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Previous studies concluded that parrots and oscine songbirds, two taxa that have independently evolved the ability to learn vocalizations, possess similar neural circuits for vocal control. These investigations suggested, however, that the vocal control systems of parrots and songbirds may also differ in several respects. Most importantly, auditory inputs to the vocal control system derive from Field L in songbirds, but this area does not appear to project to the vocal control system in parrots. The principal aims in the present study were, therefore, to determine 1) exactly how similar the vocal control system in budgerigars is to that in songbirds and 2) whether the vocal control system in budgerigars receives auditory inputs from areas other than Field L. Biotinylated and fluorescently labeled dextrans were injected into five telencephalic nuclei of the vocal control system in budgerigars and into the physiologically identified auditory portions of the frontal neostriatum and nucleus basalis. The results indicate that the forebrain vocal control system in budgerigars is only superficially similar to that in songbirds. Many of the vocal control nuclei differ between the two taxa in both cytoarchitecture and connections. The nuclei in budgerigars that are comparable to those of the accessory loop of the vocal control system in songbirds, for example, do not form an accessory loop in budgerigars. The vocal control systems in the two taxa differ most significantly in the source of their auditory inputs. In songbirds, auditory information is conveyed to the vocal control system via Field L, whereas, in budgerigars, the auditory inputs to the vocal control system derive from nucleus basalis and the frontal neostriatum. A phylogenetic analysis suggests that the midbrain and medullary vocal control pathways are homologous across all birds, but that most of the vocal control circuits in the forebrain have probably evolved independently in parrots and songbirds.
Androgens and estrogens are crucial for the differentiation and function of the vocal control system of songbirds. A major source of estrogens in songbirds is the cerebral aromatization of circulating testosterone by aromatase (ARO). In the vocal control system, songbirds have a unique estrogen receptor (ER)-containing area, the nucleus hyperstriatalis ventrale pars caudale (HVC) of the caudal neostriatum. Work in the zebra finch has demonstrated ARO expression adjacent to but not in the HVC. Compared with other songbirds, such as the canary, the HVC of adult zebra finches contains only few ERs. To determine whether the disjunctive distribution of ERs and ARO in the forebrain is a songbird-specific feature, the authors investigated ARO and ER mRNA expression in songbirds (canary, house sparrow, and zebra finch) and in nonsongbirds (budgerigar, ring dove, swift, grey partridge, and barn owl) of five avian orders. In addition, the coexpression of androgen receptor (AR) and ARO mRNAs was studied. Preoptic hypothalamic areas showed similar expression of ARO in all species. In the caudal neostriatum, ARO, AR, and ER transcripts were found only in songbirds. ARO and ER mRNA expression in the caudal forebrain was spatially separated, i.e., the HVC contained ER mRNA but very little or no ARO mRNA, and the caudomedial neostriatum contained high levels of ARO mRNA but few if any ERs. ARO and AR mRNAs, however, were coexpressed in the caudomedial neostriatum. The coexpression of ARO mRNA with AR mRNA but not with ER mRNA was found in further brain areas, such as the nucleus posterior lateralis hypothalami. The area-specific coexpression of AR, ER, and ARO suggests various possibilities for the steroid-dependent regulation of ARO and for the role of ARO in controlling AR- and ER-dependent mechanisms.
Species-typical vocal patterns subserve species identification and communication for individual organisms. Only a few groups of organisms learn the sounds used for vocal communication, including songbirds, humans, and cetaceans. Vocal learning in songbirds has come to serve as a model system for the study of brain-behavior relationships and neural mechanisms of learning and memory. Songbirds learn specific vocal patterns during a sensitive period of development via a complex assortment of neurobehavioral mechanisms. In many species of songbirds, the production of vocal behavior by adult males is used to defend territories and attract females, and both males and females must perceive vocal patterns and respond to them. In both juveniles and adults, specific types of auditory experience are necessary for initial song learning as well as the maintenance of stable song patterns. External sources of experience such as acoustic cues must be integrated with internal regulatory factors such as hormones, neurotransmitters, and cytokines for vocal patterns to be learned and produced. Thus, vocal behavior in songbirds is a culturally acquired trait that is regulated by multiple intrinsic as well as extrinsic factors. Here, we focus on functional relationships between circuitry and behavior in male songbirds. In that context, we consider in particular the influence of sex hormones on vocal behavior and its underlying circuitry, as well as the regulatory and functional mechanisms suggested by morphologic changes in the neural substrate for song control. We describe new data on the architecture of the song system that suggests strong similarities between the songbird vocal control system and neural circuits for memory, cognition, and use-dependent plasticity in the mammalian brain.
Specialized neural circuitry has evolved in groups of birds where vocal learning is known to occur, such as in the oscine suborder of the Passeriformes (songbirds) and in the order Psittaciformes (parrots). These specializations are most prominent in the telencephalon, while the midbrain and medullary portions of the vocal control circuit are generally similar in all orders of birds. Specializations in songbirds have at least four components: 1) a set of distinct and interconnected vocal control nuclei in the forebrain; 2) unique connections between the auditory system and these vocal control nuclei; 3) the occurrence of receptors for sex steroid hormones in a subset of the vocal control nuclei; and 4) unique patterns in the distribution of various markers of the major classes of neurotransmitters within the vocal control nuclei. In the order Psittaciformes, as exemplified by budgerigars (Melopsittacus undulatus), it appears that neural specializations generally similar to those described in songbirds have evolved independently. Although anatomical studies have found a system of interconnected forebrain regions in budgerigars that are roughly similar to those described in oscines, detailed connectivity studies of this forebrain system suggest that it is only superficially similar to that of songbirds, and budgerigars also have unique connections between the auditory and motor systems. Also, analyses of the distribution of markers of neurotransmitter function in the budgerigar brain reveal patterns different from those described in songbird vocal control systems. This work suggests that songbirds and parrots have evolved separate neural 'solutions' to solve the problem of vocal plasticity. There are differences between these 'solutions' but also similarities that may be the result of convergent evolution. Although vocal behavior is learned in both songbirds and parrots, it differs in many respects. By taking advantage of this 'natural' experiment one can gain insight into the hormonal and neural events that mediate these different forms of vocal plasticity.
We report on the oldest European songbird (Passeriformes), from the early Oligocene (30-34 million years ago) of Frauenweiler in Germany. The specimen represents the earliest associated remains of an early Tertiary passerine described so far. It ties the first appearance of Passeriformes in Europe to a minimum age of 30 million years. Passeriform birds are absent in Eocene deposits that yielded abundant remains of small land birds and apparently dispersed into Europe around the Eocene/Oligocene boundary (about 34 million years ago), not at the Oligocene/Miocene boundary (about 24 mya) as hitherto thought. This possibly relates the appearance of songbirds in Europe to a well-known major faunistic break at the Eocene/Oligocene boundary, called the " grande coupure". The Frauenweiler songbird most notably differs from extant Passeriformes in having a larger processus procoracoideus on the coracoid and appears to be outside Eupasseres, the taxon which includes Oscines (all modern European and most Old World songbirds) and Suboscines (most South and Central American songbirds). It shows that there were earlier dispersal events of non-oscine songbirds into Europe before the arrival of Oscines from the Australian continental plate towards the late Oligocene.
The biology of songbirds poses fundamental questions about the interplay between gene, brain, and behavior. New tools of genomic analysis will be invaluable in pursuing answers to these questions. This review begins with a summary of the broad properties of the songbird genome and how songbird brain gene expression has been measured in past studies. Four key problems in songbird biology are then considered from a genomics perspective: What role does differential gene expression play in the development, maintenance, and functional organization of the song control circuit? Does gene regulation set boundaries on the process of juvenile song learning? What is the purpose of song-induced gene activity in the adult brain? How does the genome underlie the profound sexual differentiation of the song control circuit? Finally, the range of genomic technologies currently or soon to be available to songbird researchers is briefly reviewed. These technologies include online databases of expressed genes ("expressed sequence tags" or ESTs); a complete library of the zebra finch genome maintained as a bacterial artificial chromosome (BAC) library; DNA microarrays for simultaneous measurement of many genes in a single experiment; and techniques for gene manipulation in the organism. Collectively, these questions and techniques define the field of songbird neurogenomics.
Mycoplasma gallisepticum (MG) conjunctivitis emerged in 1994 as a disease of free-ranging house finches (Carpodacus mexicanus) in North America and has also been isolated from other songbirds with conjunctivitis. Random amplification of polymorphic DNA (RAPD) of house finch and other songbird isolates has suggested that a single 'strain' initiated this outbreak. To explore the possibility of genomic variability among house finch isolates of MG and to evaluate the utility of a second technique for MG genotyping, we selected samples from our archive of reference strains and wild songbird isolates to analyze using both RAPD and amplified-fragment length polymorphism (AFLP); this is a newer technique that has been successfully used to explore the genomic variability of several Mycoplasma species. Both RAPD and AFLP results confirmed previous observations that during the initial stages of the MG epidemic in songbirds, isolates from different geographic locations and songbird species had genotypes that appeared to be highly similar, further supporting a single point source of origin. One 2001 isolate from New York was clearly different from the other songbird samples and clustered together with the vaccine and reference strains, indicating that substantial molecular evolution or a separate introduction has occurred.
Estrogens play an important role in the control and differentiation of species-typical behavior and in endocrine homeostasis of birds, but the distribution and evolution of cells that contain estrogen receptors in the avian brain are poorly understood. This study therefore surveys 26 species in the avian orders Anseriformes (1 species), Galliformes (2), Columbiformes (3), Psittaciformes (1), Apodiformes (2), and Passeriformes (3 suboscines, 14 oscines). Indirect immunocytochemistry with the estrogen receptor (ER) antibody H222Spy revealed a general pattern of ER-antibody-immunoreactive cells (ER-IRC) in all 26 species, with ER-IRC in consistent, well-defined locations in the limbic forebrain, the midbrain striatum, the hippocampus, the hindbrain, and especially in the preoptic area and the tuberal hypothalamus. For some species, the microdistribution of ER-IRC in some of these general areas differed, such as in the hippocampus and the anterior hypothalamus of suboscine species and in the preoptic area of the Japanese quail. Brains of oscine songbirds of both sexes, unlike brains of nonsongbirds, had ER-IRC in three specific structures of the nonlimbic forebrain: in the area surrounding the nucleus robustus archistriatalis; in the rostral forebrain; and, for all individuals, in the caudale neostriatum, including the nucleus hyperstriatalis ventrale, pars caudale (HVc). Among songbird families or subfamilies, adult males of the Estrildinae had much lower numbers of ER-IRC in HVc than did adult males of the Fringillidae, Paridae, Sturnidae, and Ploceinae. Differences occurred, too, among closely related species: the songbird canary (Serinus canaria) had an ER-IRC area in the rostral forebrain that was lacking in all other songbird species, including other cardueline finches. The cells with ER that are found only in the songbird forebrain but not in reptiles, nonpasserine birds, and nonoscine passerine birds very likely coevolved with steroid-dependent differentiation of vocal control areas. The songbird-specific expression of ER in the forebrain could be an example in which taxon-specific behavior is due to taxon specific neurochemical properties of the brain.
Androgens and estrogens influence the maturation and function of numerous tissues in both male and female birds, especially the brains of the oscine songbirds. Although there exist a very large number of studies that have investigated circulating sex steroids in many species of wild and captive-held songbirds, there remain a significant number of questions about the sites of synthesis of the active steroids that act on the songbird brain. Estrogens are derived from androgen. Thus, the synthesis of androgen itself is critical for both androgen- and estrogen-dependent actions in both male and female songbirds. Therefore, we have undertaken studies of the enzyme 17alpha-hydroxylase/C17,20-lyase (Cyp17), the enzyme responsible for the synthesis of androgens from their progestin or pregnane precursors via their 17alpha-hydroxy intermediates. Here we have characterized optimal conditions for measuring Cyp17 in gonads of adult zebra finches via the conversion of tritiated [3H]progesterone into 17alpha-hydroxy P (17alpha-hydroxylase activity) and androstenedione and testosterone (C17,20-lyase) activity. Cyp17 activity is abundant in testis, with lesser amounts in ovary. Low levels of Cyp17 activity were also detected in male adrenals, but not in any other tissue, including brain. Testicular Cyp17 activity is readily inhibited in vitro by ketoconazole, a specific Cyp17 inhibitor. Ketoconazole works less well in vivo. In males castrated and/or treated with fadrozole, an inhibitor of aromatase, we detected no extragonadal sites of Cyp17 activity, although fadrozole appeared to increase circulating androgens in both castrated and intact males. Thus, we still do not know the site of androgen synthesis in these males. Further studies of Cyp17 will be useful in understanding more about the mechanisms of androgen delivery to neural circuits in adult and developing songbirds.
Auditory fMRI in humans has recently received increasing attention from cognitive neuroscientists as a tool to understand mental processing of learned acoustic sequences and analyzing speech recognition and development of musical skills. The present study introduces this tool in a well-documented animal model for vocal learning, the songbird, and provides fundamental insight in the main technical issues associated with auditory fMRI in these songbirds. Stimulation protocols with various listening tasks lead to appropriate activation of successive relays in the songbirds' auditory pathway. The elicited BOLD response is also region and stimulus specific, and its temporal aspects provide accurate measures of the changes in brain physiology induced by the acoustic stimuli. Extensive repetition of an identical stimulus does not lead to habituation of the response in the primary or secondary telencephalic auditory regions of anesthetized subjects. The BOLD signal intensity changes during a stimulation and subsequent rest period have a very specific time course which shows a remarkable resemblance to auditory evoked BOLD responses commonly observed in human subjects. This observation indicates that auditory fMRI in the songbird may establish a link between auditory related neuro-imaging studies done in humans and the large body of neuro-ethological research on song learning and neuro-plasticity performed in songbirds.
Humans and songbirds are two of the rare animal groups that modify their innate vocalizations. The identification of FOXP2 as the monogenetic locus of a human speech disorder exhibited by members of the family referred to as KE enables the first examination of whether molecular mechanisms for vocal learning are shared between humans and songbirds. Here, in situ hybridization analyses for FoxP1 and FoxP2 in a songbird reveal a corticostriatal expression pattern congruent with the abnormalities in brain structures of affected KE family members. The overlap in FoxP1 and FoxP2 expression observed in the songbird suggests that combinatorial regulation by these molecules during neural development and within vocal control structures may occur. In support of this idea, we find that FOXP1 and FOXP2 expression patterns in human fetal brain are strikingly similar to those in the songbird, including localization to subcortical structures that function in sensorimotor integration and the control of skilled, coordinated movement. The specific colocalization of FoxP1 and FoxP2 found in several structures in the bird and human brain predicts that mutations in FOXP1 could also be related to speech disorders.
The brain of the budgerigar (Melopsittacus undulatus), a small parrot that acquires new vocalizations throughout life, was examined for immunoreactivity to the opioid peptide methionine enkephalin (mENK). mENK is a highly prominent feature of the chemical architecture of the forebrain vocal system of oscine songbirds. Forebrain vocal control nuclei are believed to have evolved independently in parrots and songbirds (Streidter [1994] J. Comp. Neurol. 343:35-56); however, recent studies have found similarities in the neural organization of vocal control pathways in budgerigars and songbirds (Durand et al. [1997] J. Comp. Neurol. 377:179-206). Among the similarities are the existence of recursive pathways interconnecting vocal control neurons in the archistriatum, basal ganglia (i.e., lobus parolfactorius), and dorsal thalamus. In the present study, we found that all vocal control nuclei within the budgerigar forebrain exhibit prominent mENK-like immunoreactivity (ELI) in fibers and somata. We also found striking similarities between the morphology of ELI elements in budgerigar vocal control nuclei and that described previously in songbird vocal nuclei. Despite these similarities, the budgerigar dorsal striatopallidum (lobus parolfactorius, paleostriatum augmentatum, and paleostriatum primitivum) and somatomotor (anterior) archistriatum exhibit unique patterns of ELI. The dorsal striatopallidum contained far less ELI, whereas the archistriatum contained far more than would be expected on the basis of previous studies of opioid peptides in other avian species, including pigeons, chickens, and songbirds. These differences may reflect neural specializations unique to the budgerigar that contribute to the extraordinary flexibility of the vocal motor system of this species to acquire socially significant stimuli throughout life.
The brain is now widely recognized as having the capacity to make steroids, neurosteroidogenesis. Although many functions are known for steroids that might be made in the brain, the evolution of and natural biological functions for these neurosteroids are not fully understood. In songbirds, neurosteroids may function in the development of neural circuits controlling song and may also participate in the activation of some steroid-dependent behaviors during the non-breeding season. In addition to neuroanatomical and behavioral evidence, we have physiological, molecular, and biochemical evidence for the expression and activity of steroidogenic enzymes in the brains of developing and adult songbirds. We review the evidence published so far for songbird neurosteroidogenesis and discuss why we believe songbird species are excellent models for the study of brain steroid synthesis and action.
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.
Biological systems by default involve complex components with complex relationships. To decipher how biological systems work, we assume that one needs to integrate information over multiple levels of complexity. The songbird vocal communication system is ideal for such integration due to many years of ethological investigation and a discreet dedicated brain network. Here we announce the beginnings of a songbird brain integrative project that involves high-throughput, molecular, anatomical, electrophysiological and behavioral levels of analysis. We first formed a rationale for inclusion of specific biological levels of analysis, then developed high-throughput molecular technologies on songbird brains, developed technologies for combined analysis of electrophysiological activity and gene regulation in awake behaving animals, and developed bioinformatic tools that predict causal interactions within and between biological levels of organization. This integrative brain project is fitting for the interdisciplinary approaches taken in the current songbird issue of the Journal of Comparative Physiology A and is expected to be conducive to deciphering how brains generate and perceive complex behaviors.
In both humans and songbirds, infants learn vocalizations by imitating the sounds of adult tutors with whom they interact during an early sensitive period. Vocal learning occurs in few animal taxa; similarities in the imitation process between humans and songbirds make the songbird a unique system in which vocal learning mechanisms can be studied at the neurobiological level. One theory of vocal learning proposes that early auditory experience generates auditory memories that subsequently guide vocal imitation. We now present a combination of behavioral and neurophysiological results, obtained in a songbird, that support this theory. We show that neurons in a forebrain auditory area of adult male zebra finches are selectively tuned to the song of a tutor heard early in development. Furthermore, the strength of this selectivity shows a striking correlation with the fidelity of vocal imitation, suggesting that this auditory memory may have served as the model for song learning.
The bird fauna of Madagascar includes a high proportion of endemic species, particularly among passerine birds (Aves: Passeriformes). The endemic genera of Malagasy songbirds are not allied obviously with any African or Asiatic taxa, and their affinities have been debated since the birds first were described. We used mitochondrial sequence data to estimate the relationships of 13 species of endemic Malagasy songbirds, 17 additional songbird species, and one species of suboscine passerine. In our optimal trees, nine of the 13 Malagasy species form a clade. although these birds currently are classified in three different families. In all optimal trees, the sister to this endemic clade is a group of Old World warblers including both African and Malagasy birds. The endemic Malagasy songbird clade rivals other island radiations, including the vangas of Madagascar and the finches of the Galapagos, in ecological diversity.