A bioacoustic analysis of red crossbill (Loxia curvirostra) vocalizations from the island of Newfoundland, Canada

On the basis of morphological, ecological, genetic and vocal differences, North America Red Crossbills (Loxia curvirostra) have been described as a cryptic sibling species complex consisting of at least 10 forms. These forms are most easily differentiated via spectrographic analysis of their vocalizations and are hence known as "vocal types." However, little quantitative work has been done on forms' repertoires. For example, it is unclear if vocalizations, such as those named "excitement calls" or "chitter" by others, vary acoustically depending on the social context in which they are used or the physical state of the caller; it is also not known if such calls have characteristics that are systematically informative. Further, little is known about the vocal behaviour of Red Crossbills on the island of Newfoundland, an island which is generally presumed to contain the endangered Red Crossbill subspecies, L. c. percna. I made field recordings 1000 minutes) of Red Crossbills at 10 sites on the island in order to describe and document structural and contextual variation in vocalizations and to determine categories of calls (Call Classes) that might contain systematic information useful for subsequent comparative analyses. Subjectively classified calls, made on the basis of the social, behavioural, and physical contexts of calling birds, audible qualities of calls, and general appearance of spectrograms, corresponded closely to those of crossbills (Loxia spp.) described elsewhere in North America and Europe. Ln total, adults uttered five Call Classes (I- HI, V and VI); juveniles uttered two (IV and V). Both adult males and juveniles of unknown sex also sang. Multivariate clustering of calls, based on the individual averages of 10 acoustic variables measured from 1186 calls, corresponded with the subjectively established Call Classes. Acoustic variability within individuals and across contexts was relatively low among Call Classes I and III. Call Class Il was the most individualistic. A discriminant analysis (DA) on acoustic variables of Class I calls from Newfoundland individuals (n = 83) and individuals of vocal types from elsewhere in North America and Europe (n = 31) classified 89% of birds to the correct vocal type; 98% of birds from Newfoundland were differentiated successfully by DA. Further, many birds from Newfoundland also emitted Class III calls and song motifs that were distinct. Visual examination of Class I spectrograms, including those individuals misclassified by DA, confirmed that seven individuals from Newfoundland were spectrographically and audibly more similar to vocal types 2 and 10, which have been described previously on mainland North America. Thus, the results of this study show that multiple Red Crossbill forms, including possibly L. c. percna, are present on the island of Newfoundland.

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