A flight to or from a high-tech hub, especially Silicon Valley.
"And with more business travelers turning to the red-eye, airlines have bulked up their overnight schedules to meet demand. The San Francisco Bay area, home of Silicon Valley, launches 28 so-called late-night 'nerd birds' every night: 20 out of San Francisco, seven from San Jose and one from Oakland, an increase of roughly 40 percent from just five years ago."
Amanda Lang, "Red Eye Flights," CNNfn, February 13, 2001
American Airlines will put a third 'Nerd Bird' in the air on Nov. 3, providing another link between Austin and Silicon Valley.
R. Michelle Breyer, "American plans 3rd 'Nerd Bird'," Austin American-Statesman, September 16, 1995
The original nerd birds were flights between Silicon Valley and Austin, Texas, the home of Dell Computer Corp. and other high-tech firms. This is reflected in the earliest citation I could find for nerd bird (see below).
A closely related term is Barbie bird, which refers to a flight between Silicon Valley and Los Angeles, which often has a high ratio of "actresses" and "models" who fly to Silicon Valley to work as booth bunnies in technology trade shows.