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Added October 8, 2001: Details of the tertials: According to Tundra Plovers (Byrkjedal and Thompson), juvenile PAGP have an extensive partial molt into 1B in late Oct - mid March during which some of the tertials may be replaced, then another very limited partial molt into 1A in late Feb - early May during which some of the tertials may be replaced (or not). Juvenile AMGP have a similar 1B molt to PAGP, but the molt into 1A is more extensive, starting in early March and continuing into early June - again, some tertials may be replaced (or not).
Given the energy cost of replacing feathers, it is very unlikely that any tertials replaced in the 1B molt are replaced again in the 1A molt (as they would be relatively fresh, and not need replacement). So, on a 2CY bird in mid-May, each tertial will have been replaced once or not at all, but could be from 3 different generations: juvenile (not replaced at all in the 1B and 1A molts); 1B (juv feather replaced Oct- Mar); 1A (juv feather replaced Feb - May, and possibly not yet fully- grown).
When assessing the photos of this bird, please keep in mind that the tertials are different on each side of the body: On the right side, the three lower tertials are older feathers - the pattern looks more like 1B than juvenile - which are very faded and worn, while the uppermost tertial is of a later generation (first-alternate?); on the left side the lowest tertial is missing, the 2nd-lowest (the longest, when fully-grown) is fairly fresh and appears not to be fully grown (and thus likely to be 1A), the 2nd-highest is old and extremely worn and faded (could be juv. or 1B), and slightly askew inward (so that it is not visible in most profile views), and the uppermost (shortest) tertial is fresh (1A?).
Right side:- note the three old, worn lower tertials, plus a fresher uppermost tertial; the longest (third) tertial - despite being quite worn - extends to just beyond the tip of P7, so that only 3 primary tips are visible:




Left side:- between the fresh, presumed 1A first and third tertials (the fourth/lowest tertial is missing), note the old, very worn second tertial that is angled inwards over the inner edge of the primaries; it is longer than the third tertial, indicating that this newer third tertial (the longest, when fully grown) will grow out at least a few more millimeters, thence covering the tip of P7:

Note also in this photo that the retained juvenile outer tail feather is visible - the black feather with small white lateral notches below the black/ gray-barred inner tail feathers. The size and shape of the white notches on this outer retrix are more like that of PAGP than AMGP (according to Appendix 3 of Tundra Plovers) - yet the pattern of the new tertial is more like AMGP in that same Appendix - I do not know the relative reliability of those illustrations.

note that the old, 2nd tertial is not visible in the next photo:

note in the next image that the old, worn 2nd tertial has blown up (lifting the newer first tertial with it); when laying down over the 3rd tertial (which it rarely did, as the feather was misaligned) it reached the tip of P7. Again the outer tail feather from the far side is visible here, sticking out beyond/below the barred inner tail feathers - note the pattern of the white notches on this retained juvenile outer retrix: