there was no requirement to register a birth pre 1877 - those registered pre this date are generally strong non-conformists who wanted nothing to do with established church but needed proof of legitimacy (tho a few high ranking official figures entered their children )
Some vicars did note the birthdate (often if baptism was somewhat later than usual) but this was not usual thus baptisms are the only record for 99% - there might be some private note (eg in family bibles) - many patron submitted IGI entries are incorrectly entered as births whereas they are baptisms