This chart shows the percentage of tweets in recent analyses @ZellaQuixote and I have done (246M tweets) categorized by app/service used. ~9% come from sources that indicate automation is likely (red section). 
For the sake of including a control group, here's the same figure for a set of 107K tweets containing the word "horse". The proportion likely to be automated based on app/service is similar.
Footnote: note that there are bots that work via automating the browser or phone app, and those forms of automation would not be detected by the test here.
