  We show that many topological features of level-1 species networks are
identifiable from the distribution of the gene tree quartets under the network
multi-species coalescent model. In particular, every cycle of size at least 4
and every hybrid node in a cycle of size at least 5 is identifiable. This is a
step toward justifying the inference of such networks which was recently
implemented by Sol\'is-Lemus and An\'e. We show additionally how to compute
quartet concordance factors for a network in terms of simpler networks, and
explore some circumstances in which cycles of size 3 and hybrid nodes in
4-cycles can be detected.
