For instance, it's not unusual for fledglings to show an entire row of grayish greater coverts, like this bird:
|Juvenile Ring-billed Gull. Blue Island, Illinois. 04 July 2013.|
Looking closely at the other wing coverts and scapulars, you'll notice they too have grayish centers and bases as well.
|Juvenile Ring-billed Gull. Tinley Park, Illinois. 26 June 2014. Showing a couple of lesser coverts with gray coating, and almost the entire row of greater coverts with a silver-gray (juvenile feathers).|
|Juvenile Ring-billed. East Chicago, Indiana. 27 June 2015.|
I've wondered why some juveniles show this gray color more than others. The simple answer is "variation". It does however seem more common as we get later in the season (late July and August). It could be birds that fledged a bit later develop adult-like colors (as is the case in late molters in 2nd cycle large white-headed gulls).
But I do have a few examples of juveniles that show gray greater coverts in mid-June. My earliest inland juvenile (away from Lake Michigan) is this bird:
|Juvenile Ring-billed Gull. Tinley Park, Illinois. 16 June 2015.|
Another loose theory of mine is that the wing coverts and scapulars pack more gray in their centers. As they begin to show the slightest amount of wear, the gray appears to widen and the buffy edges diminish. This can only be a function of time, as feathers loose their paler edges via wear as the weeks progress. Take this bird, for instance:
|Juvenile Ring-billed. Chicago, Illinois 26 July 2013. Intricate gray centers to most of the greater coverts, and again, along the bases of the lower tertial edges. Will this gray widen as the edges begin to wear?|
|Juvenile Ring-billed Gull. Chicago, Illinois. 26 July 2013. An atypical individual with white primary tips and boldy checkered upperparts. Still, grayish/white centers can be seen on many of the lower scapulars and wing coverts.|
|Juvenile Ring-billed Gull. 26 July 2013. A unique "ghost-type" with silvery greater coverts and whitish triangular centers to the scaps and lesser/median coverts.|
It should be apparent to the careful observer when "true" post-juvenile gray feathers have molted in.
|Ring-billed Gull undergoing prefomative molt - no longer juvenile. East Chicago, Indiana. 24 August 2015.|
Although Ring-billeds do often replace upperwing coverts in their preformative molt, it's usually limited to a few feathers here and there (as is the case with tertial replacement), like this bird:
|Mostly post-juvenile (formative) scapulars. Several lesser median covets have been molted (gray). But notice how the juvenile greater coverts are mostly brown with very little hints of gray. St. Joseph, Michigan. 01 September 2012.|
|The gray band of greater coverts are juvenile feathers. The back shows a mixture of brown juvenile scapularss and gray formative scapulars. East Chicago, Indiana. 24 August. 2015.|