Environmental regulation of the reproductive system in a flexibly breeding Sonoran Desert bird, the Rufous-winged Sparrow, Aimophila carpalis

We investigated reproductive regulation in male Rufous-winged Sparrows, Aimophila carpalis, a Sonoran Desert passerine that breeds after irregular summer rains. Field and captive data demonstrate that increased photoperiod stimulates testicular development in March and maintains it until early September. Free-living birds caught in July and placed on captive long days (16L: 8D) maintained developed testes for up to 7 months, and free-living birds caught in September, during testicular regression, redeveloped testes when placed on captive long days, indicating that these birds were still photosensitive. Captive birds on long days maintained testicular development when exposed to temperatures mimicking those occurring during regression in free-living birds. In free-living birds, testicular development was observed during spring and summer, but unless this was associated with rainfall, breeding (indicated by juveniles) did not occur. Large increases in plasma luteinizing hormone (LH) in free-living males were correlated with heavy rainfall in July/August, when the birds bred, and in November, when they did not breed. In captive birds, plasma LH concentrations were unresponsive to photoperiodic changes, but may have responded to social cues. Plasma prolactin concentrations were directly correlated with photoperiod in free-living birds, but an effect of photoperiod on prolactin secretion was not seen in captive birds. It is concluded that male Rufous-winged Sparrows use long photoperiods to stimulate and maintain testicular development, but exposure to long photoperiods does not terminate breeding by inducing absolute photorefractoriness. The specific timing of reproductive behaviors is apparently determined by elevated plasma LH coinciding with long day stimulated gonad development.     

Stimulatory effects on the reproductive axis in female songbirds by conspecific and heterospecific male song

Courtship vocalizations of male songbirds can profoundly enhance the reproductive physiology and behavior of conspecific females. However, no study has fully investigated the selectivity of conspecific song effects on reproductive development in birds. We studied the effects of conspecific and heterospecific song on reproductive development in domesticated (canaries) and wild songbirds (song sparrows). As expected, conspecific song enhanced follicular development. Unexpectedly, however, birds exposed to heterospecific song also underwent enhanced follicular development (compared to birds exposed to no song); conspecific and heterospecific songs were equally effective in enhancing ovarian development. In canaries exposed to 18L:6D, conspecific song induced oviposition earlier and at a greater frequency than in heterospecific and no song groups, with the fewest eggs being laid in the no song group. These results indicate that conspecific and heterospecific male song can enhance reproductive activity in female songbirds. Whether or not activation of the reproductive axis in female songbirds by heterospecific song occurs in the wild remains unclear. It is also unclear as to whether the ability of the reproductive axis to respond to heterospecific song performs a specific function, or whether it is simply a consequence of greater selection pressure acting upon behavioral responses to song.     

Interspecific song imitation by a Prairie Warbler

Song development in oscine songbirds relies on imitation of adult singers and thus leaves developing birds vulnerable to potentially costly errors caused by imitation of inappropriate models, such as the songs of other species. In May and June 2012, we recorded the songs of a bird that made such an error: a male Prairie Warbler (Setophaga discolor) in western Massachusetts that sang songs seemingly acquired by imitating the songs of a Field Sparrow (Spizella pusilla). Another song type in the bird's repertoire was a near‐normal Group A Prairie Warbler song, but the bird used this song in contexts normally reserved for Group B songs. Despite its abnormal singing behavior, the aberrant bird successfully defended a territory and attracted a mate that laid two clutches of eggs. Results of playbacks of the focal bird's heterospecific song suggested that neighboring conspecific males learned to associate the Field Sparrow‐like song with the focal male, and responded to the song as if it were a Prairie Warbler song. Our evidence suggests that the focal bird's aberrant singing evoked normal responses from potential mates and rivals. If such responses are widespread among songbirds, the general failure of heterospecific songs, once acquired, to spread through populations by cultural transmission is probably not attributable to a lack of recognition by conspecifics of the songs of heterospecific singers.     

Plasticity of the Rufous-winged Sparrow, Aimophila carpalis, song control regions during the monsoon-associated summer breeding period

In most temperate zone songbirds, exposure to increasing photoperiod in the spring stimulates the reproductive system and induces reproductive behaviors. Additionally, the brain regions that control singing (song control regions; SCRs) are larger during the breeding season, thus paralleling changes in the activity of the reproductive system. However, in some birds, environmental factors other than photoperiod initiate breeding. For example, free-living male Rufous-winged Sparrows develop their testes in March due to increasing photoperiod, but have relatively low plasma T until after they begin to breed, usually in July, during the monsoon period when day length is declining. We tested the hypothesis that SCRs grow and singing behavior increases after the monsoon rains begin. We captured adult male Rufous-winged Sparrows in July 2002, 7 days before and 20 days after the monsoon rains began, euthanized birds in the field, collected their brains, and measured SCR volumes from sections immunostained for the neuronal marker NeuN. In June and July 2006, we measured song rates in the field before and after the monsoon rains. SCR volumes were larger and singing behavior increased after the onset of the monsoon rains, coinciding with the initiation of breeding. Unlike in other species studied so far, SCR volumes grew as day length was decreasing. Comparative studies utilizing species that do not breed when day length is increasing may provide information on the relative contributions of various environmental factors to SCR neuroplasticity.     

Immediate early gene (ZENK) responses to song in juvenile female and male zebra finches: effects of rearing environment

Accurate song perception is likely to be as important for female songbirds as it is for male songbirds. Male zebra finches (Taeniopygia guttata) show differential ZENK expression to conspecific and heterospecific songs by day 30 posthatch in auditory perceptual brain regions such as the caudomedial nidopallium (NCM) and the caudomedial mesopallium (CMM). The current study examined ZENK expression in response to songs of different qualities at day 45 posthatch in both sexes. Normally reared juvenile zebra finches showed higher densities of immunopositive nuclei in both the dorsal and ventral areas of NCM and CMM (formerly cmHV), but not HA, a visual area, in response to normal song over untutored song or silence. Male and female patterns of ZENK expression did not differ. We next compared responses of birds reared without exposure to normal song (untutored) to those of normally reared birds. Untutored birds did not show higher responses to normal song than to untutored song in the three song perception areas. Furthermore, untutored birds of both sexes showed lower densities of immunopositive nuclei in all four areas than did normally reared birds. In addition, ZENK expression was greater in untutored females than in males in the dorsal portion of NCM and in CMM. Our findings suggest that at least some neural mechanisms of song perception are in place in socially reared female and male finches at an early age. Furthermore, early exposure to song tutors affects responses to song stimuli.     

The Role of Song Phonology and Syntax in Vocal Learning Preferences in the Song Sparrow,Melospiza melodia

Are young songbirds ready to learn virtually any song, or are they predisposed to learn songs of their own species? To explore this question tests were conducted on the equipotentiality of auditory song learning stimuli in the song sparrow. 23 males reared as nestlings were exposed to tape recordings of their own and other species songs in early life and subsequent song production was analyzed for imitations. Birds exposed to natural song sparrow songs, including their fathers', and equal numbers of swamp sparrow songs, strongly preferred conspecific songs. They neither favored nor eschewed paternal songs despite having had access to them for 6–10 days as nestlings. In three other experiments synthetic songs were used in which some properties were held constant and others were systematically varied. Birds were exposed to 1–4 segmented songs varying in phrase order, tempo and syllable number, each synthesized in two versions, one from conspecific and the other from heterospecific (swamp sparrow) song syllables. With one-segmented songs (alien syntax) subjects favored conspecific over heterospecific syllable songs. Heterospecific syllables were rendered more acceptable by incorporation into two-segmented trilled songs (more song sparrow-like syntax). Heterogeneous summation of phonological and syntactical cues appeared to occur. There was also evidence of interaction between phonology and syntax. When another phrase type, the note complex, was added, in three- and four-segmented songs, a preference for conspecific syllables reappeared. Heterospecific syllables may be more readily accepted as a trilled sequence than without repetition, as in a note-complex. When phrase structure within four-segmented songs was varied, birds favored patterns most like normal conspecific song. We conclude that there are innate learning preferences in the song sparrow, based on note and syllabic structure (phonology and syllabic syntax), and temporal organization of phrases (segmental syntax), differing from those of the closely related swamp sparrow, Melospiza georgiana, in which song syntax plays no role in learning preferences.     

Sexual differentiation and seasonal variation in response to conspecific and heterospecific acoustic signals

Interspecific territoriality is frequently reported between closely related species; however, few studies have demonstrated interspecific territoriality between distantly related species living in sympatry. We conducted playback experiments to investigate territorial behaviour in male and female White‐bellied Wrens (Uropsila leucogastra) in response to simulated conspecific and heterospecific intruders during the breeding and non‐breeding seasons. We explored whether heterospecific songs of the Happy Wren (Pheugopedius felix), a distantly related species and ecological competitor, elicited antagonistic responses from focal White‐bellied Wrens, and whether such responses differed between the sexes. We also examined whether male and female responses to conspecific and heterospecific rivals varied with season. We found that male White‐bellied Wrens always responded to conspecific song, and responded significantly more to heterospecific song compared to a control stimulus (Tropical Parula, Setophaga pitiayumi). In contrast, although female White‐bellied Wrens responded strongly to conspecific song, their response to heterospecific song did not differ significantly from the control stimulus. The proportion of males that responded to heterospecific songs and the proportion of females that responded to conspecific songs varied seasonally, showing significantly lower responses during the breeding season. The intense responses of male White‐bellied Wrens to playback of heterospecific songs suggest that they recognise ecological competitors based on their vocal signals. Furthermore, the decrease in agonistic interactions during the breeding season is in line with the hypothesis that aggressive behaviour may be detrimental to reproductive and parental activity, and the hypothesis that heterospecific animals pose less of a threat during the breeding season.     

Cognitive tactics of Bengalese finch (<Emphasis Type="Italic">Lonchura striata </Emphasis>var. <Emphasis Type="Italic">domestica</Emphasis>) for song discrimination in a go/no-go operant task

Twelve Bengalese finches (Lonchura striata var. domestica) were trained to discriminate between a conspecific and a heterospecific song in a go/no-go operant task. Training the birds to go for the conspecifics song or to go for the heterospecific song required the same number of training sessions. Nine possible cognitive tactics could be used to solve this task, but probe tests revealed that the birds used only four. Six birds memorized only the “no-go” stimulus and responded to the rest of the stimuli (no-go memory), two birds classified songs according to the species category (open-ended categorization), one bird memorized both of the training stimuli but responded by chance to the probe stimuli (rote categorization), and two birds combined open-ended and rote categorization tactics (combined categorization). These tactics were related to the number of sessions needed to reach the species-discrimination criterion. Our results suggest that investigators should consider individual cognitive tactics and the pitfalls of go/no-go discrimination when interpreting the results of operant discrimination tasks.     

Developmental experience alters information coding in auditory midbrain and forebrain neurons

In songbirds, species identity and developmental experience shape vocal behavior and behavioral responses to vocalizations. The interaction of species identity and developmental experience may also shape the coding properties of sensory neurons. We tested whether responses of auditory midbrain and forebrain neurons to songs differed between species and between groups of conspecific birds with different developmental exposure to song. We also compared responses of individual neurons to conspecific and heterospecific songs. Zebra and Bengalese finches that were raised and tutored by conspecific birds, and zebra finches that were cross‐tutored by Bengalese finches were studied. Single‐unit responses to zebra and Bengalese finch songs were recorded and analyzed by calculating mutual information (MI), response reliability, mean spike rate, fluctuations in time‐varying spike rate, distributions of time‐varying spike rates, and neural discrimination of individual songs. MI quantifies a response's capacity to encode information about a stimulus. In midbrain and forebrain neurons, MI was significantly higher in normal zebra finch neurons than in Bengalese finch and cross‐tutored zebra finch neurons, but not between Bengalese finch and cross‐tutored zebra finch neurons. Information rate differences were largely due to spike rate differences. MI did not differ between responses to conspecific and heterospecific songs. Therefore, neurons from normal zebra finches encoded more information about songs than did neurons from other birds, but conspecific and heterospecific songs were encoded equally. Neural discrimination of songs and MI were highly correlated. Results demonstrate that developmental exposure to vocalizations shapes the information coding properties of songbird auditory neurons. © 2009 Wiley Periodicals, Inc. Develop Neurobiol 70: 235–252, 2010.     

Changes in plasma levels of testosterone during male-male interactions in the song sparrow,Melospiza melodia: time course and specificity of response

Summary Previous investigations have shown that malemale interactions over territory boundaries result in an increase in plasma levels of testosterone (T). In order to determine the time course of this increase following an agonistic challenge, male song sparrows, Zonotrichia (= Melospiza) melodia, were exposed to simulated territorial intrusions. Responding males were captured at intervals after onset of the intrusion for up to 60 min. Plasma levels of T were elevated by 10 min after onset of the challenge, and remained high in birds sampled 10–60 min after onset of the intrusion. In addition, plasma levels of luteinizing hormone (LH) increased and were significantly elevated after 10 min of stimulation. The results suggest that at least the prolonged elevation of T secretion precipitated by a male-male interaction was mediated via an increase in LH release. However, the initial surge of T levels in plasma may be LH independent. p Investigations of captive birds confirmed that circulating LH and T levels increased when male song sparrows were challenged by a conspecific male. Challenges involving a male house sparrow, Passer domesticus, had no effect suggesting that responses to male intrusions were species characteristic and not a result of general arousal. Further, auditory stimuli (vocalizations) alone, and visual stimuli (a devocalized male) alone were less effective in stimulating an increase in secretion of T than a combination of auditory and visual stimuli.Abbreviations DHT dihydrotestosterone - LH luteinizing hormon; - T testosterone     

Up to the challenge? Hormonal and behavioral responses of free-ranging male Cassin's Sparrows,Peucaea cassinii,to conspecific song playback

The Challenge Hypothesis postulates that male vertebrates can respond to social challenges, such as simulated territorial intrusions, by rapidly increasing their concentrations of plasma androgens, such as testosterone (T). This increase may facilitate the expression of aggressive behavior and lead to persistence of this behavior even after withdrawal of the challenge, thus potentially promoting territoriality and the probability of winning future challenges. The scope of the Challenge Hypothesis was tested by exposing free-ranging male Cassin's Sparrows, Peucaea cassinii, to conspecific song playback (SPB) at the beginning of the vernal nesting season. Exposure to SPB stimulated aggressive behavior but did not influence plasma T. Furthermore, plasma T did not correlate with the duration of exposure to SPB, and the behavioral response to SPB did not differ in males that were challenged a second time shortly after the first challenge. As birds were investigated at a stage of their reproductive cycle when plasma T is presumably seasonally high due to photostimulation, the lack of hormonal response to SPB may have been due to the hypothalamus-pituitary-gonadal axis secreting hormones at maximum rates. This was not the case, however, because administration of gonadotropin-releasing hormone I rapidly stimulated the secretion of luteinizing hormone (LH) and T, and treatment with ovine LH rapidly stimulated T secretion.     

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.     

Discrimination of calling song models by the cricket,Teleogryllus oceanicus: the influence of sound direction on neural encoding of the stimulus temporal pattern and on phonotactic behavior

Summary Recordings were made from an identified auditory neuron, the omega neuron, in the cricketTeleogryllus oceanicus. Models of the conspecific calling song and of the song of another species were presented either singly or simultaneously, and the degree to which the temporal pattern of the conspecific model was encoded in the neuron's spike train was determined. When a single stimulus was presented alone, its temporal pattern was faithfully reflected by the cells's spiking activity, no matter what the azimuth of the broadcasting loudspeaker (Fig. 3). When two stimuli were presented simultaneously from opposite sides, encoding of the pattern ipsilateral to the recorded neuron was interfered with only slightly by the contralateral pattern, as long as the two loudspeakers were sufficiently separated (Figs. 2, 3, 4). When the loudspeakers were each 15° from the cricket's midline, however, the encoding of the temporal pattern of the ipsilateral song model was severely disrupted (Figs. 3, 4). Bilateral interactions are important in determining the response level of the neuron, but do not appear to contribute to the direction-selective encoding of the stimulus temporal pattern (Figs. 5, 6).Phonotactic steering movements of tethered, flying crickets were recorded under stimulus conditions similar to those used in the neurophysiological tests. Under one-stimulus conditions, crickets attempted to turn towards the conspecific model for all tested speaker locations. The heterospecific model elicited reliable steering behavior when it was broadcast from azimuths of 90° and 60°, but often failed to elicit consistent responses when the speaker was positioned closer to the cricket's midline (Figs. 7, 8A and 8B). Responses to the heterospecific pattern were smaller in amplitude than those to the conspecific song model (Figs. 7, 8B). Under two-stimulus conditions, the conspecific model was consistently preferred over the heterospecific song for all tested speaker locations in half the tested crickets. In the remaining animals, preference for the conspecific pattern was only evident for the larger loudspeaker azimuths (Figs. 7, 8C).These results demonstrate that simultaneouslypresented stimuli can be represented separately in the nervous system as a consequence of auditory directionality. It is postulated that the cricket's ability to choose between these stimuli may result from the interactions between two bilaterallypaired song recognizers, each of which may be driven primarily by sound stimuli from one side.     

Altered auditory BOLD response to conspecific birdsong in zebra finches with stuttered syllables

How well a songbird learns a song appears to depend on the formation of a robust auditory template of its tutor's song. Using functional magnetic resonance neuroimaging we examine auditory responses in two groups of zebra finches that differ in the type of song they sing after being tutored by birds producing stuttering-like syllable repetitions in their songs. We find that birds that learn to produce the stuttered syntax show attenuated blood oxygenation level-dependent (BOLD) responses to tutor's song, and more pronounced responses to conspecific song primarily in the auditory area field L of the avian forebrain, when compared to birds that produce normal song. These findings are consistent with the presence of a sensory song template critical for song learning in auditory areas of the zebra finch forebrain. In addition, they suggest a relationship between an altered response related to familiarity and/or saliency of song stimuli and the production of variant songs with stuttered syllables.     

The effect of auditory distractors on song discrimination in male canaries (Serinus canaria)

Male songbirds such as canaries produce complex learned vocalizations that are used in the context of mate attraction and territory defense. Successful mate attraction or territorial defense requires that a bird be able to recognize individuals based on their vocal performance and identify these songs in a noisy background. In order to learn more about how birds are able to solve this problem, we investigated, with a two-alternative choice procedure, the ability of adult male canaries to discriminate between conspecific song segments from two different birds and to maintain this discrimination when conspecific songs are superimposed with a variety of distractors. The results indicate that male canaries have the ability to discriminate, with a high level of accuracy song segments produced by two different conspecific birds. Song discrimination was partially maintained when the stimuli were masked by auditory distractors, but the accuracy of the discrimination progressively declined as a function of the number of masking distractors. The type of distractor used in the experiments (other conspecific songs or different types of artificial white noise) did not markedly affect the rate of deterioration of the song discrimination. These data indicate that adult male canaries have the perceptual abilities to discriminate and selectively attend to one ongoing sound that occurs simultaneously with one or more other sounds. The administration of a noradrenergic neurotoxin did not impair markedly the discrimination learning abilities although the number of subjects tested was too small to allow any firm conclusion. In these conditions, however, the noradrenergic lesion significantly increased the number failures to respond in the discrimination learning task suggesting a role, in canaries, of the noradrenergic system in some attentional processes underlying song learning and processing.     

Cues for Early Discrimination of Conspecific Song in the White-Crowned Sparrow (Zonotrichia leucophrys)

In song learning, white-crowned sparrows ( Zonotrichia leucophrys ) begin memorizing conspecific song models at around 20 d of age. Even prior to song memorization, however, between 10 and 20 d of age, these birds respond differently to playbacks of conspecific and heterospecific songs. To investigate the acoustic cues underlying this early song discrimination, we measured the vocal responses of newly fledged young to playbacks of modified conspecific and heterospecific songs. Fledgling white-crowned sparrows responded more strongly to songs containing conspecific notes than to songs containing notes from other species. In contrast, the presence or placement of an introductory whistle, present in all white-crowned sparrow songs, did not affect response levels. A whistle has previously been shown to serve as an acoustic cue for song memorization and production in this species. Such different responses to the species-typical introductory whistle at different stages suggests that developmental changes occur in the template system underlying song learning, the factors governing song recognition, or both.     

Neural mechanisms of auditory species recognition in birds

Auditory communication in humans and other animals frequently takes place in noisy environments with many co‐occurring signallers. Receivers are thus challenged to rapidly recognize salient auditory signals and filter out irrelevant sounds. Most bird species produce a variety of complex vocalizations that function to communicate with other members of their own species and behavioural evidence broadly supports preferences for conspecific over heterospecific sounds (auditory species recognition). However, it remains unclear whether such auditory signals are categorically recognized by the sensory and central nervous system. Here, we review 53 published studies that compare avian neural responses between conspecific versus heterospecific vocalizations. Irrespective of the techniques used to characterize neural activity, distinct nuclei of the auditory forebrain are consistently shown to be repeatedly conspecific selective across taxa, even in response to unfamiliar individuals with distinct acoustic properties. Yet, species‐specific neural discrimination is not a stereotyped auditory response, but is modulated according to its salience depending, for example, on ontogenetic exposure to conspecific versus heterospecific stimuli. Neuromodulators, in particular norepinephrine, may mediate species recognition by regulating the accuracy of neuronal coding for salient conspecific stimuli. Our review lends strong support for neural structures that categorically recognize conspecific signals despite the highly variable physical properties of the stimulus. The available data are in support of a ‘perceptual filter’‐based mechanism to determine the saliency of the signal, in that species identity and social experience combine to influence the neural processing of species‐specific auditory stimuli. Finally, we present hypotheses and their testable predictions, to propose next steps in species‐recognition research into the emerging model of the neural conceptual construct in avian auditory recognition.     

Conspecific and heterospecific song discrimination in male zebra finches with lesions in the anterior forebrain pathway

Adult zebra finches can produce normal song in the absence of Area X, lMAN, or DLM, nuclei that constitute the anterior forebrain pathway of songbirds. Here, we address whether lesions involving Area X and lMAN affect adult male zebra finches' ability to discriminate between conspecific or heterospecific songs. Intact birds and lesioned birds were trained on an operant GO/NOGO conditioning paradigm to discriminate between hetero- or conspecific songs. Both lesioned and intact birds were able to learn all discriminations. Lesioned and intact birds performed equivalently on canary song discriminations. In contrast, discriminations involving bird's own song took significantly more trials to learn for lesioned birds than for intact birds. Discrimination between conspecific songs in general also took longer in the lesioned birds, but missed significance level. Birds with control lesions medial to Area X did not show any differences from intact animals. Our results suggest that an intact anterior forebrain pathway is not required to discriminate between heterospecific songs. In contrast, Area X and lMAN contribute to a male zebra finch's ability to discriminate between its own song and that of other zebra finches. © 1998 John Wiley & Sons, Inc. J Neurobiol 36: 81–90, 1998     

Song discrimination by nestling collared flycatchers during early development

Pre-zygotic isolation is often maintained by species-specific signals and preferences. However, in species where signals are learnt, as in songbirds, learning errors can lead to costly hybridization. Song discrimination expressed during early developmental stages may ensure selective learning later in life but can be difficult to demonstrate before behavioural responses are obvious. Here, we use a novel method, measuring changes in metabolic rate, to detect song perception and discrimination in collared flycatcher embryos and nestlings. We found that nestlings as early as 7 days old respond to song with increased metabolic rate, and, by 9 days old, have increased metabolic rate when listening to conspecific when compared with heterospecific song. This early discrimination between songs probably leads to fewer heterospecific matings, and thus higher fitness of collared flycatchers living in sympatry with closely related species.     

Rapid effects of hearing song on catecholaminergic activity in the songbird auditory pathway

Catecholaminergic (CA) neurons innervate sensory areas and affect the processing of sensory signals. For example, in birds, CA fibers innervate the auditory pathway at each level, including the midbrain, thalamus, and forebrain. We have shown previously that in female European starlings, CA activity in the auditory forebrain can be enhanced by exposure to attractive male song for one week. It is not known, however, whether hearing song can initiate that activity more rapidly. Here, we exposed estrogen-primed, female white-throated sparrows to conspecific male song and looked for evidence of rapid synthesis of catecholamines in auditory areas. In one hemisphere of the brain, we used immunohistochemistry to detect the phosphorylation of tyrosine hydroxylase (TH), a rate-limiting enzyme in the CA synthetic pathway. We found that immunoreactivity for TH phosphorylated at serine 40 increased dramatically in the auditory forebrain, but not the auditory thalamus and midbrain, after 15 min of song exposure. In the other hemisphere, we used high pressure liquid chromatography to measure catecholamines and their metabolites. We found that two dopamine metabolites, dihydroxyphenylacetic acid and homovanillic acid, increased in the auditory forebrain but not the auditory midbrain after 30 min of exposure to conspecific song. Our results are consistent with the hypothesis that exposure to a behaviorally relevant auditory stimulus rapidly induces CA activity, which may play a role in auditory responses.