Sibling Species Empidonax flycatchers, especially species such as the Dusky and the Gray Flycatchers, are notoriously difficult to identify. In general, birds of this group are more readily separated by their songs than by their appearance; indeed, it is suspected that even the birds depend heavily on song to sort out who is who in order to avoid hybridizing. These flycatchers are examples of sibling species -- species that are extremely similar in appearance but are nonetheless reproductively isolated from one another. Sibling species are often thought to be the result of fairly recent differentiation -- relatively new products of the speciation process. If, however, new species remain subject to very similar selection pressures, there is no reason why they cannot remain siblings for a very long time. SEE: Species and Speciation; Natural Selection; Interspecific Territoriality. Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.