Most of these clues have multiple possible answers. However. the title and flavor text suggest that the answer to every clue is one letter away from the name of a bird. The answers and their associated birds are:
BUCKLING / DUCKLING
CRACKLE / GRACKLE
HURON / HERON
MANAGER / TANAGER
SANDPAPER / SANDPIPER
STERLING / STARLING
STOCK / STORK
TEEN / TERN
THICKEN / CHICKEN
There is only one way to fit these answers into the grid. At this point, solvers will note that the changed letters each appear in a green square.
Reading the new letters from top to bottom gives the word CARTRIDGE. Following the same rule as the rest of the puzzle, one letter can be changed to give the name of a bird, PARTRIDGE.