In Birdsong by Sebastian Faulks, what is the significance of the "birds" as a trope throughout the novel?

"Birdsong" and birds themselves are used as a central motif, throughout the novel. This is first highlighted by Stephen’s phobia of birds. The horror for Stephen of the flapping birds wings is described in the telling of a recurring childhood dream, whereby the birds ‘brought their beaks towards his face’. His phobia is heightened when he sits with Isabelle and reacts to the presence of a pigeon and when he has to return the canary back to its cage. These birds represent a form of primitivism that Stephen is not able to control or repress. This carnal primitivism is one which is further exploited through the constant war motif, throughout the novel specifically also as the canary (another form of bird) is used to detect early danger when Jack is in the tunnel.
This is therefore contrasted again whereby "birdsong" can also be used to evoke the feelings of innocence - because of this Stephen’s horror is all the more noticeable and unexpected. The associations with birds are manifold then and Faulks draws on these multiple meanings to maintain an ambiguity. Birds are often regarded as ill omens (a crow for example). Conversely birds such as doves are inseparable with the idea of beauty and tranquility and peace perhaps due to the bible, whereby Faulks draws on similar thoughts to English Romantic, John Keats' ‘Ode to a Nightingale’ in which a bird is used to represent transcendence. 
The sound of birds is constantly referred to throughout the novel and is often used to register the end of events, for example after Stephen and Isabelle make love or when Stephen has to return to the British line at the end of the war. The birds can be heard when the violence stops - they fill the silence that is left. Their continuing sound implies that life continues despite tragedy. The novel ends with the ‘ambiguous call’ of a crow that can be heard ‘by those still living.’ The ambiguity suggests the continued presence of nature despite subsequent wars as well as the fear that Stephen felt. It could also suggest the continued haunting of the war to the soldiers that survive - Stephen’s phobia remains.