cname.bitly.com has no NS records.
NS records for this zone are defined at bitly.com.
NS records for this zone are defined at bitly.com.
Question and response
Question
dig cname.bitly.com. NS
Answer
The response has no answer records.
Authority
bitly.com. 900 SOA ns-880.awsdns-46.net. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0