This black/blue and white hirundine looks superficially like the well-known House Martin with its white rump, but a closer look reveals a white line from above the bill to the eye. The White-rumped Swallow is a bird which occurs in Uruguay. Its description is as follows:- Above (except rump) black with blue sheen. Rump and underparts white. Variable white line from forehead to brow. 

This dusky hirundine looks like a female Purple Martin but this species is not regular in Uruguay, and why would the designer choose a female? A close look at the hirundines in the Checklist reveals a very good match: Grey-breasted Martin which is described as bluish black above; throat and breast grey with whitish wash; rest of underparts whitish.

This looks like a Great Black-back but with a black band on its white tail. The Checklist gives an excellent illustration of this which could have been "lifted" and shown on the stamp. Description: Yellow bill with red tip and black subterminal band right across. Yellow legs. Head, neck, underparts, rump and tail white, the tail with a broad black subterminal band. Back and wing coverts black, primaries black, secondaries grey with white tip.

This looks like a White-rumped Sandpiper! Description: Non-breeding plumage: upper parts grey, slightly flecked brown. Rump and upper tail-coverts white. Eyebrow and belly white; breast pale grey flecked darker, flecking extending onto the flanks.