Most birds and primates live high above - in the air or in trees - and those that don´t walk on two feet. They all need good eyes and vocal communication, traveling long distances and communication requires complex brains. Complex brains could favour evolution towards monogamy: Mating behaviour (finding a partner and keeping him/her is a challenge - for all creatures). It could also be the other way round: Two parents can foster their offspring better, so there is evolutionary pressure towards higher intelligence. That is convergent evolution of four traits in parallel between primates and birds.