I cannot offer any advice, but if young birds are dying in the nest at between 5 and 15 days old - then there is something seriously wrong there, be it streptococcus, salmonella - or something else -(YBS seems doubtful to be honest, as they usually need to mix with other birds) I would say that the advice you have been given is reasonable - but would go for amoxicillin (not baytril) as a treatment, as baytril will not treat strep.