Mechanisms of Cultural Evolution in the Songs of Wild Bird Populations

Young songbirds draw the source material for their learned songs from parents, peers, and unrelated adults, as well as from innovation. These learned songs are used for intraspecific communication, and have well-documented roles for such functions as territory maintenance and mate attraction. The songs of wild populations differ, forming local "dialects" that may shift over time, suggesting that cultural evolution is at work. Recent work has focused on the mechanisms responsible for the cultural evolution of bird songs within a population, including drift, learning biases (such as conformity and rare-form copying), and selection (including sexual selection). In many songs or song repertoires, variability is partitioned, with some songs or song segments being stable and consistent, while others vary within the population and across time, and still others undergo population-wide transitions over time. This review explores the different mechanisms that shape the cultural evolution of songs in wild populations, with specific reference to a long-term investigation of a single population of philopatric Savannah sparrows. Males learn a single four-segment song during their 1st year and sing the same song thereafter. Within this song, the buzz segment is a population marker, and may be stable for decades - variant forms occur but eventually disappear. In contrast, the middle segment is highly variable both within the population and over time; changes in relative prevalence of different forms may be due to cultural drift or a rare-form learning bias. Within the introductory segment, a high note cluster was replaced by a click train between 1982 and 2010, following an S-shaped trajectory characteristic of both selective sweeps in population genetics and the replacement of one form by another in human language. In the case of the Savannah sparrows, this replacement may have been due to sexual selection. In subsequent generations, the number of clicks within trains increased, a form of cultural directional selection. In contrast to the narrowing of a trait's range during directional selection in genetic systems, variation in the number of clicks in a train increased as the mean value shifted because improvisation during song learning allowed the range of the trait to expand. Thus, in the single short song of the Savannah sparrow, at least four different mechanisms appear to contribute to three different types of cultural evolutionary outcomes. In the future, it will be import to explore the conditions that favor the application of specific (and perhaps conditional) learning rules, and studies such as the ongoing song seeding experiment in the Kent Island Savannah sparrow population will help in understanding the mechanisms that promote or repress changes in a population's song.

The uses and implications of avian vocalizations for conservation planning

There is a growing recognition that animal behavior can affect wildlife conservation, but there have been few direct studies of animal behavior in conservation programs. However, a great deal of existing behavioral research can be applied in the context of conservation. Research on avian vocalizations provides an excellent example. The conspicuous nature of the vocal behavior of birds makes it a useful tool for monitoring populations and measuring biodiversity, but the importance of vocalizations in conservation goes beyond monitoring. Geographic song variants with population-specific signatures, or dialects, can affect territory formation and mate choice. Dialects are influenced by cultural evolution and natural selection and changes can accumulate even during the timescale of conservation interventions, such as translocations, reintroductions, and ex situ breeding. Information from existing research into avian vocalizations can be used to improve conservation planning and increase the success of interventions. Vocalizations can confer a number of benefits for conservation practitioners through monitoring, providing baseline data on populations and individuals. However, the influence of cultural variation on territory formation, mate choice, and gene flow should be taken into account because cultural differences could create obstacles for conservation programs that bring birds from multiple populations together and so reduce the success of interventions.

Black-capped chickadees (Poecile atricapillus) use temperature as a cue for reproductive timing

Reliable environmental cues, such as photoperiod, act as initial predictive cues that allow birds to time reproduction to match peak food abundance for their offspring. More variable local cues, like temperature, may, however, provide more precise information about the timing of food abundance. Non-migratory birds, in particular, should be sensitive to temperature cues and use them to modulate their reproductive timing. We conducted two experiments to examine the effect of temperature on reproductive condition (gonad size and circulating androgen levels) in non-migratory black-capped chickadees (Poecile atricapillus). First, we exposed groups of birds in outdoor aviaries to three different over-winter temperature treatments and assessed gonad size in the spring. Second, we manipulated temperature in environmental chambers under photostimulatory and non-photostimulatory photoperiodic conditions and assessed gonad size and circulating testosterone levels. Temperature had no independent effect on gonad size or testosterone levels, but when photostimulated birds exposed to warmer conditions became reproductively ready earlier than birds experiencing cooler conditions. We conclude that temperature acts as a supplementary cue that modulates the photoperiod-driven timing of reproduction.

Effects of a social cue on reproductive development and pre-alternate molt in seasonally breeding migrant and resident female songbirds (Zonotrichia leucophrys)

To time reproduction optimally, birds have evolved diverse mechanisms by which they respond to environmental changes that help them anticipate and prepare for the breeding season. While residents initiate reproductive preparation and breed in the same geographic location, migrant birds simultaneously prepare for breeding and migration far from their breeding grounds. As a result, it is hypothesized that migrant and resident birds use environmental cues differently to prepare to breed and that there is adaptive specialization in mechanisms regulating reproductive preparation. Specifically, residents are expected to rely more on non-photic cues (e.g. food, temperature, social cues) than migrants. We tested this general prediction using a social cue manipulation. First, we compared the effects of subspecies-appropriate recorded male song on reproductive development in migrants and residents on a naturally increasing photoperiod. Second, we tested the sensitivity of migrant-specific life history events (fattening and pre-alternate molt) to song treatment. After 82 days, residents had higher luteinizing hormone and greater ovarian development than migrants, but song treatment had no effect on these metrics in either subspecies. Song advanced pre-alternate molt but had no effect on fattening in migrants. While our study does not support specialization in social cue use in migrants and residents, it is consistent with findings in the literature of specialization in photoperiodic response. It also demonstrates for the first time that social cues can influence molt in a migrant species. Additional findings from a pilot study looking at responses to a live male suggest it is important to test other kinds of social cues.

Genetic population structure in an equatorial sparrow: roles for culture and geography

Female preference for local cultural traits has been proposed as a barrier to breeding among animal populations. As such, several studies have found correlations between male bird song dialects and population genetics over relatively large distances. To investigate whether female choice for local dialects could act as a barrier to breeding between nearby and contiguous populations, we tested whether variation in male song dialects explains genetic structure among eight populations of rufous-collared sparrows (Zonotrichia capensis) in Ecuador. Our study sites lay along a transect, and adjacent study sites were separated by approximately 25 km, an order of magnitude less than previously examined for this and most other species. This transect crossed an Andean ridge and through the Quijos River Valley, both of which may be barriers to gene flow. Using a variance partitioning approach, we show that song dialect is important in explaining population genetics, independent of the geographic variables: distance, the river valley and the Andean Ridge. This result is consistent with the hypothesis that song acts as a barrier to breeding among populations in close proximity. In addition, songs of contiguous populations differed by the same degree or more than between two populations previously shown to exhibit female preference for local dialect, suggesting that birds from these populations would also breed preferentially with locals. As expected, all geographic variables (distance, the river valley and the Andean Ridge) also predicted population genetic structure. Our results have important implications for the understanding whether, and at what spatial scale, culture can affect population divergence.

Sex and seasonal differences in hippocampal volume and neurogenesis in brood-parasitic brown-headed cowbirds (Molothrus ater)

Brown-headed cowbirds (Molothrus ater) are one of few species in which females show more complex space use than males. Female cowbirds search for, revisit, and parasitize host nests and, in a previous study, outperformed males on an open field spatial search task. Previous research reported a female-biased sex difference in the volume of the hippocampus, a region of the brain involved in spatial memory. Neurons produced by adult neurogenesis may be involved in the formation of new memories and replace older neurons that could cause interference in memory. We tested for sex and seasonal differences in hippocampal volume and neurogenesis of brood-parasitic brown-headed cowbirds and the closely related non-brood-parasitic red-winged blackbird (Agelaius phoeniceus) to determine whether there were differences in the hippocampus that reflected space use in the wild. Females had a larger hippocampus than males in both species, but hippocampal neurogenesis, measured by doublecortin immunoreactivity (DCX+), was greater in female than in male cowbirds in the absence of any sex difference in blackbirds, supporting the hypothesis of hippocampal specialization in female cowbirds. Cowbirds of both sexes had a larger hippocampus with greater hippocampal DCX+ than blackbirds. Hippocampus volume remained stable between breeding conditions, but DCX+ was greater post-breeding, indicating that old memories may be lost through hippocampal reorganization following breeding. Our results support, in part, the hypothesis that the hippocampus of cowbirds is specialized for brood parasitism. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1275-1290, 2016.

Is it useful to view the brain as a secondary sexual characteristic?

Many sex differences in brain and behavior related to reproduction are thought to have evolved based on sexual selection involving direct competition for mates during male-male competition and female choice. Therefore, certain aspects of brain circuitry can be viewed as secondary sexual characteristics. The study of proximate causes reveals that sex differences in the brain of mammals and birds reflect organizational and activational effects of sex steroids as articulated by Young and collaborators. However, sex differences in brain and behavior have been identified in the cognitive domain with no obvious link to reproduction. Recent views of sexual selection advocate for a broader view of how intra-sexual selection might occur including such examples as competition within female populations for resources that facilitate access to mates rather than mating competition per se. Sex differences can also come about for other reasons than sexual selection and recent work on neuroendocrine mechanisms has identified a plethora of ways that the brain can develop in a sex specific manner. Identifying the brain as sexually selected requires careful hypothesis testing so that one can link a sex-biased aspect of a neural trait to a behavior that provides an advantage in a competitive mating situation.

Modulation of sensory-motor integration as a general mechanism for context dependence of behavior

Social communication is context-dependent, with both the production of signals and the responses of receivers tailored to each animal's internal needs and external environmental conditions. We propose that this context dependence arises because of neural modulation of the sensory-motor transformation that underlies the social behavior. Neural systems that are restricted to individual behaviors may be modulated at early stages of the sensory or motor pathways for optimal energy expenditure. However, when neural systems contribute to multiple important behaviors, we argue that the sensory-motor relay is the likely site of modulation. Plasticity in the sensory-motor relay enables subtle context dependence of the social behavior while preserving other functions of the sensory and motor systems. We review evidence that the robust responses of anurans to conspecific signals are dependent on reproductive state, sex, prior experience, and current context. A well-characterized midbrain sensory-motor relay establishes signal selectivity and gates locomotive responses to sound. The social decision-making network may modulate this auditory-motor transformation to confer context dependence of anuran reproductive responses to sound. We argue that similar modulation may be a general mechanism by which vertebrates prioritize their behaviors.

Female Lincoln's sparrows modulate their behavior in response to variation in male song quality

Sexually reproducing organisms should mate with the highest quality individuals that they can. When female songbirds choose a mate, they are thought to use several aspects of male song that reflect his quality. Under resource-limited environmental conditions, male Lincoln's sparrows (Melospiza lincolnii) vary among one another in several aspects of song quality, including song length, song complexity, and trill performance. In a 2-pronged approach, we tested whether variation in song quality of male Lincoln's sparrows influences the behavior of females that are in a reproductive-like state. Over two trials, we exposed females to songs from the high and low ends of the distribution of naturally occurring song quality variation and found a higher level of behavioral activity in females exposed to high-quality songs, especially when they had first been exposed to low-quality songs. We also examined female phonotaxis toward antiphonally played songs with experimentally elevated and reduced trill performance and found that females moved preferentially toward the songs with elevated trill performance. Contrary to most studies investigating the behavioral responses of wild, female songbirds to variation in male song, we obtained our results without administering exogenous estradiol, which can artificially perturb the female's physiology. Our results demonstrate that the behavior of female Lincoln's sparrows is modulated by the quality of male songs to which they are exposed and that trill performance plays a significant role in this behavioral modulation. Furthermore, as the order of song quality presentation matters, it appears that recent song experience also influences female behavior.

Effects of social cues on GnRH-I, GnRH-II, and reproductive physiology in female house sparrows (Passer domesticus)

In all vertebrates, at least two forms of gonadotropin-releasing hormone (GnRH) are present: GnRH-I and GnRH-II. GnRH-I directly influences the reproductive axis whereas the function of GnRH-II is less clear. The present experimental objectives were to determine the effect(s) of male social cues on the peripheral and neural responses of female house sparrows (Passer domesticus). We hypothesized that male breeding status would significantly influence the amount of immunoreactive GnRH-II in female house sparrow brains. In order to test this hypothesis, females were caged with a breeding male, a non-breeding male, or caged alone. The presence of breeding males did not significantly influence ovary development, luteinizing hormone, or estradiol levels, but male presence increased female body mass, and male presence and condition interacted to influence ovarian follicle size. Using immunocytochemistry, GnRH-I and GnRH-II immunoreactivity was measured in order to evaluate the neuroendocrine response to breeding status in males. When females were housed with breeding males, there were stable numbers of immunoreactive GnRH-I and -II cells but significantly lower amounts of immunoreactive GnRH-I fibre staining within the preoptic area compared to females housed with non-breeding males. Moreover, immunoreactive GnRH-II fibres in the preoptic area, ventromedial nucleus, and medial septum were significantly greater in females housed alone in chamber with non-breeding males. The data demonstrate that the GnRH system in songbirds is modulated by social context. These finding provide novel insight into the mechanisms involved with regulating avian reproductive physiology.

Auditory stimulation of reproductive function in male Rufous-winged Sparrows, Aimophila carpalis

Prolonged exposure to conspecific song stimulates gonadal function and reproductive hormone secretion in female birds but few studies have investigated the physiological effects of conspecific song exposure on males outside of short-term, aggressive interactions. We exposed male Rufous-winged Sparrows, Aimophila carpalis, either to conspecific song (CS Song), to heterospecific song (Black-throated Sparrow, Amphispiza bilineata; HS Song), or to no recorded song (No Song) for 59 consecutive days (two h per day). Birds were exposed to short days (8L:16D) for the first 21 days of treatment and were then transferred to long days (13L:11D) for the remaining 38 days. During long day exposure, CS Song birds experienced faster growth of testes than HS Song and No Song birds. HS Song birds also grew their testes faster than No Song birds. Plasma luteinizing hormone (LH) and testosterone did not differ between CS Song and No Song birds. However, plasma LH was higher in HS Song birds compared to other groups. There were no differences in hypothalamic immunocytochemical labeling for gonadotropin-releasing hormone, its precursor proGnRH, or gonadotropin-inhibitory hormone, nor were there differences in two song control nuclei volumes (HVC and RA) between CS Song and No Song treatment groups. Furthermore, we found no effect of heterospecific song on free-living Rufous-winged Sparrow aggressive behaviors. These data indicate that long-term exposure to auditory stimuli, such as song, can influence the reproductive system of male songbirds and different types of auditory stimuli can have differential effects on reproductive function.

Effects of learning on song preferences and Zenk expression in female songbirds

Male songbirds learn to produce their songs, and females attend to these songs during mate choice. The evidence that female song preferences are learned early in life, however, is mixed. Here we review studies that have found effects of early song learning on adult song preferences, and those that have not. In at least some species, early experience with song can modify adult song preferences. Whether this learning needs to occur during an early sensitive phase, akin to male imitative vocal learning, or not remains an open question. Studies of the neural bases for female song preferences highlight activity (as measured by immediate-early gene induction) in regions of the auditory forebrain as often, but not always, being associated with song preferences. Immediate-early gene induction in these regions, however, is not specific to songs experienced early in life. On the whole, inherited factors, early experience, and adult experience all appear to play a role in shaping female songbirds preferences for male songs.

Complementary neural systems for the experience-dependent integration of mate-choice cues in European starlings

Choice of a particular mate phenotype may arise out of experience with the very phenotypes under consideration. Female European starlings (Sturnus vulgaris) prefer males that sing predominantly long-bout songs over males that sing predominantly short-bout songs, and thus, song-bout length is a phenotypic parameter instrumental in releasing the female's mate choice. The preferred long-bout songs induce higher expression of the immediate early gene (IEG) ZENK in the female auditory telencephalon than short-bout songs do, but this sensitivity to song length depends on the female's recent song experience. Here, we compared the experience-dependent modulation of ZENK with that of another IEG, FOS, and report that ZENK and FOS expression in the caudomedial mesopallium and caudomedial nidopallium show different modulation properties that complement natural variation in song-bout length. As reported previously, ZENK expression was greater in response to novel long-bout than to novel short-bout songs following a 1-week experience with long-bout but not short-bout songs. In contrast, FOS expression was greater in response to novel long-bout than to novel short-bout songs following a 1-week experience with short-bout but not long-bout songs. Thus, the ZENK and FOS signaling pathways are made sensitive to variation in song length by experiences with songs at opposite ends of the starling song-variation continuum, suggesting the presence of complementary neural systems made sensitive in register with the natural axis of phenotypic variation fundamental to the female's mate choice.

Effects of early song experience on song preferences and song control and auditory brain regions in female house finches (Carpodacus mexicanus)

We examined the effects of song tutoring on adult song preferences, volume of song-control brain regions, and activity of auditory brain regions in female house finches (Carpodacus mexicanus). Hand-reared females were tutored with local songs, foreign songs, or no song. We then examined adult song preferences, determined the Nissl-defined volume of the song-control nuclei HVc, Area X, and RA, and compared the number of cells immunoreactive for Zenk protein in the auditory regions NCM and cmHV, following playback of songs heard early in life (Tutor/Playback Match) versus not heard (Tutor/Playback Nonmatch). All hand-reared birds exhibited preferences for locally recorded song over foreign or heterospecific song. We found no difference in the volume of song-control nuclei among the three groups. As well, we found no difference in the number of Zenk immunoreactive cells in NCM and cmHV between females in the Tutor/Playback Match group and females in the Tutor/Playback Nonmatch group. Isolate-reared birds showed greater Zenk immunoreactivity following song playback than either tutored group. Thus, early auditory experience may not play a role in adult geographic song preferences, suggesting that genetic factors can lead to preferences for songs of local dialects. Song tutoring did not influence the size of song-control regions nor Zenk induction levels following song playback, suggesting that early experience with particular songs does not influence Zenk expression. However, overall greater activation in isolate females in auditory areas suggests that exposure to song early in life may increase the selectivity of Zenk activation to song playback in auditory areas.

Recent experience modulates forebrain gene-expression in response to mate-choice cues in European starlings

Mate-choice decisions can be experience dependent, but we know little about how the brain processes stimuli that release such decisions. Female European starlings (Sturnus vulgaris) prefer males with long-bout songs over males with short-bout songs, and show higher expression of the immediate early gene (IEG) ZENK in the auditory forebrain when exposed to long-bout songs than when exposed to short-bout songs. We exposed female starlings to a short-day photoperiod for one of three durations and then, on an increased photophase, exposed them to one week of long-bout or short-bout song experience. We then examined their IEG response to novel long-bout versus novel short-bout songs by quantifying ZENK protein in two song-processing areas: the caudo-medial hyperstriatum ventrale and the caudo-medial neostriatum. ZENK expression in both areas increased with tenure on short-day photoperiods, suggesting that short days sensitize females to song. The ZENK response bias toward long-bout songs was greater in females with long-bout experience than in females with short-bout experience, indicating that the forebrain response bias toward a preferred trait depends on recent experience with that category of trait. This surprising level of neuroplasticity is immediately relevant to the natural history and fitness of the organism, and may underlie a mechanism for optimizing mate-choice criteria amidst locally variable distributions of secondary sexual characteristics.

Bird song, ecology and speciation

The study of bird song dialects was once considered the most promising approach for investigating the role of behaviour in reproductive divergence and speciation. However, after a series of studies yielding conflicting results, research in the field slowed significantly. Recent findings, on how ecological factors may lead to divergence in both song and morphology, necessitate a re-examination. We focus primarily on species with learned song, examine conflicting results in the literature and propose some potential new directions for future studies. We believe an integrative approach, including an examination of the role of ecology in divergent selection, is essential for gaining insight into the role of song in the evolution of assortative mating. Habitat-dependent selection on both song and fitness-related characteristics can lead to parallel divergence in these traits. Song may, therefore, provide females with acoustic cues to find males that are most fit for a particular habitat. In analysing the role of song learning in reproductive divergence, we focus on post-dispersal plasticity in a conceptual framework. We argue that song learning may initially constrain reproductive divergence, while in the later stages of population divergence it may promote speciation.