In my opinion, the sentence is trying to compare the vanishing of birds with that of the depletion of grassy fields and old pastures.
1.) I think there are two errors:
---> a) It should be sandpiper'
s vanishing, NOT sandpiper vanishing. (Apostrophe missing)
---> b) Even if the first error is corrected, it still doesn't remain parallel with the first part, 'the grassy fields...'
2.) Here it makes the correct comparision.
the bird themselves' is incorrect usage here.
4.) Incorrect comparision of depletion of grassy fields
with a region
5.) Same reason as 4.)
+++ Believe me, it doesn't take much of an effort to underline SC questions. Just try it out. +++
+++ Please tell me why other options are wrong. +++
~~~ The only way to get smarter is to play a smarter opponent. ~~~