Spatial and age-related variation in use of locally common song elements in dawn singing of song sparrows Melospiza melodia: old males sing the hits

Long-distance dependencies in birdsong syntax

Songbird syntax is generally thought to be simple, in particular lacking long-distance dependencies in which one element affects choice of another occurring considerably later in the sequence. Here, we test for long-distance dependencies in the sequences of songs produced by song sparrows (Melospiza melodia). Song sparrows sing with eventual variety, repeating each song type in a consecutive series termed a 'bout'. We show that in switching between song types, song sparrows follow a 'cycling rule', cycling through their repertoires in close to the minimum possible number of bouts. Song sparrows do not cycle in a set order but rather vary the order of song types from cycle to cycle. Cycling in a variable order strongly implies long-distance dependencies, in which choice of the next type depends on the song types sung over the past cycle, in the range of 9-10 bouts. Song sparrows also follow a 'bout length rule', whereby the number of repetitions of a song type in a bout is positively associated with the length of the interval until that type recurs. This rule requires even longer distance dependencies that cross one another; such dependencies are characteristic of more complex levels of syntax than previously attributed to non-human animals.

Acoustic localization of terrestrial wildlife: Current practices and future opportunities

Autonomous acoustic recorders are an increasingly popular method for low-disturbance, large-scale monitoring of sound-producing animals, such as birds, anurans, bats, and other mammals. A specialized use of autonomous recording units (ARUs) is acoustic localization, in which a vocalizing animal is located spatially, usually by quantifying the time delay of arrival of its sound at an array of time-synchronized microphones. To describe trends in the literature, identify considerations for field biologists who wish to use these systems, and suggest advancements that will improve the field of acoustic localization, we comprehensively review published applications of wildlife localization in terrestrial environments. We describe the wide variety of methods used to complete the five steps of acoustic localization: (1) define the research question, (2) obtain or build a time-synchronizing microphone array, (3) deploy the array to record sounds in the field, (4) process recordings captured in the field, and (5) determine animal location using position estimation algorithms. We find eight general purposes in ecology and animal behavior for localization systems: assessing individual animals' positions or movements, localizing multiple individuals simultaneously to study their interactions, determining animals' individual identities, quantifying sound amplitude or directionality, selecting subsets of sounds for further acoustic analysis, calculating species abundance, inferring territory boundaries or habitat use, and separating animal sounds from background noise to improve species classification. We find that the labor-intensive steps of processing recordings and estimating animal positions have not yet been automated. In the near future, we expect that increased availability of recording hardware, development of automated and open-source localization software, and improvement of automated sound classification algorithms will broaden the use of acoustic localization. With these three advances, ecologists will be better able to embrace acoustic localization, enabling low-disturbance, large-scale collection of animal position data.

Repertoire structure, song sharing, reproductive success, and territory tenure in a population of Canada Warblers (Cardellina canadensis) in central New Hampshire

Sexual selection for larger repertoires and the social advantages of sharing songs with territorial neighbours are two forces that may drive the evolution of complex song repertoires in songbirds. To evaluate the influence of these two selective pressures on repertoire evolution in a species with a complex repertoire, we examined repertoire structure, song sharing, reproductive success, and territory tenure in a Canada Warbler (Cardellina canadensis (L., 1766)) population in New Hampshire. Over two breeding seasons, we recorded 63 singing males, classified their song repertoires, quantified male song sharing, and determined male reproductive success and territory tenure. Male Canada Warblers had complex repertoires averaging 12 phrases (particular sequences of song elements) and 55 variants (songs composed of particular sequences of phrases). Song sharing decreased significantly with distance between territories, all of which were <1.75 km apart. Network analysis revealed clusters of male neighbours with high variant sharing, which was significantly associated with longer territory tenure. Overall pairing and fledging success were high, but were not related to repertoire size or song sharing. Our results suggest that song sharing aids in male territory acquisition and defence, and that females may therefore select mates based on their ability to retain a high-quality territory.

Song repertoire size, not territory location, predicts reproductive success and territory tenure in a migratory songbird

In territorial animals occupying environments that vary in quality over the landscape, high-quality individuals are predicted to monopolize high-quality territories. Thus, in many cases it may be difficult to disentangle the relative effects of individual quality from those of territory quality on long-term fitness. We used a 9-year field data set from a migratory population of Eastern Song Sparrows (Melospiza melodia melodia (A. Wilson, 1810)) to evaluate the relative contributions of male song quality (as measured by song repertoire size) and territory location to fitness components including annual reproductive success, overwinter return rates, and between-year territory tenure. Song repertoire size did not predict territory location, allowing us to evaluate territory location and song quality separately. Song repertoire size, but not territory location, predicted annual reproductive success. Moreover, males with larger repertoires moved smaller distances between subsequent breeding seasons, suggesting more successful territory tenure. There was no effect of either repertoire size or territory location on overwinter return. We conclude that intrinsic male phenotype, indicated by song repertoire size, is an important predictor of male fitness, independent of breeding-territory location in this migratory population, and that the value of specific territories may depend largely on previous experience.

Developmental stress, condition, and birdsong: a case study in song sparrows

Sexual-selection theory posits that ornaments and displays can reflect a signaler's condition, which in turn is affected both by recent and developmental conditions. Moreover, developmental conditions can induce correlations between sexually selected and other traits if both types of traits exhibit developmental phenotypic plasticity in response to stressors. Thus, sexually selected traits may reflect recent and/or developmental characteristics of signalers. Here, we review data on the relationships between birdsong, a sexually selected trait, and developmental and current condition of birds from a long-term study of a population of song sparrows (Melospiza melodia). Field studies of free-living birds indicate that the complexity of a male's songs, a permanent trait, reflects the size of a song-control region of his brain (HVC), and is correlated with body size and several parameters of immunity, specifically investment in protective proteins. However, the performance of a male's songs, a dynamic trait, is not correlated to immune investment. Complexity of song is correlated with the glucocorticoid stress-response, and in some years response to stress predicts overwinter survival. Experimental manipulations have revealed that stressors in early life impair development of HVC, but that HVC recovers in size by adulthood. These manipulations result in impaired song-complexity and song-learning, but not song-performance. Experimental developmental stressors also affect growth, endocrine physiology, metabolism, and immune-function, often in a sex-specific manner. Combined, these studies suggest that song-complexity provides reliable information about early developmental experience, and about other traits that have critical developmental periods. Birdsong thus provides a multi-faceted sexually selected trait that may be an indicator both of developmental and recent conditions.