I could tell when my dog was pregnant. You can tell when a cat or a horse is pregnant. Why don't we ever see a pregnant bird?
-- Dweezil (not that Dweezil), out there
Well, Dweeze, you'll have to look fast. (I'm assuming by "pregnant" you mean hauling around an egg...) Most birds don't waste much time getting the thing out and into the nest. In fact, until the calendar says it's time to mate, birds' reproductive organs (all internal, for most species) are shriveled to practically nothing. When spring comes, they puff up to about ten times non-breeding size. After they've found a mate and done it a few times, an ovum attached to the egg yolk is sucked into the beginning of the oviduct, where it's fertilized. In chickens, this takes about 18 minutes. Then it's squeezed down the tube to an area called the magnum, where the egg "white" (albumen) is added. Figure three hours at step two. Then it moves on to the shell-membrane stage (one hour), then to the last stop, the uterus, for the final egg shell and shell coloring (about 20 hours). Most birds labor in eggbirth for less than five minutes, though turkeys can take a couple of hours. So, beginning to end, most birds are pregnant for a little over 24 hours. And a lot of that time is spent hanging curtains and painting the nursery for the big event; the extra egg weight makes them vulnerable, so they're not usually very active. When breeding season is over, the oviduct shrinks so the female doesn't have to haul around useless weight for 10 months of the year. And birds can't carry twins. Only one oviduct enlarges to prevent two eggs from crushing each other and having Mama lay an omelet.