all the words listed can be used in a speech, there is no too formal word, actually, it totally depends on which kind of situation that u use them.

i sincerely suggest u never should use"一个鸟",it can be really impolite cos it will mean something nasty,ya..that's true..

一个鸟 is kinda weird man.... u may consider using 只 for all smaller animals... 个 is used more for inanimate objects

Okay... :P Darn. I don't mind remembering 一杯咖啡 and 一本书 but stuff like 一只鸟 ... I'd rather just say 一个鸟. :P I was surprised by some of the uses earlier like 一口人 instead of 一个人 ...

mmhm. Basically, yes. n_n Unless someone wants to give off the impression that their grammar is absolutely horrendous by saying "个" all the time for everything. I, for one, have been corrected a gazillion times for saying "个" excessively. haha anyways, increased vocabulary is a plus.