A person of no religious belief, or whose religion does not provide for an oath, affirms.
The person swears by whatever binds his conscience but if his preferred means of doing so is impractical , he affirms. That stops the inconvenience if his oath would involve sacrificing a chicken!However, the major courts do try. There is one religion of the Chinese which involves breaking a plate or saucer,in taking an oath. The Old Bailey still keeps some breakable saucers for use in that event.
Jewish men are supposed to cover their head when taking the oath.Jewish judges, in particular, have been known to insist on this because intending Jewish perjurers have been known to avoid it, so their conscience is clear, though their evidence is lies. Strange thinking!
Can't say that swearing on the bible has any noticeable effect on the honesty of the witness though. It only serves to remind them of the seriousness of their evidence. A brief reminder of the law of perjury would do as well or as badly for most.



