In the first stages of juvenile feather growth, some 1st basic feathers may grow out weak, defective, or even be tugged at by adults. These feathers are dropped, adventitiously. Take for instance this hatch year Ring-billed with a renewed mid-primary:
|Ring-billed Gull (1st cycle) with adult-like primary flight feather. Chicago, IL. 05 Nov 2011.|
This is not to say that hatch-year gulls shouldn't show new, adult-like, gray feathers. Many deliberately replace their feathers via a strategic molt soon after leaving their nest site. For example, here's a recently fledged juvenile Ring-billed showing a couple of gray upperwing coverts:
|Ring-billed (juvenile). Tinley Park, IL. 23 July 2014.|
Some of our gulls replace most or all of their scapulars via a prealternate (or preformative) molt in their hatch year, like these individuals:
|California Gull (1st cycle) with replaced, gray, scapulars. New Buffalo, MI. Oct 2012.|
|Ring-billed (1st cycle) with replaced scapulars, some upperwing coverts AND a few upper tertials. Chicago, IL. Oct 2012.|
|Bonaparte's Gull (1st cycle) with replaced scapulars. Chicago, IL. Oct 2010.|
Note that the gray greater and median coverts on the Bonaparte's above are not preformative feathers. These are juvenile feathers that were originally grown as gray. This is a variable trait found in a number of species. One example that comes to mind are the silvery gray-tinged greater coverts of some Ring-billeds.
|Ring-billed Gull (1st cycle) with extensive gray on juvenile greater coverts. Chicago, IL. 04 July 2013.|
An extreme example of juveniles that begin with a sizable percentage of their upperparts showing adult-like gray, are kittiwakes. Take this Black-legged Kittiwake, for instance:
|Black-legged Kittiwake (1st cycle) with gray juvenile upperwing coverts and scapulars. Marseilles, IL. Dec 2013.|
Knowing which feathers are supposed to be replaced "when" is lots of fun to monitor and observe throughout the year. Fortunately, gulls and their feather tracts are large enough, and usually cooperative enough, for us to study in the field with some ease.