No abstract

The process through which young male songbirds learn the characteristics of the songs of an adult male of their own species has strong similarities with speech acquisition in human infants. Both involve two phases: a period of auditory memorization followed by a period during which the individual develops its own vocalizations. The avian 'song system', a network of brain nuclei, is the probable neural substrate for the second phase of sensorimotor learning. By contrast, the neural representation of song memory acquired in the first phase is localized outside the song system, in different regions of the avian equivalent of the human auditory association cortex.

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.

No abstract

Presumably, due to a rapid early diversification, major parts of the higher-level phylogeny of birds are still resolved controversially in different analyses or are considered unresolvable. To address this problem, we produced an avian tree of life, which includes molecular sequences of one or several species of ∼ 90% of the currently recognized family-level taxa (429 species, 379 genera) including all 106 for the non-passerines and 115 for the passerines (Passeriformes). The unconstrained analyses of noncoding 3-prime untranslated region (3’UTR) sequences and those of coding sequences yielded different trees. In contrast to the coding sequences, the 3’UTR sequences resulted in a well-resolved and stable tree topology. The 3’UTR contained, unexpectedly, transcription factor binding motifs that were specific for different higher-level taxa. In this tree, grebes and flamingos are the sister clade of all other Neoaves, which are subdivided into five major clades. All non-passerine taxa were placed with robust statistical support including the long-time enigmatic hoatzin (Opisthocomiformes), which was found being the sister taxon of the Caprimulgiformes. The comparatively late radiation of family-level clades of the songbirds (oscine Passeriformes) contrasts with the attenuated diversification of non-passeriform taxa since the early Miocene. This correlates with the evolution of vocal production learning, an important speciation factor, which is ancestral for songbirds and evolved convergent only in hummingbirds and parrots. Since 3’UTR-based phylotranscriptomics resolved the avian family-level tree of life, we suggest that this procedure will also resolve the all-species avian tree of life

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. J. Comp. Neurol. 407:115–129, 1999. © 1999 Wiley‐Liss, Inc.

The present investigation used stable area-specific, neuronal properties instead of Nissl stain to delineate the boundaries of the nucleus hyperstriatalis caudal c (HVc) in the telencephalon of the adult male canary. Immunocytochemical procedures combined with retrograde tracing labeled a large population of perennial long-projecting neurons that contain estrogen receptors in the canary HVc. The HVc area defined by the distribution of these neurons was congruent with the HVc area defined in Nissl-stained sections during the breeding period. The HVc area defined in Nissl-stained preparations showed an extensive seasonal change in size, confirming previous results (Nottebohm: Science, 214:1368-1370, '81). In contrast, the HVc area defined by the distribution of the estrogen receptor containing long-projection neurons showed little or no seasonal change in size. Because these neurons are permanent, the HVc seems to be of rather constant size year round. The internal morphology of the HVc, however, undergoes seasonal alterations, which are reflected in changes in size of the HVc area distinguishable in Nissl-stained sections. The combination of cytoarchitectural criteria of Nissl-stained preparations with area-specific cytochemical and hodological markers to delineate the boundaries of a brain nucleus might give new insights in the partitioning and neuronal plasticity of brain areas.

The expression of brain-derived neurotrophic factor (BDNF) mRNA is increased significantly within the high vocal center (HVc) of male but not female zebra finches from posthatching day 30-35 on. The population of HVc cells expressing BDNF mRNA included 35% of the neurons projecting to the nucleus robustus of the archistriatum (RA). In the RA and in RA-projecting neurons of the lateral portion of the magnocellular nucleus of the anterior neostriatum, BDNF mRNA was expressed at very low levels in both sexes. The BDNF-receptor trkB mRNA was expressed in the RA, in RA-projecting neurons of lateral portion of the magnocellular nucleus of the anterior neostriatum, and in the HVc, except in most of its RA-projecting neurons. Premature stimulation and an inhibitory effect on the normal increase of the BDNF mRNA expression in juvenile males occurred after treatments with 17␤-estradiol and the aromatase inhibitor fadrozole, respectively. The up-regulation of the BDNF expression in the HVc could be a mechanism by which estrogen triggers the differentiation of cells within and connected to the HVc of male zebra finches.