If a bird is perched on your shoulder you cannot use nether portals until you remove them by "losing height" which can cause a player to get temporarily trapped in the nether until you are able to remove the perched birds. Bird being perched on your shoulder stops you from using the nether portal from either side (regular world or nether) and after removing the birds and leaving the nether a new portal was created even though I had one that was already linked. Birds can accompany you through the nether and the portals as long as they aren't on your character.