For me, it's less the memory card than the camera battery - I've been known to go through three rechargeable batteries a day. I usually only have to change the multi-gigabyte memory card once during the trip.
And I probably end up with fewer birds than you'd think, because they are up there high in the trees, moving and ducking behind bunches of leaves or flying away faster than my camera and I can focus. Tropical butterflies seem to flutter even faster than my local ones, so I have fewer photos than I like. Maybe that's why I'm taking more photos of plants and flowers and sedentary bugs than I used to.