Apex domains cannot have CNAME records.
Question and response
Question
dig 237.19.71.in-addr.arpa. NS
Answer
237.19.71.in-addr.arpa. 5 NS ns2.birdhosting.com.
237.19.71.in-addr.arpa. 5 NS ns1.birdhosting.com.
Authority
The response has no authority records.
Additional
ns1.birdhosting.com. 5 A 204.11.96.10
ns2.birdhosting.com. 5 A 204.11.102.254
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0