Elegy Written in a Country Churchyard Stanza 5

Lines 17-20

The breezy call of incense-breathing Morn,
The swallow twitt'ring from the straw-built shed,
The cock's shrill clarion, or the echoing horn,
No more shall rouse them from their lowly bed.

• If you hadn't figured it out from the previous stanza, the speaker wants to clarify that the sleeping guys are not going to wake up. Here's how he explains it:
• The first three lines of this stanza list different things that normally would wake a person up (at least, in the days before alarm clocks and cell phones).
  (1) The delicious smells of the breeze first thing in the morning ("incense" is a substance that you burn to make a room smell good).
  (2) Birds twittering and singing in their straw nests.
  (3) The rooster's cock-a-doodle-doo ("clarion" = "alarm"), or the echoes of a horn blown by a hunter or a shepherd.
• Having listed all those things in the first three lines, the speaker tells us that none of those things are going to wake up the dead guys anymore. Okay, speaker!
• We get it! They're dead, not just sleeping!