The term ROADRUNNER RELATIVE is a crossword solution that refers to the CUCKOO, a six-letter bird. The CUCKOO is a species of bird that is related to the roadrunner, which is a large, long-legged bird native to North and South America. The CUCKOO is found in many parts of the world, including Europe, Asia, and Africa. It is known for its distinctive call, which is often heard during the spring and summer months.