Tomahawk cruise missile image credit here.
I'm hating on this expression, and as a civilian employee of the Defense Department, believe me, I hear this term used a LOT.
Examples: We launched a lot of birds (Tomahawk cruise missiles) at Libya. Or, The birds landed at bin Laden's compound to insert the Navy Seals.
Mostly "bird" is used by present and former flyboys, flyboy-wannabees, missile shooters, and missile providers via the supply chain. Additional subsets include aficionados of both fixed wing and rotary wing craft (which I mention only because I wonder why they don't just say airplanes and helicopters, respectively), and NASA types. There are undoubtedly more user categories, I'm certain.
Wait, I know why people say this--it's the sexy lure of MIL-Speak. Tossing around insider military terms identifies you as, well, an insider. Well, my entire career has been spent in the Department of Defense--30+ years--and not once did I ever utter the word "bird." Nor will I. I just have standards, I guess, about the English language (see here for a recent example).
But back to "birds." That term is particularly annoying to me, because I love real birds and it seems a slight to them to prostitute the term. A flying war machine can never be as wondrous and magical as a living, breathing bird.
House wren image credit here.