What are freedom birds and why do men feel guilty about dreaming of them?

The freedom birds were sleek silver birds with feathers...... they were the transport of the dead. Dreaming of the birds caused them to feel guilty for leaving their comrades behind but also happy to be free of the burdens of war.

At night, on guard, staring into the dark, they were carried away by jumbo jets. They felt the rush of takeoff. Gone! they yelled. And then velocity—wings and engines—a smiling stewardess—but it was more than a plane, it was a real bird, a big sleek silver bird with feathers and talons and high screeching. They were flying. The weights fell off; there was nothing to bear. They laughed and held on tight, feeling the cold slap of wind and altitude, soaring, thinking It's over, I'm gone!—they were naked, they were light and free—it was all lightness, bright and fast and buoyant, light as light, a helium buzz in the brain, a giddy bubbling in the lungs as they were taken up over the clouds and the war, beyond duty, beyond gravity and mortification and global entanglements—Sin loi! they yelled. I'm sorry, motherfuckers, but I'm out of it, I'm goofed, I'm on a space cruise, I'm gone!—and it was a restful, unencumbered sensation, just riding the light waves, sailing that big silver freedom bird over the mountains and oceans, over America, over the farms and great sleeping cities and cemeteries and highways and the golden arches of McDonald's, it was flight, a kind of fleeing, a kind of falling, falling higher and higher, spinning off the edge of the earth and beyond the sun and through the vast, silent vacuum where there were no burdens and where everything weighed exactly nothing—Gone! they screamed. I'm sorry but I'm gone!—and so at night, not quite dreaming, they gave themselves over to lightness, they were carried, they were purely borne.