Brown-throated parakeet vocalization data

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 five 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 three 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 two
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, while the lower
frequency of island calls could either be a response to the very 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 very 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.