Suppose a user tweets:
@twitter_dev I think you are wonderful, but I have a tough life
Will this tweet have the field in_reply_to_screen_name filled?
I thought this would only be filled if it is a reply, i.e., suppose @twitter_dev replies to above tweet: @nilanjanb you really have an ugly face
in that case in_reply_to_screen_name would be nilanjanb
See these tweets - these are not replies. They are the first tweet sent out in the conversation:
However, they have in_reply_to_screen_name filled