Absence of Referential Alarm Calls in Long-term Allopatry from the Referent: A Case Study with Galapagos Yellow Warblers.

Animals across diverse lineages use referential calls to warn of and respond to specific threats, and the ability to understand these calls may be dependent on experience with the threat being referenced. Yellow warblers (Setophaga petechia) produce referential 'seet' calls towards brood parasitic brown-headed cowbirds (Molothrus ater), which threaten the warblers' reproductive success. Seet calls are produced frequently in populations sympatric with cowbirds, but rarely in allopatric populations, even when those populations are genetically similar, begging the question of the role of personal experience in anti-parasitic responsiveness in aggression and propensity to seet call towards brood parasites. Here we tested for seet call responses from a yellow warbler population on the Galapagos Islands (subspecies aureola), which has been geographically isolated from the mainland and obligate brood parasites for ~ 300,000 years. We presented playbacks of brown-headed cowbird calls (allopatric brood parasite), seet calls (North American yellow warbler's referential anti-parasitic call), chip calls (yellow warbler's general alarm call), sympatric predator calls, and harmless allopatric and sympatric control songs to breeding yellow warblers, and compared behavioral and vocal responses between treatments. We found that in response to playbacks signaling brood parasitic risk (seet and cowbird calls), Galapagos yellow warblers showed aggression comparable to controls, and much lower compared to chip or predator playbacks. Galapagos yellow warblers never produced any seet calls in response to the playbacks. Our results suggest that in geographic isolation from cowbirds, Galapagos yellow warblers do not produce or respond to referential alarm calls indicating mainland brood parasitic nest threats. Significance Statement: Communication signals that denote specific objects in the environment, known as referential signals, are shaped by several ecologically important drivers, such as the extent of geographic overlap between signalers and referents, social learning, and direct experience with the referent. The yellow warbler is a useful focal species to explore questions about the contexts in which referential alarm calls occur because of the specificity for production of its anti-parasitic "seet" calls and because multiple populations of yellow warblers exist with varying exposure to obligate brood parasites. Our study explores referential alarm calling in a context without personal or social experience/learning (due to ~ 300,000 year insular separation from mainland brood parasites), and the findings are starkly different, as no anti-parasitic calls were produced at all on the Galapagos Islands, compared to mainland warblers allopatric from brood parasites for only ~ 6000 years, which are still able to produce referential anti-parasitic calls. [ABSTRACT FROM AUTHOR]