In diverse communities, recognizing other species’ alarm signals is critical for evading predators. Recognition among community members is thought to build up predictably and quickly as individuals learn to associate previously unrecognized calls with the presence of a predator. Here, we use a natural range expansion in which a songbird species, the pied flycatcher (Ficedula hypoleuca), is being gradually displaced on a Baltic island by an ecologically similar congeneric, the collared flycatcher (Ficedula albicollis), to test this prediction. We conducted 2 field experiments to evaluate the abilities of both species to recruit local heterospecifics to antipredator groups, called mobs. First, we stimulated mobs by exposing breeding collared and pied pairs to taxidermied predators. Second, to isolate the effect of alarm call recognition from other potential confounds, such as behavioral cues or differences in the locations of pied and collared flycatcher nests, we broadcast alarm calls of both species in breeding areas and measured the responses of heterospecifics. We found that pied flycatcher pairs were more likely to attract heterospecifics than were collared flycatcher pairs. This difference is driven by weak responses of community members to collared flycatcher alarm calls: The alarm calls of the native pied flycatcher were much more likely to attract heterospecifics than those of the colonist, collared flycatchers. Our results show that subtle changes in species composition may have large, unpredictable consequences on community-wide communication. Because many avian breeding communities are heavily affected by predation, disturbed communication networks may, in turn, have cascading effects on community composition.