Learned vocalizations play a key role in parrot social dynamics and vocal dialects have been documented for several mainland species, but to date no studies of geographically structured call variation in parrot species have examined the role of isolation on islands. In a study of the Brown-throated Parakeet (Eupsittula pertinax), which inhabits 5 small Caribbean islands as well as the adjacent mainland, we found that the contact calls of island and mainland parakeets show divergence in vocal characters as well as in call variability. We assessed call variation using 3 approaches: frequency measurements, spectrogram cross-correlation (SPCC) analyses, and call duration measurements. Island parakeets' calls were longer and had lower mean frequencies, and calls from different islands were distinguishable from each other as well as from mainland calls using measures derived from the SPCCs. In addition, we measured call variability at 2 different levels—within-location and within-individual. We found calls to be more variable for island parakeets for SPCC and duration measures, but less variable for frequency measures. The observed call differentiation among locations may be due to drift, whereas the lower frequency of island calls could either be a response to the windy environment on the islands or a consequence of the island subspecies' larger body sizes. We also hypothesize that the isolation of parakeet populations on small islands may have resulted in reduced selection for local call convergence, allowing island parakeets to produce more variable calls. We suggest that due to poor signal transmission in the windy island environment, selective pressures may favor variability in more easily perceived call features (like call duration) rather than more subtle features, like frequency shifts. Experimental tests are required to determine whether observed call patterns translate into similarly structured patterns in the responses to vocal variants.
LAY SUMMARY
When populations of birds become isolated on islands, their vocalizations often become different from those of their mainland relatives.
Two types of vocal divergence have been documented: changes in acoustical measures (character shifts), and changes in variability (variance shifts).
Nearly all studies of island bird vocalizations have involved songbirds, and little is known about the vocalizations of parrots isolated on islands.
We recorded Brown-throated Parakeet contact calls on five small islands as well as on the adjacent mainland, and compared the calls from different individuals and different locations.
We documented character shifts between calls from different locations, and by most measures the calls from islands are more variable (demonstrating variance shifts).
We hypothesize that these changes in island parakeets' calls may be a response to poor sound transmission in the windy environment on the islands, and may also reflect a reduced need to encode geographic information for parakeets living on small islands.
