Here is some info on doses for birds.I've no certain number to throw as the ideal, but ... there's bound to be a study for that ~'-)
As our good ol' family doctor puts it, there's a tendency to 'shoot rabbits w/ cannons' where meds are concerned, but using dosages under certain minimums can ultimately do far greater harm -- not just to the one living creature, but to all: That's where the resistant forms of everything are comin' from.
Recommended dose rates for metronidazole in dogs are 44 mg/kg, PO, followed by 22 mg/kg, qid for anaerobic infections; 25 mg/kg, PO, bid for giardiasis; and 66 mg/kg, PO, sid for trichomoniasis. Courses of therapy are generally 5-7 days. Both PO and IV preparations are available.
Source: http://www.harrisonsbirdfoods.com/avmed/cam/09_therapeutic_agents.pdf