Stupid question but...

[speckledhen]

Well, caffeine is really bad for them, but if I didn't watch where I put my cup outside, they'd all be in it. Just like a bunch of rotten puppies.

 

[seminolewind]

My horses like diet coke

 

[SpottedCrow]

Everything tastes better with birdie slobber in it...

Obelisk drank my coffee once, and only once. I don't use sugar and obviously she would want some in hers...She spit it out onto me and wiped her beak on my sleeve for good measure. She hasn't touched my coffee again.
Truthfully, a sip of coffee won't do any harm, but it's not at all good for them.