In Flanders fields the poppies blow
Between the crosses, row on row,
That mark our place and in the sky,
The larks, still bravely singing, fly,
Scarce heard amid the guns below.
We are the Dead; short days ago
We lived, felt dawn, saw sunset glow,
Larks are small/medium-sized birds that are known for their extravagant songs. 
 
 In this context the larks represent the souls of the fallen, with the significant qualifying adjective “bravely”.  However, larks have long enjoyed a place in literature as a symbol of hope, happiness, good luck, creativity, and spiritual transcendence.  They are also a sign of dawn; there is a common expression “up with the lark”. This is ironic, as the soldiers will have no more dawns. 
 McCrae’s use of larks here has a religious connotation, especially in conjunction with the crosses a few lines earlier.  The larks circle above the battlefield and sanctify the soldiers' battle below. 
 See  this source  for a more detailed breakdown of different types of larks in literary history. 
