It's probably been a decade since reading this book but it's worth posting for anyone who has not read it yet. The subtitle is "A novel of love and war" which hits the mark.
The war in this question is WWI and the central characters move between England and France. Before the war, there is love discovered, love gained and lost. After the war there is more love gained. These gains and losses aren't always with the same people. Sounds pretty trite. However, like others of its class, the result is anything but trite. The characters are believable, complex and interesting. The second part of the book, that mostly takes place in the trenches is very well done. You get a sense of the horror and boredom involved with that existance and the scenes near the end are gripping. A beautifully written story about different types of love and life in a horrendous war. A great story and great literature.
If you like this form of story, try "Cold Mountain" by Charles Frazier. It's basically the Iliad set in Civil War North Carolina. Although a bit less complex of a story, it's a good read. For stories about WWI at a level with "Birdsong", try "The Four Horsemen Of The Apocalypse" by Vincente Blasco Ibanez. It's a translation of a 1918 Spanish novel that is similar in scope to "Birdsong." An equally well written and engrossing novel about WWI, without the love story but with a very good twist at the end, is "A Fable" by William Faulkner. It won a 1955 Pulitzer Prize and National Book Award and is a distinct change from his southern-based stories. I think it's one of his best and equal to "Birdsong" and "The Four Horsement Of The Apocalypse." Enjoy.