What ARE birds?

Ok, I'm going to just jump over most of that and just state that othodoncetes are not birds - but it is difficult to say when they stopped being birds precisely
Also I just found a study that suggests mysticetes are in fact birds all along and my original instincts were right

mysticetes birds, othodoncetes not birds yes good let's move on

Is the dumpster behind the outback steakhouse a bird?

there may be birds and not birds inside the dumpster behind that infernal place but the dumpster itself is not a bird