The large birds like ostriches were previously thought to have blood-based erection mechanisms similar to humans. The Yale University, US, scientists confirmed that the birds really expand their penises with lymph fluid. According to Dr. Patricia Brennan Yale University the majority of birds reproduce with a 'cloacae kiss' - touching together their cloacae for long enough for sperm to transfer from the males to the females.
Seeing as all other birds with penises have lymphatic erection mechanisms, I always thought that it was strange that the ostrich would be blood vascular. All other birds with a penis attain erections in this way, leading scientists to judge the mechanism evolved in their antique ancestors.
Several species of bird, such as ducks, are known for their volatile erections achieved while lymph fluid is forced into the penis to increase pressure for a short time. The penis of the ostrich is fundamentally very different from emu and rhea because it is made out of a dense collagen matrix, but the lymphatic machinery is all there.