Fitness correlates of song repertoire size in free-living song sparrows (Melospiza melodia)

Models of sexual selection propose that exaggerated secondary sexual ornaments indicate a male's own fitness and the fitness of his offspring. These hypotheses have rarely been thoroughly tested in free-living individuals because overall fitness, as opposed to fitness components, is difficult to measure. We used 20 years of data from song sparrows (Melospiza melodia) inhabiting Mandarte Island, British Columbia, Canada, to test whether a male's song repertoire size, a secondary sexual trait, predicted overall measures of male or offspring fitness. Males with larger song repertoires contributed more independent and recruited offspring, and independent and recruited grandoffspring, to Mandarte's population. This was because these males lived longer and reared a greater proportion of hatched chicks to independence from parental care, not because females mated to males with larger repertoires laid or hatched more eggs. Furthermore, independent offspring of males with larger repertoires were more likely to recruit and then to leave more grandoffspring than were offspring of males with small repertoires. Although we cannot distinguish whether observed fitness variation reflected genetic or environmental effects on males or their offspring, these data suggest that female song sparrows would gain immediate and intergenerational fitness benefits by pairing with males with large song repertoires.     

Inbreeding effects on immune response in free-living song sparrows (Melospiza melodia)

The consequences of inbreeding for host immunity to parasitic infection have broad implications for the evolutionary and dynamical impacts of parasites on populations where inbreeding occurs. To rigorously assess the magnitude and the prevalence of inbreeding effects on immunity, multiple components of host immune response should be related to inbreeding coefficient (f) in free-living individuals. We used a pedigreed, free-living population of song sparrows (Melospiza melodia) to test whether individual responses to widely used experimental immune challenges varied consistently with f. The patagial swelling response to phytohaemagglutinin declined markedly with f in both females and males in both 2002 and 2003, although overall inbreeding depression was greater in males. The primary antibody response to tetanus toxoid declined with f in females but not in males in both 2004 and 2005. Primary antibody responses to diphtheria toxoid were low but tended to decline with f in 2004. Overall inbreeding depression did not solely reflect particularly strong immune responses in outbred offspring of immigrant-native pairings or weak responses in highly inbred individuals. These data indicate substantial and apparently sex-specific inbreeding effects on immune response, implying that inbred hosts may be relatively susceptible to parasitic infection to differing degrees in males and females.     

Age-related changes in song repertoire size and song type sharing in the Whinchat Saxicola rubetra

In many oscine passerines males’ songs, the repertoire size increases with age. At the same time it often remains unknown when and where males learn new songs. To infer the Whinchat Saxicola rubetra song learning strategy, we described and catalogued song-type repertoire, revealed age differences and examined song sharing strategies among neighbouring and distant males. We recorded song vocalizations of 40 males in a limited (104 ha) study plot during four years. Whinchats produce short and discrete songs with clear intersong pauses. In total 45 song types were allocated, the individual repertoire size averaged 23.5 ± 7.6 song types (range 9–34 song types). The males’ age significantly influenced the song-type repertoire size. The second calendar year (first breeding) males had a lower repertoire size than the older males. The majority of song types were shared by less than half of males in our sample. The Jaccard similarity indexes varied from 0.5 to 0.7. We could not find a relationship between males’ song sharing and geographic distances between their nests. We assume that Whinchat males learned new songs in the local population before territory establishment.     

Heritability of female extra-pair paternity rate in song sparrows (Melospiza melodia)

The forces driving the evolution of extra-pair reproduction in socially monogamous animals remain widely debated and unresolved. One key hypothesis is that female extra-pair reproduction evolves through indirect genetic benefits, reflecting increased additive genetic value of extra-pair offspring. Such evolution requires that a female's propensity to produce offspring that are sired by an extra-pair male is heritable. However, additive genetic variance and heritability in female extra-pair paternity (EPP) rate have not been quantified, precluding accurate estimation of the force of indirect selection. Sixteen years of comprehensive paternity and pedigree data from socially monogamous but genetically polygynandrous song sparrows (Melospiza melodia) showed significant additive genetic variance and heritability in the proportion of a female's offspring that was sired by an extra-pair male, constituting major components of the genetic architecture required for extra-pair reproduction to evolve through indirect additive genetic benefits. However, estimated heritabilities were moderately small (0.12 and 0.18 on the observed and underlying latent scales, respectively). The force of selection on extra-pair reproduction through indirect additive genetic benefits may consequently be relatively weak. However, the additive genetic variance and non-zero heritability observed in female EPP rate allow for multiple further genetic mechanisms to drive and constrain mating system evolution.     

Song characterization in the spectacled warbler (Sylvia conspicillata): a circum-Mediterranean species with a complex song structure

The spectacled warbler (Sylvia conspicillata) is a small passerine with a patchy distribution throughout the circum-Mediterranean region, including the North Atlantic archipelagos of Madeira, Canary Islands and Cape Verde. Here we characterize the species song structure on the island of Fuerteventura, quantifying repertoire size, inter- and intra-individual spectrographic variation, to determine whether acoustic variation occurred within an island population. Male song display was organized in song bouts of a variable number of song phrases, which in turn were made up of 4–69 syllables. We classified syllable types to derive a measure of repertoire size (number of different syllables) per song bout, and then used rarefaction methods to calculate the estimated repertoire size for our population of males. Three categories of song bout length were considered in analyses: short song bouts of 10 phrases, average bouts of 19 phrases and long bouts of ≥ 29 phrases. The observed and estimated repertoire size per male (between 43 and 126 syllables per male) increased with song bout duration, although the relationship was not significant for the estimated values. To test whether songs could be individually specific, we measured 11 spectrotemporal parameters of the song. A discriminant analysis using these variables performed poorly in classifying songs to the individuals that uttered them, but we found less variation in the individual than in the population for three out of the 11 variables. These individually specific variables, involving the first or the most common syllable of the song, the trill, were the duration of the first syllable of the phrase, the duration and the dominant frequency of the trill syllable. Our study emphasizes the complexity of spectacled warbler songs, in which males continuously add novel syllables over the entire song bout. This complexity appears to be determined by individual innovation capabilities rather than by the behaviour of copying neighbour repertoires, since songs of close birds were not more similar than songs from far-away territories.     

Genetic and phenotypic correlations in a natural population of song sparrows

We estimated heritabilities, and genetic and phenotypic correlations between beak and body traits in the song sparrow ( Melospiza melodia ). We compared these estimates to values for the same traits in the Galápagos finches, Geospiza (Boag, 1983; Grant, 1983). Morphological variance is low in the song sparrow, and our results show that genetic and phenotypic correlations are considerably lower than correlations in the morphologically more variable Geospiza. Comparison using a larger sample of Galapagos populations confirms the existence of an association between variance and correlation for phenotypic values. We suggest two possible explanations for this association. First, most traits studied are functionally related, and the joint evolution of variance and correlation may have resulted from stabilizing selection about a line of optimal allometry between traits. Alternatively, introgression between populations and species could have caused correlation and variance to evolve jointly. Both selection and introgression were probably influential in producing the observed pattern, but it is not possible to estimate their relative importance with current data. Genetic and phenotypic correlations were correlated in the song sparrow, but heritabilities of traits varied greatly. As a result, the genetic variance-covariance matrix for traits is not simply a constant multiple of the phenotypic matrix. Evolutionary response to natural selection cannot, therefore, be predicted from the measurement of phenotypic characteristics alone.     

Social context affects testosterone‐induced singing and the volume of song control nuclei in male canaries (Serinus canaria)

The contribution of social factors to seasonal plasticity in singing behavior and forebrain nuclei controlling song, and their interplay with gonadal steroid hormones are still poorly understood. In many songbird species, testosterone (T) enhances singing behavior but elevated plasma T concentrations are not absolutely required for singing to occur. Singing is generally produced either to defend a territory or to attract a mate and it is therefore not surprising that singing rate can be influenced by the sex and behavior of the social partner. We investigated, based on two independent experiments, the effect of the presence of a male or female partner on the rate of song produced by male canaries. In the first experiment, song rate was measured in dyads composed of one male and one female (M‐F) or two males (M‐M). Birds were implanted with T‐filled Silastic capsules or with empty capsules as control. The number of complete song bouts produced by all males was recorded during 240 min on week 1, 2, 4, and 8 after implantation. On the day following each recording session, brains from approximately one‐fourth of the birds were collected and the volumes of the song control nuclei HVC and RA were measured. T increased the singing rate and volume of HVC and RA but these effects were affected by the social context. Singing rates were higher in the M‐M than in the M‐F dyads. Also, in the M‐M dyads a dominance‐subordination relationship soon became established and dominant males sang at higher rates than subordinates in T‐treated but not in control pairs. The differences in song production were not reflected in the size of the song control nuclei: HVC was larger in M‐F than in M‐M males and within the M‐M dyads, no difference in HVC or RA size could be detected between dominant and subordinate males. At the individual level, the song rate with was positively correlated with RA and to a lower degree HVC volume, but this relationship was observed only in M‐M dyads, specifically in dominant males. A second experiment, carried out with castrated males that were all treated with T and exposed either to another T‐treated castrate or to an estradiol‐implanted female, confirmed that song rate was higher in the M‐M than in the M‐F condition and that HVC volume was larger in heterosexual than in same‐sex dyads. The effects of T on singing rate and on the volume of the song control nuclei are thus modulated by the social environment, including the presence/absence of a potential mate and dominance status among males. 2006 Wiley Periodicals, Inc. J Neurobiol, 2006     

Early nutritional stress impairs development of a song-control brain region in both male and female juvenile song sparrows (Melospiza melodia) at the onset of song learning

Birdsong is a sexually selected trait and is often viewed as an indicator of male quality. The developmental stress hypothesis proposes a model by which song could be an indicator; the time during early development, when birds learn complex songs and/or local variants of song, is of rapid development and nutritional stress. Birds that cope best with this stress may better learn to produce the most effective songs. The developmental stress hypothesis predicts that early food restriction should impair development of song-control brain regions at the onset of song learning. We examined the effect of food restriction on song-control brain regions in fledgling (both sexes, 23-26 days old) song sparrows (Melospiza melodia). Food restriction selectively reduced HVC volume in both sexes. In addition, sex differences were evident in all three song-control regions. This study lends further support to a growing body of literature documenting a variety of behavioural, physiological and neural detriments in several songbird species resulting from early developmental stress.     

Heterosis and outbreeding depression in descendants of natural immigrants to an inbred population of song sparrows (Melospiza melodia)

We studied heterosis and outbreeding depression among immigrants and their descendants in a population of song sparrows on Mandarte Island, Canada. Using data spanning 19 generations, we compared survival, seasonal reproductive success, and lifetime reproductive success of immigrants, natives (birds with resident-hatched parents and grandparents), and their offspring (F1s, birds with an immigrant and a native parent, and F2s, birds with an immigrant grandparent and resident-hatched grandparent in each of their maternal and paternal lines). Lifetime reproductive success of immigrants was no worse than that of natives, but other measures of performance differed in several ways. Immigrant females laid later and showed a tendency to lay fewer clutches, but had relatively high success raising offspring per egg produced. The few immigrant males survived well but were less likely to breed than native males of the same age that were alive in the same year. Female F1s laid earlier than expected based on the average for immigrant and native females, and adult male F1s were more likely to breed than expected based on the average for immigrant and native males. The performance differences between immigrant and native females and between F1s and the average of immigrants and natives are consistent with the hypothesis that immigrants were disadvantaged by a lack of site experience and that immigrant offspring benefited from heterosis. However, we could not exclude the possibility that immigrants had a different strategy for optimizing reproductive success or that they experienced ecological compensation for life-history parameters. For example, the offspring of immigrants may have survived well because immigrants laid later and produced fewer clutches, thereby raising offspring during a period of milder climatic conditions. Although sample sizes were small, we found large performance differences between F1s and F2s, which suggested that either heterosis was associated with epistasis in F1s, that F2s experienced outbreeding depression, or that both phenomena occurred. These findings indicate that the performance of dispersers may be affected more by fine-scale genetic differentiation than previously assumed in this and comparable systems.     

Song and immunological condition in male barn swallows (Hirundo rustica)

Male secondary sexual characters may have evolved as intra-or intersexual signals of male phenotypic or genetic quality.In birds, singing performance may have the function to honestlyreveal health and vigor of individual males. Infectious diseasesand poor body conditions would therefore be expected to negativelyinfluence singing performance. Since bird pathogens are knownto elicit both a humoral and a cell-mediated immune response,it can be predicted that a negative relationship exists betweensinging performance and activity of the immune system. Thisprediction was tested for the first time in this correlationalstudy. The relationships between song rate and features andhematological variables (concentration of leukocytes in peripheralblood, ratio of gamma-globulins to total plasma proteins, bloodcell sedimentation rate, hematocrit) and body condition wereanalyzed in a population of bam swallows (Hirundo rustica).Song rate was negatively correlated with lymphocyte concentrationand with the ratio of gamma-globulins to plasma proteins. Spectrographicanalysis showed that features of song were not significantlycorrelated with hematological variables or body condition. Thelevel of circulating testosterone was not correlated with songrate nor hematological variables. This study is the first toshow a correlation between a bird's singing performance andhematological profile and suggests that song rate of male barnswallows may reflect their health status. Song in this speciesmight thus have evolved because it allows prospecting femalesto assess aspects of phenotypic and/or genetic quality of potentialmates     

Immimohistochemical localization of corticotropin-releasing factor in selected brain areas of the European starling (Sturnus vulgaris) and the song sparrow (Melospiza melodia)

Summary Corticotropin-releasing factor (CRF) was localized in the brains of two passerine species, the European starling (Sturnus vulgaris) and the song sparrow (Melospiza melodia), by means of immunohistochemistry. The hypothalamic distribution of this peptide in these species includes a complex of immunoreactive perikarya observed in the paraventricular nucleus (PVN), in both its medial and lateral divisions. Nerve fibers were also seen running from these areas to the anterior median eminence (AME) where a terminal field is apparent. A wide variety of extra-hypothalamic nuclei containing CRF-immunoreactive cells and fibers were identified. An apparent CRF terminal field can be visualized in the lateral septum. A dense fiber plexus is present in the nucleus accumbens (Ac) and more caudally in the nucleus of the stria terminalis (nST). In colchicinepretreated animals, it was revealed that these areas also contain CRF-stained perikarya. The pattern of CRF immunoreactivity in the Ac-nST complex is continuous, with no distinction apparent between the nuclei. The medial preoptic area (mPOA) and the adjacent diagonal band of Broca contain CRF-fibers, while cells are apparent in the mPOA. In the mesencephalon, cells were visualized in the midbrain central gray; a terminal field and scattered positively stained perikarya were found in areas more ventral to the central grey that are adjacent to the third cranial nerve. Scattered cells were also seen at the border of the nucleus intercollicularis-nucleus mesencephalicus lateralis, pars dorsalis complex. In contrast to mammalian studies, no immunoreactive nerve fibers or perikarya were observed in telencephalic areas homologous to the mammalian neocortex. These studies confirm the presence of a CRF path-way regulating pituitary function and suggest a broad role played by CRF as a neuromodulator or neurotransmitter in autonomic and possibly behavioral activities in these species.     

Conventional and network analyses of song organization and complexity in northern House Wrens (Troglodytes aedon parkmanii)

Among songbirds with large song-type repertoires, there may be functional variation in how individuals organize and display song-type diversity over time. Past studies focusing on conventional measures of song production have been extremely productive. However, network analysis offers a novel set of tools to quantify additional, previously unstudied elements of song organization and display. We studied protracted bouts of singing by 10 male House Wrens (Troglodytes aedon) to (1) test functional hypotheses of variation in song diversity in this species, and (2) evaluate the utility of network metrics in such research. Our analysis included a variety of conventional measures of song production and several standard metrics from network theory to quantify how variably the many song types in a male’s repertoire could be connected to one another and the limitations or diversity of their song sequences. Analysis of conventional variables showed that males produced more and longer songs, at shorter intervals, containing more syllables and more different syllable types, and also more different song types, prior to than after pairing and early in the morning regardless of breeding stage. These results are consistent with the hypothesis that song diversity functions in mate attraction and possibly in territory signaling. In contrast, analyses of network metrics revealed variety in song sequencing by males, but comparatively few and weak effects associated with either breeding stage or time of day. Overall, most song types connected to only a few others and a relatively small proportion of all possible song-type transitions actually occurred. Hence, much of the variety in song sequencing that was possible with the large song repertoires of males was not realized. The latter outcomes, brought to light via network analyses, highlight an important paradox for future research on this and related species with large song repertoires.     

Autosomal variation for male body size and sperm competition phenotypes is uncorrelated in Drosophila melanogaster

Correlations between male body size and phenotypes impacting post-copulatory sexual selection are commonly observed during the manipulation of male body size by environmental rearing conditions. Here, we control for environmental influences and test for genetic correlations between natural variation in male body size and phenotypes affecting post-copulatory sexual selection in Drosophila melanogaster. Dry weights of virgin males from 90 second-chromosome and 88 third-chromosome substitution lines were measured. Highly significant line effects (p<0.001) documented a genetic basis to variation in male body size. No significant correlations were identified between male body size and the components of sperm competitive ability. These results suggest that natural autosomal variation for male body size has little impact on post-copulatory sexual selection. If genetic correlations exist between male body size and post-copulatory sexual selection then variation in the sex chromosomes are likely candidates, as might be expected if sexually antagonistic coevolution was responsible.     

Offspring sex ratio is related to male body size in the great tit (Parus major)

Sex allocation theory predicts that the allocation of resourcesto male and female function should depend on potential fitnessgain realized through investment in either sex. In the greattit (Parus major), a monogamous passerine bird, male resourceholdingpotential (RHP) and fertilization success both depend on malebody size (e.g., tarsus length) and plumage traits (e.g., breaststripe size). It is predicted that the proportion of sons ina brood should increase both with male body size and plumage traits,assuming that these traits show a father—offspring correlation. Thiswas confirmed in our study: the proportion of sons in the brood increasedsignificantly with male tarsus length and also, though not significantly,with the size of the breast stripe. A sex ratio bias in relationto male tarsus length was already present in the eggs because(1) the bias was similar among broods with and without mortalitybefore the nestlings' sex was determined, and (2) the bias remainedsignificant when the proportion of sons in the clutch was conservativelyestimated, assuming that differential mortality before sex determinationcaused the bias. The bias was still present among recruits.The assumption of a father—offspring correlation was confirmedfor tarsus length. Given that both RHP and fertilization successof male great tits depend on body size, and size of father andoffspring is correlated, the sex ratio bias may be adaptive.