Sheldon's fear of birds has long been established since The Friendship Algorithm (2009). His fear of nets is a new one, possibly caused by him being caught under an electrified net in The Bozeman Reaction (2010).
Sheldon says this bird is not a terrifying bird like a swan or a goose. Swans and geese are both territorial and aggressive, frequently attacking people. A swan's beak can break an adult man's femur.
Sheldon's encounter with an angry chicken that once chased him into a tree was mentioned before in The Jiminy Conjecture (2009) and The Cruciferous Vegetable Amplification (2010).
Penny tries to humiliate Leonard in front of another woman by revealing that he has not one but two Star Trek uniforms. In The Prestidigitation Approximation (2011), we learned that he has an everyday uniform and a dress uniform.
Title reference: Ornithophobia is a type of specific phobia defined as an abnormal, irrational fear of birds. Sheldon finally manages to 'diffuse' (getting less concentrated) his fear of birds in this episode.