Complex sentence: They are pollinated by sunbirds, which use the spathe as a perch when visiting the flowers; the weight of the bird on the spathe opens it to release the pollen onto the bird's feet, which is then deposited on the next flower it visits.
Split into simpler sentences: They are pollinated by sunbirds, which use the spathe as a perch when visiting the flowers. The weight of the bird when standing on the spathe opens it to release the pollen onto the bird's feet, which is then deposited on the next flower it visits.