Variation in the Non-mimetic Vocalizations of Brood-Parasitic Indigobirds and Their Potential Role in Speciation

Indigobirds (Vidua spp.) are obligate brood parasites in which imprinting on heterospecific hosts shapes adult vocal behavior and mating preferences. Adult male indigobirds mimic the songs and other vocalizations of their respective hosts, which signals their own host environment to prospective mates and has important implications for speciation. In this study, we examined variation within and among indigobird species in the non-mimetic components of their vocal behavior, including both chatter calls and their impressive repertoires of intricate non-mimicry songs. We test whether indigobird species in Tanzania (V. chalybeata, V. codringtoni, V. funerea, and V. purpurascens) differ consistently in general features of their non-mimetic vocalizations, and we test whether local ecological conditions influence vocal behavior. Indigobird non-mimetic song repertories are learned from and shared with other males of the same species. We find that local dialect “neighborhoods” are variable in size among species and regions, depending on habitat continuity and the distribution of male territories. Despite the complete turnover of the specific songs comprising non-mimicry song repertoires from one local dialect to the next, we find significant species effects for more general measures of non-mimicry songs such as repertoire size and diversity, frequency, song length, and pace. For some traits, we also found significant regional differences, which may be mediated by significant relationships between elevation and morphometrics. Chatter calls were broadly similar across both species and localities, but we found significant species and region effects for frequency and to a lesser extent pace. We discuss the possibility that learning and mimicking the vocalizations of different hosts might influence the production of non-mimetic vocalizations and explain many of the species differences we detected. Whether these species differences are purely due to phenotypic plasticity or also reflect genetic divergence in traits influencing sound production and/or female preferences, they may contribute to reproductive isolation among nascent and recently evolved indigobird species.

Sexual and temporal variation in New Zealand bellbird song repertoires

How song repertoires vary within species and change over time is well studied in male songbirds. However, variation in female song repertoires remains largely unstudied despite female song being much more common and complex than once assumed. We investigated the song syllable repertoire of the New Zealand bellbird (Anthornis melanura), a species where both sexes have complex but sexually dimorphic song. We compared songs at individual and population levels to investigate sex and temporal variation of syllable repertoires. We detected 96 syllable types in the population over four years, of which 58% were unique to males, 32% unique to females and 9% were shared between the sexes. The population syllable repertoire of both sexes changed substantially across years with similar turnover rates (Jaccard’s similarity coefficients; female 52.9–69.0%; male 58.6–73.7%). Furthermore, many syllable types, unique to each sex, varied in prevalence within the population across years. The syllable repertoire sizes of individuals were higher for males than females (13-32, n = 7 and 6-16, n = 8, respectively). Although these sample sizes were low, the temporal variation in syllable prevalence and turnover for individuals were similar to patterns at the population level. Overall, male and female bellbirds exhibited similarities in temporal patterns of yearly repertoire composition, with rapid changes in syllable prevalence, but females had fewer syllable types than males. We suggest that these similarities and differences are consistent with male and female song repertoires being driven by similar but not identical selection pressures.

Correlated evolution between repertoire size and song plasticity predicts that sexual selection on song promotes open-ended learning

Some oscine songbird species modify their songs throughout their lives (‘adult song plasticity’ or ‘open-ended learning’), while others crystallize their songs around sexual maturity. It remains unknown whether the strength of sexual selection on song characteristics, such as repertoire size, affects adult song plasticity, or whether adult song plasticity affects song evolution. Here, we compiled data about song plasticity, song characteristics, and mating system and then examined evolutionary interactions between these traits. Across 67 species, we found that lineages with adult song plasticity show directional evolution toward increased syllable and song repertoires, while several other song characteristics evolved faster, but in a non-directional manner. Song plasticity appears to drive bi-directional transitions between monogamous and polygynous social mating systems. Notably, our analysis of correlated evolution suggests that extreme syllable and song repertoire sizes drive the evolution of adult song plasticity or stability, providing novel evidence that sexual selection may indirectly influence open- versus closed-ended learning.

An Analysis of the full song of five free-living urban European Blackbirds (Turdus merula); a network approach.

Is the song repertoire of male blackbirds safe-guarded against loss of variability during the breeding season? In order to answer this question the repertoires of five males were analysed from the viewpoint of network theory. In four of the five males a strong coherence of the repertoire was found to exist in anastomoses between strophe types, same elements being reached from different beginnings. The succession of strophe types in song sessions betrayed a mixture of relatively predictable cycles and chaotic connections. The former should facilitate repeated retrieval of a considerable part of the repertoire, while the chaotic component keeps the whole repertoire readily accessible from any point in a singing session. The same mixture may be considered optimal for binding the attention of conspecific listeners. It is concluded that the existing structure of blackbird song repertoires is favourable for the conservation of its richness, but it is as yet uncertain whether this is due to evolutionary (i.c. sexual) adaptation.

The songs of male pied flycatchers: exploring the legacy of the fathers

Singing is a key element of songbirds’ behavioral repertoire, particularly for males, which sing during the breeding season to defend resources against other males and to attract females. Different song traits may convey honest information about males’ qualities or conditions, which may be used by females to select their mates. Traits under strong sexual selection have an important component of additive genetic variation (i.e., the main genetic inheritance from parents), and so relatively high heritability; therefore, it can be expected that song traits also do. Although the act of singing is an innate behavior, and thus, genetically determined, songbirds need to learn their songs and therefore the genetic contribution to song traits may be reduced by the effect of environmental factors. We tested this hypothesis in seven song traits recorded in the long-distance migratory bird, the pied flycatcher (Ficedula hypoleuca). From a 23-year database (1992–2015), we obtained songs for 28 father–son pairs, and for each song trait we applied parent–offspring regressions to estimate heritability. The type of syllables sung are learned from tutors, and here we also determined the cultural contribution of fathers to the song repertoires of their sons, by quantifying the percentage of syllables that sons shared with their fathers, and compared this with what sons shared with other males in the population (e.g., neighbors). The heritabilities of song traits were highly variable (ranging from −0.22 to 0.56), but most of these were around zero and none of them were significant. These results indicate that the seven song traits are most likely determined by environmental factors. Sons shared more syllables with their fathers than with neighbors (21% vs. 3%), suggesting that fathers are important song tutors during the nestling period. We conclude that there is a cultural inheritance from fathers to their sons’ syllable repertoires, but there is no strong evidence for a genetic contribution of fathers to the seven song traits studied.