Dispersal influences genetic and acoustic spatial structure for both males and females in a tropical songbird

Animals exhibit diverse dispersal strategies, including sex‐biased dispersal, a phenomenon common in vertebrates. Dispersal influences the genetic structure of populations as well as geographic variation in phenotypic traits. Patterns of spatial genetic structure and geographic variation may vary between the sexes whenever males and females exhibit different dispersal behaviors. Here, we examine dispersal, spatial genetic structure, and spatial acoustic structure in Rufous‐and‐white Wrens, a year‐round resident tropical bird. Both sexes sing in this species, allowing us to compare acoustic variation between males and females and examine the relationship between dispersal and song sharing for both sexes. Using a long‐term dataset collected over an 11‐year period, we used banding data and molecular genetic analyses to quantify natal and breeding dispersal distance in Rufous‐and‐white Wrens. We quantified song sharing and examined whether sharing varied with dispersal distance, for both males and females. Observational data and molecular genetic analyses indicate that dispersal is female‐biased. Females dispersed farther from natal territories than males, and more often between breeding territories than males. Furthermore, females showed no significant spatial genetic structure, consistent with expectations, whereas males showed significant spatial genetic structure. Overall, natal dispersal appears to have more influence than breeding dispersal on spatial genetic structure and spatial acoustic structure, given that the majority of breeding dispersal events resulted in individuals moving only short distances. Song sharing between pairs of same‐sex animals decreases with the distance between their territories for both males and females, although males exhibited significantly greater song sharing than females. Lastly, we measured the relationship between natal dispersal distance and song sharing. We found that sons shared fewer songs with their fathers the farther they dispersed from their natal territories, but that song sharing between daughters and mothers was not significantly correlated with natal dispersal distance. Our results reveal cultural differences between the sexes, suggesting a relationship between culture and sex‐biased dispersal.     

Digit ratios, secondary sexual characters and condition in barn swallows Hirundo rustica

The second and fourth digit length ratio (2D:4D) is sexuallydimorphic in many vertebrates. This ratio has been suggestedto provide an estimate of steroid levels encountered duringprenatal development, which may have organizational consequencesfor physiology and behavior of adults. However, recent studiesshowed that the relation between digit ratio and steroids seemsinconsistent and may vary among species and populations. Wetested the hypothesis that digit ratios would be correlatedwith the expression of secondary sexual characters, using thebarn swallow (Hirundo rustica) as a model system. This was doneby testing whether variation in 2D:4D ratio was correlated withtail length and features of song, which are important secondarysexual characters positively correlated with circulating steroidconcentration in adult birds. Furthermore, we examined the predictionthat male and female digit ratios would correlate with bodymass in an antagonistic way. There was no significant sexualdimorphism in digit ratio, which may be due to low levels ofsexual selection in this population. Adult right 2D:4D ratiowas negatively linked to tail length but not to male song output.Moreover, right 2D:4D ratio was negatively correlated with bodymass in male and positively in females. These results are consistentwith high digit ratios reflecting low levels of testosteronein this bird species.     

Sheep in wolf's clothing: host nestling vocalizations resemble their cowbird competitor's

Nestlings of many avian brood parasites are virtuosos at mimicking host nestling vocalizations, which, like egg mimicry, presumably ensures acceptance by host parents. Having been accepted, parasitic nestlings then often exaggerate the aspects of the host's display to increase parental care. Host nestlings may, in turn, exaggerate their vocalizations to keep up with the parasite, though this possibility has not been evaluated. We experimentally parasitized song sparrow (Melospiza melodia) nests with a brown-headed cowbird (Molothrus ater) chick to evaluate how host nestlings respond. Vocalizations emitted from experimentally parasitized nests were higher in frequency, and louder, than those from unparasitized nests, consistent with the cowbird exaggerating its signalling. In response, host nestlings exaggerated the frequency and amplitude of their vocalizations, such that they resembled the cowbird's while they 'scaled back' on calls per parental provisioning bout. Sparrows in parasitized nests were fed equally often as sparrows in unparasitized nests, suggesting that exaggerating some aspects of vocalization while scaling back on others can help host nestlings confronted with a cowbird. Our results support the recently proposed hypothesis that signalling in parasitized nests involves a dynamic interaction between parasitic and host nestlings, rather than a one-way process of mimicry by the parasite.     

Behavioral Sequence Leading to Sexual Isolation Between <Emphasis Type="Italic">Drosophila ananassae</Emphasis> and <Emphasis Type="Italic">D. pallidosa</Emphasis>

Drosophila ananassae and D. pallidosa are closely related, sympatric species that lack postmating isolation. Sexual isolation has been considered important in maintaining them as independent species. To clarify the behavioral processes leading to sexual isolation, we analyzed behavioral sequences and examined the effect of courtship song on mating success and on behaviors of both sexes by surgically removing male wings (song generators), female aristae (song receivers), or female wings (means of fluttering). We found that heterospecific courtship songs evoked female wing fluttering, whereas conspecific courtship song did not. Furthermore, female wing fluttering made courting males discontinue courtship. These findings suggest that strong sexual isolation is achieved through the following behavioral sequence: heterospecific song→female wing fluttering→courtship discontinuation.     

Yolk testosterone, postnatal growth and song in male canaries

Avian eggs contain substantial amounts of maternal yolk androgens, which have been shown to modulate offspring phenotype. The first studies on the functional consequences of maternal yolk androgens have focused on early life stages and their role in sibling competition. However, recent longitudinal studies reported long-lasting effects of maternal yolk androgens on offspring phenotype, mostly concerning traits that are sensitive to androgens. This suggests that maternal yolk androgens could play an important role in sexual selection, since the expression of many male sexual characters is testosterone-dependent. Using male canaries as a model, we examined the consequences of an experimental elevation of yolk testosterone concentrations on early development as well as long-lasting effects particularly on song, which is one of the most important sexual characters in male songbirds. Elevated yolk testosterone concentrations inhibited male growth, possibly in interaction with an existent ectoparasite exposure. Males hatched from testosterone-treated eggs (T-males) did not have enhanced competitive skills, in contrast to previous studies. The elevation of yolk testosterone concentrations delayed song development but did not affect adult song phenotype. This is intriguing, as yolk testosterone possibly induced developmental stress, which is known to reduce song quality. We hypothesize that yolk testosterone has either no direct effect on adult song phenotype, or that positive effects are merged by the negative effects of developmental stress. Finally, females mated with T-males invested more in their clutch indicating that females either assess T-males as more attractive (differential allocation hypothesis) or compensated for lower offspring viability (compensation hypothesis).     

Elevated yolk androgen levels and the expression of multiple sexually selected male characters

Maternal hormones in bird eggs modulate not only offspring development, but recently it has also been shown that these effects can persist into adult life. A number of long-lasting effects concern traits of which the expression or development is modulated by androgens. This suggests that the nature of yolk hormone-mediated maternal effects may be organizational. Maternal androgens may therefore play an important role in sexual selection, since the expression of sexually selected male characters is often androgen-dependent.We experimentally manipulated the yolk androgen concentrations of European Starling (Sturnus vulgaris) eggs. Subsequently we followed 49 unrelated males from hatching until year of first reproduction. We investigated the expression of multiple sexually selected male characters (song, beak color and throat feather characteristics), taking into account whether a trait is androgen-dependent.Elevated levels of yolk androgens affected the length of the embryonic period, but did not modify the expression of either androgen-dependent or androgen-independent sexually selected male characters including song phenotype at adulthood. Thus the most important function of yolk androgens in starlings and possibly other bird species may relate to the early developmental period. The outcome of our study together with the results of our meta-analysis indicates that the effects of yolk androgens on sexually selected male characters may be comparatively small. Our results suggest that this may relate to the numerous other environmental and/or genetic factors influencing the expression of sexually selected male characters.     

Effects of urban noise on song and response behaviour in great tits

Acoustic communication is fundamental in avian territory defence and mate attraction. In urban environments where sound transmissions are more likely to be masked by low-frequency anthropogenic noise, acoustic adaptations may be advantageous. However, minor modifications to a signal could affect its efficacy. While recent research has shown that there is divergence between songs from noisy and quiet areas, it is unknown whether these differences affect the response to the signal by its receivers. Here, we show that there is a difference in spectral aspects of rural and urban song in a common passerine, the great tit Parus major, at 20 sites across the UK. We also provide, to our knowledge, the first demonstration that such environmentally induced differences in song influence the response of male territory holders. Males from quiet territories exhibited a significantly stronger response when hearing song from another territory holder with low background noise than from those with high background noise. The opposite distinction in response intensity to homotypic versus heterotypic song was observed in males from noisy territories. This behavioural difference may intensify further signal divergence between urban and rural populations and raises important questions concerning signal evolution.     

Mate choice for nonadditive genetic benefits and the maintenance of genetic diversity in song sparrows

The lek paradox asserts that strong directional selection via female choice should deplete additive genetic variation in fitness and consequently any benefit to females expressing the preference. Recently, we have provided a novel resolution to the paradox by showing that nonadditive genetic effects such as overdominance can be inherited from parent to offspring, and populations with females that express a mating preference for outbred males maintain higher genetic variation than populations with females that mate randomly. Here, we test our dynamic model using empirical data previously published from a small island population of song sparrows (Melospiza melodia). The model assumes that fitness and male trait expression display overdominance effects. The results demonstrate that female choice for outbred males mediated by directional selection on song repertoire size provides a heritable benefit to offspring through reduced inbreeding depression. Within the population, we estimate the heritability of the inbreeding coefficient to be 0.18 ± 0.08 (SD). Furthermore, we show that mate choice for outbred males increases fitness‐related genetic variation in the population by 12% and thereby reduces inbreeding depression by 1% per generation in typical years and upwards of 15% in severe years. Thus, mate choice may help to stave off population extinction in this and other small populations.     

The deterrent effect of bird song in territory defense

Using the responses of territory owners to playback to inferthe territorial function of acoustic signals is common practice.However, difficulties with interpreting the results of suchexperiments have obscured our understanding of territorial signalling.For instance, a stronger response to playback is often interpretedas more aggressive, but there is no consensus as to whetherthis should be in response to the least or most threateningsimulated intruder. Rather than following a gradual increaseor decrease, the relationship between signal intensity and responsestrength may instead describe a peaked curve. We manipulatedbanded wren (Thryophilus pleurostictus) songs to simulate low-,median-, and high-performance singers and used these songs asstimuli in playback experiments. Banded wrens were less likelyto approach the high-performance stimulus compared with thelow- and median-performance stimuli. However, the birds thatdid approach the high-performance stimulus sang more than thosethat approached the low-performance stimulus. In addition, birdswere more likely to match the songs when exposed to the median-and high-performance stimuli compared with the low-performancestimuli, and song matching predicted approach behavior. Theseresults are in accordance with theoretical models of aggressiveencounters in which low-performance opponents are challengedwithout further assessment. Median- and high-performance opponents,however, may require further assessment, and the latter maybe perceived as too intimidating for approach.     

鸟类鸣声地理变异的形成机制

鸣声在鸟类的通讯和性选择中具有重要的意义.鸣声地理变异广泛存在于鸟类中,对其研究有助于了解近缘种的分类和演化历史.鸣声地理变异格局的形成是一个复杂的过程,可能是某一因素起主导、多因素协同作用的结果.主要影响因素有奠基者效应、鸣声漂变、遗传差异、性选择、生境等.年际间鸣声的稳定性和种群内鸣声的变异性,是鸣声地理变异研究中应该重视的问题,通过鸣声学习和扩散机制来研究鸣声地理变异,可能是将来本领域的研究热点.