Trips should group different birdlists from the same country or region. For example, if you go to Kenya for 15 days, you should create just one Trip and call it (for example) “Kenya summer 2014”.
Then, for each different place, you should create a Birdlist. For example “Masai Mara”, “Nairobi (near Airport)”, etc.. Each of these birdlists will contain the sightings made at that specific place.
From the Display data page, you will be able to list the species from the whole trip, or just of one specific birdlist, etc.