A white-striped White-throated Sparrow
One of the most important aspects of banding birds is figuring out how old they are. Most techniques on that subject involve looking for molt limits on the wing. A molt limit among the greater coverts on the Yellow-rumped (Myrtle) Warblers (below, left) is easy to see. A molt limit is the point within one or between two feather tracts where old (retained) and new (replaced) feathers meet. Many first year birds only undergo a partial molt and are easy to age in the spring as second year (SY) when they show the sometimes obvious product of a purposeful "half-molt". Five of the inner greater coverts on the top bird are replaced, showing black centers, blue-gray edging and bluish white tips. These contrast strongly with the retained feathers to the right which show paler gray centers, buffy tips and edging, and more pronounced wear. The replaced greater coverts also contrast with the primary coverts and flight feathers, all of which are retained (compare to this bird). The bird below shows a similar pattern, but only replaced two greater coverts.
The female Black-and-white Warbler (above, right) can be aged in the spring as an after second year (ASY) by the smaller alula feather which shows white wrapping around the tip and a black center that does not separate the white and reach the tip. Both sexes of Black-and-white Warbler can be aged using this trick (compare the same feather on this SY male). A uniformly dark wing, especially the very black primary coverts and primaries help to cement this birds age as an ASY.
Sometimes molt limits are extremely obvious, even in the field on a closed wing. Summer Tanagers are a fantastic example of this. After their first prebasic molt, SY males in the spring are left with a patchwork of red and yellow feathers. The bird below replaced its tertials, two outermost secondaries, and its greater coverts. It's almost easier to see on the closed wing. This bird was the second of two Summer Tanagers we caught on April 25th during a prolonged southern slingshot weather event that littered the south coast and islands with early warblers and more. We saw additional evidence of this at Manomet with a Blue Grosbeak, Hooded Warbler, and Common Nighthawk on the property.
Two more birds below that can show fairly straightforward molt limits are SY male Eastern Towhees and Northern Cardinals. This towhee shows brown retained primaries and primary coverts contrasting nicely with the especially black replaced greater coverts. The cardinal shows a more random pattern with the most clear limit showing in the greater coverts. The inner coverts are retained along with the tertials. These feathers contrast with the redder replaced coverts and several secondaries.