2-P3 have been dropped and the outer primaries P4-P10 are still old, worn juvenile flight feathers. In this early stage, all secondaries and rectrices remain old juvenile. Outermost secondary S1 will be dropped once P6 is growing and only P9-P10 are old feathers. By waiting for that moment, birds remain manouvrable, by preventing a large molt gap to appear in the wing.
When P1 is dropped, this is not immediately apparent in resting birds with closed wings. But when P2-P3 are dropped as well, they also start to shed the median coverts (MC). By the time P1 is growing, soon the innermost fresh MC start to appear, and this is a good indication for rersting birds as well to wait for a bird to preen or to stretch and to check the inner primaries.

above: California Gull californicus PB2 molt, June 27 2015, San Leandro, CA (John Cant).
Bird in complete molt.
In resting birds the state of molt in the inner primaries is not always visible. The condition of feathers in the wing-covert panel and in the tertials can be an indication. they can be categorized in:
- very worn and bleached juvenile feathers. See the worn fringes and bleached centres of the brown, old juvenile tertials. see also the central GC, where feathers are no more than the shafts, all barbs are worn away.
- older 2nd gen coverts. These feathers were replaced last autumn, in the post-juvenile molt, and by spring the paler parts are bleached and the fringes slightly worn. See e.g. the innermost MC and innermost GC, as well the upper TT.
- fresh 2nd gen feathers. Fresh feathers are in excellent condition and show crisp fringes. In this individual, they also appear more adult-like in pattern, but this is not the case in all birds (see next bird, below, in which the pattern is barred).

Molt in wing-coverts and tertials follow a fixed startegy, as described HERE in the Herring Gull section. California Gull is somewhat different. Like its European cousin Yellow-legged michahellis, it may include inner wing-coverts and upper tertials in the post-juvenile molt in autumn. In late spring, in the very early stage of the PB molt, they follow the same "Herring Gull molt strategy", except for skipping the old 2nd gen coverts (commonly the inner coverts) from the sequence. In mid-summer, these 'old 2nd gen coverts' are replaced anyway, hence should then be labeled 3rd gen.

Even if you didn't notice P5 is missing, the wing-covert panel strongly suggests this bird is in active PB2 molt. The very crisp inner wing-coverts (LLC and MC) obviously stand out from the adjacent old worn juvenile wing-coverts. Knowing this bird is in PB molt, technically we should call this "2nd cycle".

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Resting birds: retarded
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Resting birds: PB2 molt obviously started
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