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, 11 (6), e0156578
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Can Nocturnal Flight Calls of the Migrating Songbird, American Redstart, Encode Sexual Dimorphism and Individual Identity?

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Can Nocturnal Flight Calls of the Migrating Songbird, American Redstart, Encode Sexual Dimorphism and Individual Identity?

Emily T Griffiths et al. PLoS One.

Erratum in

Abstract

Bird species often use flight calls to engage in social behavior, for instance maintain group cohesion and to signal individual identity, kin or social associations, or breeding status of the caller. Additional uses also exist, in particular among migrating songbirds for communication during nocturnal migration. However, our understanding of the information that these vocalizations convey is incomplete, especially in nocturnal scenarios. To examine whether information about signaler traits could be encoded in flight calls we quantified several acoustic characteristics from calls of a nocturnally migrating songbird, the American Redstart. We recorded calls from temporarily captured wild specimens during mist-netting at the Powdermill Avian Research Center in Rector, PA. We measured call similarity among and within individuals, genders, and age groups. Calls from the same individual were significantly more similar to one another than to the calls of other individuals, and calls were significantly more similar among individuals of the same sex than between sexes. Flight calls from hatching-year and after hatching-year individuals were not significantly different. Our results suggest that American Redstart flight calls may carry identifiers of gender and individual identity. To our knowledge, this is the first evidence of individuality or sexual dimorphism in the flight calls of a migratory songbird. Furthermore, our results suggest that flight calls may have more explicit functions beyond simple group contact and cohesion. Nocturnal migration may require coordination among numerous individuals, and the use of flight calls to transmit information among intra- and conspecifics could be advantageous. Applying approaches that account for such individual and gender information may enable more advanced research using acoustic monitoring.

Conflict of interest statement

Competing Interests: Emily T. Griffiths is employed by Golden-crowned Consultants. There are no patents, products in development or marketed products to declare. This does not alter the authors' adherence to all the PLOS ONE policies on sharing data and materials, as detailed online in the guide for authors.

Figures

Fig 1
Fig 1. American redstart flight call variants, Hann Window, FFT 256, ovlp = 85%.
See supplementary material for more detailed descriptions. a) Type A, a typical redstart flight call as described in Evans and O'Brien (2002); b) Type G, an acute, convex inflection, followed by a shallow, obtuse inflection and modulated tail; c) Type M, two convex inflections at the beginning and tail of call with a concave inflection in the middle; d)Type S, obtuse, concave inflection in middle of call while tail of call is a lower frequency than the beginning; e)Type V, approximately equal in duration on either side of acute concave inflection.
Fig 2
Fig 2. 2-D Principal Coordinate Analysis (PCoA) plot based on extracted feature measurements showing multivariate homogeneity of group dispersions between all calls (N = 180),with 99% confidence ellipse based on the standard errors of the axis score averages.
Calls from the same sex are plotted in relation to their centroids (M = Male, F = Female). Differences between sexes were shown to be statistically significant (MANOVA; F = 12.46, R2 = 0.059, p < 0.001). Two orthogonal axes summarize the variability in the data set.
Fig 3
Fig 3. 2-D Principal Coordinate Analysis (PCoA) plot based on extracted feature measurements showing multivariate homogeneity of group dispersions between all calls (N = 180), with 99% confidence ellipse based on the standard errors of the axis score averages.
Calls from the same variant class are plotted in relation to their centroids. Differences between classes were shown to be statistically significant (MANOVA; F = 4.51, R2 = 0.086, p < 0.001). Two orthogonal axes summarize the variability in the data set. Note: The M centroid is behind the V centroid, but has the large confidence ellipse.
Fig 4
Fig 4. Similarity matrix generated by calculating random forest distance between quantitative features measured from flight calls.
Birds are labeled by the bird band assigned given to the bird when the recording was made at the Powdermill bird banding station (S4 Table). The individual pixels in the matrix represent the pairwise similarity values between the 180 flight calls, and the dark grid lines between pixels separate the calls from different individuals (N = 36 individuals, with 5 calls from each). Darker pixels indicate higher pairwise similarity.

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Grant support

This project was funded by multiple sources, including the National Science Foundation (IIS-1125098), Department of Defense Legacy Resource Management Program (05-245, 06-245, 07-245, and 10-245), and the Leon Levy Foundation. Private donations, such as the Kenneth L. Harder Trust, the Scott and Karen Harder Family, and several anonymous private donations to the Cornell Laboratory of Ornithology provided additional funding for this research. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. That being said, though they had no active role, the Harder family's interest and enthusiasm in flight calls helped push this project forward. No additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript was undertaken.
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