Short- and long-term repeatability and pseudo-repeatability of bird song: sensitivity of signals to varying environments

Abstract

To provide reliable information about individual-specific characteristics, sexual signals should be consistently displayed within an individual at least within a particular social context or time window. However, some male traits, like bird song, depict extreme within-individual variations even within a very short time frame. To investigate the extent by which acoustic displays in male collared flycatchers (Ficedula albicollis) reveal consistent individual-specific attributes or more flexible characteristics, we assessed the repeatability of various song traits at different sampling regimes (within-day, between-day, and between-year contexts). Repertoire size showed considerably high and significant repeatability (R > 0.25) in all temporal contexts suggesting that it can potentially indicate an individual attribute that is shaped by genetic background, permanent environmental effects, or long-term experience. Song rate, song length, mean frequency and frequency bandwidth, tempo of syllables, and within-song complexity had small-to-moderate repeatability (R < 0.25) that was significant for the within-day scenario only. Environmental effects can confound repeatability estimates, as males that changed nest boxes between song recordings performed with lower consistency than those that systematically sang on the same territory. Hence, the characteristics of particular songs could reveal aspects that are sensitive to changes in the environment rendering a signal function to the within-individual variance of songs. The consistency of acoustic signals and their sensitivity to the environment determines what kind of information can be conveyed to the receivers. We investigated the repeatability of several song traits in the collared flycatcher to determine signal consistency in different temporal contexts: within days, between days, and between years. We found that repeatability greatly varied across traits, but it generally showed a decreasing tendency as sampling window increased. Repeatability was confounded by effects arising from territory identity, because individuals that changed residency between days sang with less consistency. This result highlights the often-neglected scenario that pseudo-repeatability may arise due to unstandardized environment. In general, our findings imply that the consistency of behaviors and the signal value of song traits may depend on the considered temporal and/or environmental context. The degree of plasticity of acoustic signals determines whether information on a long-term aspect of individual quality or on an immediate state can be conveyed to the receivers.