Quite exceptional is when primaries are actively growing via one molt (i.e., the prebasic molt), when then an independent molt simultaneously crops up and initiates another wave of primary growth. The latter has traditionally been attributed to an extensive prealternate molt.
One large white-headed gull that is thought to undergo this alteration, to some degree, is Yellow-footed Gull (L. livens). Several workers have noted this phenomenon in the literature (Pyle 2008, Howell 2010). However, few exemplars or specifics have been given, particularly from the Fall season when both prebasic and prealternate molts regularly overlap.
Below are several examples of adult-type Yellow-footed Gulls that exhibit two waves of primary growth. All are from the Salton Sea in southern California during the last week of September. It's presumed these birds originated from Baja California where the species breeds and then makes a post-breeding dispersal into the Colorado Desert. No assumptions are made as to whether these individuals have successfully bred and/or their exact ages.
|1A. Primary molt on right wing shows two waves. Wave 1: p5-p10, with p9-p10 growing. Wave 2: p1-p4, with p4 growing.|
|1B. Similar molt pattern on left wing.|
|2A. Primary molt on left wing shows two waves. Wave 1: p5-p10, with p9-p10 growing. Wave 2: p1-p4, with p4 growing.|
|2B. Very similar to individual 1 but note p4 is shorter with no black markings.|
|3A. Primary molt on left wing shows two waves. Wave 1: p4-p10, with p9-p10 growing. Wave 2: p1-p3, with p3 growing.|
|3B. Asymmetric to left wing. Wave 1: p6-p10, with p9-p10 growing. Wave 2: p1-p5, with p5 growing.|
|4A. Primary molt on right wing shows two waves. Wave 1: p7-p10, with p9-p10 growing. Wave 2: p1-p6, with p5-p6 growing. |
|4B. Asymmetric to left wing. One molt wave with p1-p8 fully grown. p9-p10 growing. Presumably basic primaries.|
A special thanks to Peter Pyle for patiently entertaining my questions on this subject. His knowledge is most inspiring.