Since Google imposes restrictions on how frequently an add-on can trigger itself (at most, once per hour), Birdfeeder has quietly suffered a lack of utility. Not being able to use it more than once per hour makes it pretty much useless. However, when running a script from the script editor, the code can trigger itself as frequently as once per minute. This means anyone who has the source code can use Birdfeeder as it was written to be used. Rather than keep this for myself, I've decided to publish the source code in its entirety. The only change is to remove the consumer key and consumer secret of the Twitter app it uses, but these can easily be created by anyone with a twitter account. There's no cost involved, and it's a pretty easy process.
If you want to implement Birdfeeder, feel free to copy the source code into your own spreadsheet's script editor, and execute it the same way you would any other add-on. The big difference will be that Birdfeeder will only appear in that one spreadsheet, rather than all your sheets (but since it's designed to operate from a single spreadsheet, this won't be a problem).
I've posted the source to github. It's licensed under Creative Commons, so if you decide to use it or share it, please leave the attribution in there. You're also welcome to fork the repo and makes changes to it as you'd like (and if you don't know what that means, don't worry about it).