In fact, they have trouble telling themselves apart and hybrids occur. The most obvious difference between them is their songs. Black-capped sings a two note song while Carolina sings a four note song.
Hybrids sing a three note song.
Mixed deciduous-coniferous forests, woodland edges, clearings, fields, thickets, orchards, suburbs. Often seen foraging in low branches of trees. Will come to feeders. Mating system is monogamous. Require 10- 14 days to excavate and line cavity.