verb

(typically of something regarded as good) become smaller, fewer, or less; decrease.

the birth rate continued to decline

decrease,reduce,get smaller,grow smaller,lessen,get less,diminish,wane,dwindle,contract,shrink,fall off,taper off,tail off,peter out,drop,fall,go down,sink,slump,plummet,plunge,nosedive,take a nosedive,take a header,go into a tailspin,crash