Male Songbird Indicates Body Size with Low-Pitched Advertising Songs

<div><p>Body size is a key sexually selected trait in many animal species. If size imposes a physical limit on the production of loud low-frequency sounds, then low-pitched vocalisations could act as reliable signals of body size. However, the central prediction of this hypothesis – that the pitch of vocalisations decreases with size among competing individuals – has limited support in songbirds. One reason could be that only the lowest-frequency components of vocalisations are constrained, and this may go unnoticed when vocal ranges are large. Additionally, the constraint may only be apparent in contexts when individuals are indeed advertising their size. Here we explicitly consider signal diversity and performance limits to demonstrate that body size limits song frequency in an advertising context in a songbird. We show that in purple-crowned fairy-wrens, <i>Malurus coronatus coronatus</i>, larger males sing lower-pitched low-frequency advertising songs. The lower frequency bound of all advertising song types also has a significant negative relationship with body size. However, the average frequency of all their advertising songs is unrelated to body size. This comparison of different approaches to the analysis demonstrates how a negative relationship between body size and song frequency can be obscured by failing to consider signal design and the concept of performance limits. Since these considerations will be important in any complex communication system, our results imply that body size constraints on low-frequency vocalisations could be more widespread than is currently recognised.</p> </div

FeedingRates

File used for the analyses of feeding rates by subordinates purple-crowned fairy-wrens

Only low-frequency trill types (*) are negatively related to tarsus length.

<p>The table shows Mixed Model predictions for each trill type (column 1), of mean frequency (column 2; average s.e.d. = 0.29) and the effect of tarsus on frequency (column 3; average s.e.d. = 0.48), as well as sample sizes (column 4, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0056717#s3" target="_blank">Results</a> for further details of Mixed Model output).</p

InheritanceProbability

File used to calculate inheritance probability of subordinate purple-crowned fairy-wrens when opposite sex breeders disappeared, in relation to the relatedness between the subordinate and remaining breeder

Body size limits low-frequency songs.

<p>a) Low-frequency trill song types (Trills 1, 2 and 3) were sung at lower frequencies by large than small males (regression line: y = −0.58x+17.49). Points represent mean peak frequency for each male, including Trills 1, 2 and 3. b) The lower bound of the relationship between frequency and body size had a negative slope (y = −0.51x+15.60, regression line fitted through black points, with minimum frequency in each tarsus length bin, see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0056717#s2" target="_blank">Methods</a>). Each point represents a song. c) Tarsus length does not predict the mean frequency of all a male's trill song types (y = 0.17x+0.77). Points represent mean peak frequency for each male, including all trill types.</p

Signal designs of trill songs.

<p>Sonagrams illustrating the five common trill song types recorded in the population. In these examples, Trill 2 and Trill 5 contain introductory syllables preceding the main trill phrase. Go to Supporting Information Audio S1, S2, S3, S4, S5 to listen to the trill songs illustrated here.</p

Data_Kingma_etal_ProspectingCosts

Body condition data of resident and prospecting individuals. All additional data used in the analyses are included

Data file from Adult sex ratios and their implications for cooperative breeding in birds

Data used for meta analyse

Pre-breeding moult and divorce data from male purple-crowned fairy-wrens

The data file includes data on pre-breeding moult timing and the outcome of divorces in male purple-crowned fairy-wrens, collected in the field over a six-year period

Pre-breeding moult data from male purple-crowned fairy-wrens

The data file includes pre-breeding moult profiles of male purple-crowned fairy-wrens based on plumage colouration scores collected in the field over a six-year period, as well as life-history-related information for each individual