Heavy-metal concentrations in female laying great tits (Parus major) and their clutches

Egg laying may be an additional excretory pathway for heavy metals available to laying female birds. In this study, we examined the relationships between tissue concentrations (both internal organs and feathers) of 10 breeding female great tits (Parus major) and the concentrations in their eggs. We also investigated differences in metal concentrations (Ag, Al, As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Zn) among eggs as a result of the laying order of the eggs. Heavy-metal concentrations in internal tissues were highest in bone (Cr, Ni, Pb, Zn), kidney (Cd), liver (Cu), blood (Al, As, Co, Hg), and intestine (Ag, Mn). In the egg contents, relatively high concentrations of the essential elements Cr, Co, Cu, Mn, Ni, and Zn were found. In eggshells, concentrations of Ag, Al, As, Co, Cr, and Ni were high compared with internal tissue concentrations. Metal concentrations in the egg contents and eggshells were poorly correlated with metal concentrations in internal tissues and feathers; significant positive correlations were found only for cadmium and lead. We found few significant differences in metal concentrations among eggs, and none were according to the laying order of the eggs. The egg volume tended to decrease significantly as egg laying progressed. Our results seem to suggest that no special sampling strategies (e.g., sampling the first egg or pooling eggs from one clutch) are required for most metals.

Prolonged moderate elevation of corticosterone does not affect hippocampal anatomy or cell proliferation rates in mountain chickadees (Poecile gambeli)

Chronic stress and corresponding chronic elevations of glucocorticoid hormones have been widely assumed to have deleterious effects on brain anatomy and functions such as learning and memory. In particular, it has been suggested that chronic elevations of glucocorticoid hormones result in death of hippocampal neurons and in reduced rates of hippocampal neurogenesis. It is not clear, however, if any increase in glucocorticoid levels has negative effects on hippocampal anatomy as many animals regularly maintain moderately elevated levels of glucocrticoids over long periods of time under natural energetically demanding conditions. We used unbiased stereological methods to investigate whether mountain chickadees (Poecile gambeli) implanted for 49 days with continuous time-release corticosterone pellets, designed to approximately double the baseline corticosterone levels, differed from placebo-implanted chickadees in their hippocampal anatomy and cell proliferation rates. We found no significant differences between corticosterone and placebo-implanted birds in either telencephalon volume, volume of the hippocampal formation, or the total number of hippocampal neurons. Cell proliferation rates, measured as the total number of BrdU-labeled cells in the ventricular zone adjacent either to the hippocampus or to the mesopallium, were also not significantly different between corticosterone and placebo-implanted chickadees. Our results suggest that prolonged moderate elevation of corticosterone might not provide the suggested deleterious effects on hippocampal anatomy and neurogenesis in food-caching birds and, as we have shown previously, it actually enhances spatial memory.

Androgens, interspecific competition and species replacement in hybridizing warblers

The steroid hormone testosterone regulates aggressive behaviour in many vertebrates and is important for territorial defence among males of the same species. However, its role in mediating interspecific competition, and ultimately species distributions, is unknown. We show that testosterone may influence the geographical replacement of one species by another. Townsend's warblers (Dendroica townsendi) have replaced hermit warblers (D. occidentalis) over a vast portion of their historical range, partly because Townsend's males are more aggressive than hermit males and outcompete them for territories in areas of sympatry. We report differences in plasma androgen levels that parallel these aggressive asymmetries and the historical pattern of species replacement between Townsend's and hermits. Using hybrids, we provide evidence that these hormonal differences are partially genetically based and thus may have evolved through sexual selection during Pleistocene glacial maxima. Hormone-behaviour mechanisms can therefore have important effects on species distributions and can even influence the pathways underlying extinction.

Thyroid function in nestling tree swallows and eastern bluebirds exposed to non-persistent pesticides and p, p'-DDE in apple orchards of southern Ontario, Canada

This study examined the associated effects of pesticides and persistent residues of p,p'-DDE on thyroid function in 16-day-old tree swallows (Tachycineta bicolor) and 12-day-old eastern bluebirds (Sialia sialis). Apple orchards sprayed with pesticides in current use and reference sites were chosen for study in southern Ontario, Canada, during 2000-2001. We assessed thyroid hormone concentrations (plasma and muscle thyroxine [T4], and triiodothyronine [T3]) and the response to a challenge with thyroid stimulating hormone (TSH) as well as parameters of thyroid histology. Individual nests were exposed to as many as seven individual pesticide applications and up to five pesticide mixtures. Concentrations of p,p'-DDE in eggs from both species were highest in orchard sites, and ranged from 0.05 to 5.44 mg/kg wet weight for tree swallows, and from 0.17 to 95.4 mg/kg for eastern bluebirds. In 2000, tree swallows from sprayed orchards had elevated plasma T4 concentrations, and thyroid glands with collapsed follicles, hypertrophic epithelia, and higher follicular epithelial cells relative to chicks from reference sites. Plasma T4 concentration was positively correlated with the total number of pesticide mixtures applied during egg incubation through chick rearing. In 2001, basal plasma T3 concentration in tree swallow chicks did not differ between orchard and reference sites, though there were differences among orchards; T3 was positively correlated with total number of individual sprays applied during egg incubation through chick rearing. Tree swallows challenged with TSH showed no significant difference in thyroid hormone concentration or follicular epithelial cell height between exposure groups; however, a significantly greater proportion of chicks from reference sites were found to have thyroids with focal or diffuse hypertrophic epithelia relative to orchard chicks. In 2001, bluebirds from sprayed orchards had significantly higher T3 concentrations in response to TSH challenge; this response was positively correlated with the total number of pesticides applied during egg incubation through chick rearing.

Accumulation of selenium and lack of severe effects on productivity of American dippers (Cinclus mexicanus) and spotted sandpipers (Actitis macularia)

Selenium has been found at elevated concentrations in water, sediments, and aquatic biota in the Elk River (British Columbia, Canada) and some of its tributaries downstream of several coal mines. Selenium water concentrations in those areas exceed Canadian and British Columbia guidelines and are above levels at which adverse effects to fish and waterfowl could occur. We compared selenium concentrations in the eggs of two riverine waterbirds, American dippers and spotted sandpipers, with measures of productivity: the number of eggs laid, egg hatchability, and nestling survival. In American dippers, the mean egg selenium concentration from the exposed areas, 1.10 +/- SE 0.059 microg/g wet weight, was indistinguishable from the reference areas, 0.96 +/- SE 0.059 microg/g wet weight. For spotted sandpipers, the mean egg selenium concentration in the exposed areas, 2.2 +/- 0.5 microg/g wet weight, was significantly higher than in the reference areas, 1.2 +/- 0.14 microg/g wet weight, but less than reported thresholds for waterfowl and other shorebirds. There were no significant differences in egg hatchability between dippers in reference and exposed areas, but reduced hatchability was apparent for sandpipers in exposed locations. Despite the slightly reduced hatchability in sandpipers, overall productivity was higher than regional norms for both species; thus, selenium did not affect the number of young recruited to local populations. We did not observe teratogenic effects in either species, although none was expected at these concentrations. Despite moderately high selenium concentrations in the water, mean egg selenium concentrations were less than predicted from uptake models. We hypothesise that the relatively low uptake of selenium into the eggs of the two waterbirds in this study is likely due to their lotic environment's low biological transformation and uptake rates.

Activation and habituation of extracellular signal-regulated kinase phosphorylation in zebra finch auditory forebrain during song presentation

The sound of tape-recorded birdsong triggers a set of behavioral and physiological responses in zebra finches, including transcriptional activation of the zenk gene in the auditory forebrain. Song repetition leads to the stimulus-specific habituation of these responses. To gain insight into the mechanisms that couple auditory experience to gene regulation, we monitored the phosphorylation of the zebra finch extracellular signal-regulated kinase (ERK) protein by immunoblotting. Initial presentations of novel song (but not tones or noise) resulted in a rapid increase in ERK phosphorylation, followed by a return to basal levels within 5 min. This response was localized to the auditory forebrain where the zenk gene is activated. Sustained repetition of one song caused a selective habituation of the ERK response: a different song triggered another cycle of ERK phosphorylation without altering the habituated response to the first. To test directly for a role of ERK in experience-dependent zenk gene regulation, we infused an inhibitor of mitogen-activated and extracellular-regulated protein kinase kinase (MEK-1; the enzyme responsible for ERK activation) unilaterally into one auditory lobule just before song stimulation. The song-induced increase in zenk mRNA was blocked on the side of the injection, but not on the contralateral (uninfused) side. These results show that ERK phosphorylation is necessary for the initiation of the zenk gene response to novel song and identify ERK as a plausible site of signal integration underlying the selective habituation of genomic responses to a repeated song.

Calcium-binding proteins define interneurons in HVC of the zebra finch (Taeniopygia guttata)

Nucleus HVC of the avian song system is essential to song patterning and is a prime site for auditory-vocal integration important to vocal learning. These processes require precise, high-frequency action potential activity, which, in other systems, is often correlated with the expression of calcium-binding proteins. To characterize any such functional specializations in HVC, we retrogradely labeled projection neurons innervating HVC's known targets, namely, area X or nucleus robustus arcopallialis (RA), then stained HVC sections with antibodies to the calcium-binding proteins parvalbumin, calbindin, and calretinin. Under epifluorescent illumination, neither projection neuron type exhibited detectable levels of calcium-binding protein immunoreactivity, whereas a third cell type, made up of nonprojection neurons (interneurons), was immunopositive for one, two, or all three of the calcium-binding proteins. In fact, most of these interneurons were either doubly or triply labeled. To explore the link between the electrical and calcium-binding protein properties of individual HVC neurons, we used intracellular methods in brain slices to record from identified HVC cell types based on their intrinsic electrical properties. Intracellular neurobiotin combined with immunostaining revealed that fast-spiking interneurons, but not the slower-spiking projection neurons, were positive for one or more calcium-binding proteins. Confocal microscopy confirmed these results and also revealed that RA-projecting cells might contain very low levels of parvalbumin. These results indicate that HVC interneurons are specialized in their calcium-binding proteins and suggest how it might be possible to resolve the details of HVC microcircuits underlying song selectivity and auditory-vocal learning.

Basal metabolic rate, food intake, and body mass in cold- and warm-acclimated Garden Warblers

We address the question of whether physiological flexibility in relation to climate is a general feature of the metabolic properties of birds. We tested this hypothesis in hand-raised Garden Warblers (Sylvia borin), long-distance migrants, which normally do not experience great temperature differences between summer and winter. We maintained two groups of birds under cold and warm conditions for 5 months, during which their body mass and food intake were monitored. When relatedness (siblings vs. non-siblings) of the experimental birds was taken into account, body mass in cold-acclimated birds was higher than in warm-acclimated birds. BMR, measured at the end of the 5-month temperature treatment, was also higher in the cold- than the warm-acclimated group. Migrant birds thus seem to be capable of the same metabolic cold-acclimation response as has been reported in resident birds. The data support the hypothesis that physiological flexibility is a basic trait of the metabolic properties of birds.

Properties of dopamine release and uptake in the songbird basal ganglia

Vocal learning in songbirds requires a basal ganglia circuit termed the anterior forebrain pathway (AFP). The AFP is not required for song production, and its role in song learning is not well understood. Like the mammalian striatum, the striatal component of the AFP, Area X, receives dense dopaminergic innervation from the midbrain. Since dopamine (DA) clearly plays a crucial role in basal ganglia-mediated motor control and learning in mammals, it seems likely that DA signaling contributes importantly to the functions of Area X as well. In this study, we used voltammetric methods to detect subsecond changes in extracellular DA concentration to gain better understanding of the properties and regulation of DA release and uptake in Area X. We electrically stimulated Ca(2+)- and action potential-dependent release of an electroactive substance in Area X brain slices and identified the substance as DA by the voltammetric waveform, electrode selectivity, and neurochemical and pharmacological evidence. As in the mammalian striatum, DA release in Area X is depressed by autoinhibition, and the lifetime of extracellular DA is strongly constrained by monoamine transporters. These results add to the known physiological similarities of the mammalian and songbird striatum and support further use of voltammetry in songbirds to investigate the role of basal ganglia DA in motor learning.

How the house sparrow Passer domesticus absorbs glucose

According to the hypothesis that most glucose absorption occurs passively across intestinal tight junctions (paracellular absorption), one would predict fairly similar rates of in vivo absorption of L-glucose, the stereoisomer of D-glucose that is absorbed only passively and is not catabolized, and of 3-O-methyl-D-glucose (3OMD-glucose), the D-glucose analogue that is actively and passively transported and not catabolized. In house sparrows Passer domesticus, we applied a pharmacokinetic method to measure simultaneous in vivo absorption of [14C]L-glucose and [3H]3OMD-glucose in a situation in which intestinal glucose transporters were relatively saturated (gavage solution contained 200 mmol l(-1) 3OMD-glucose). Fractional absorptions (F) were not significantly different between [3H]3OMD- and [14C]L-glucose (0.80 vs 0.79), and the apparent rates of absorption did not differ significantly. When we performed the same experiment on other sparrows in a situation in which intestinal glucose transporters were relatively unsaturated (200 mmol l(-1) mannitol replaced 3OMD-glucose in the gavage solution), the apparent rate of absorption was significantly reduced for [14C]l-glucose by 39% and for [3H]3OMD-glucose by 26%. A simulation model showed that a reduction is not predicted if most of the [3H]3OMD-glucose is actively absorbed, because the absorption rate of the tracer should increase when competitive inhibitor (unlabeled 3OMD-glucose) is removed. The similar extent and rates of absorption of [3H]3OMD- and [14C]L-glucose, and the acceleration of their rates of absorption in the presence of luminal 3OMD-glucose, are most consistent with Pappenheimer's hypothesis that the majority of dietary D-glucose is absorbed passively.

Bmp4 and morphological variation of beaks in Darwin's finches

Darwin's finches are a classic example of species diversification by natural selection. Their impressive variation in beak morphology is associated with the exploitation of a variety of ecological niches, but its developmental basis is unknown. We performed a comparative analysis of expression patterns of various growth factors in species comprising the genus Geospiza. We found that expression of Bmp4 in the mesenchyme of the upper beaks strongly correlated with deep and broad beak morphology. When misexpressed in chicken embryos, Bmp4 caused morphological transformations paralleling the beak morphology of the large ground finch G. magnirostris.

Relationships between metal concentrations in great tit nestlings and their environment and food

Metal concentrations (Ag, As, Cd, Cu, Hg, Ni, Pb, Zn) were determined in the feathers and excreta of nestling great tits (Parus major), in their main invertebrate prey (Lepidoptera larvae) and in vegetation samples, all collected from four sites along a pollution gradient. Metal contamination in vegetation samples increased significantly towards the pollution source. The Ag, As, Hg, Ni and Pb concentrations in food samples were significantly higher at the site closest to the pollution source compared to the other three sites. Great tit nestlings from the site closest to the pollution source had significantly higher concentrations of Ag, As, Hg and Pb in their excreta than did nestlings at the other three sites. For five metals (Ag, As, Cu, Ni and Pb), we found concentrations in caterpillars to be significantly positively correlated with vegetation samples. We also found clear significant positive correlations between excreta and caterpillars for Ag, As, Hg and Pb and between feathers and caterpillars for As and Pb. Our data suggest that excreta are a good monitor for the presence and concentrations of non-essential metals in the food and the environment of passerine birds.

Differential expression of glutamate receptors in avian neural pathways for learned vocalization

Learned vocalization, the substrate for human language, is a rare trait. It is found in three distantly related groups of birds-parrots, hummingbirds, and songbirds. These three groups contain cerebral vocal nuclei for learned vocalization not found in their more closely related vocal nonlearning relatives. Here, we cloned 21 receptor subunits/subtypes of all four glutamate receptor families (AMPA, kainate, NMDA, and metabotropic) and examined their expression in vocal nuclei of songbirds. We also examined expression of a subset of these receptors in vocal nuclei of hummingbirds and parrots, as well as in the brains of dove species as examples of close vocal nonlearning relatives. Among the 21 subunits/subtypes, 19 showed higher and/or lower prominent differential expression in songbird vocal nuclei relative to the surrounding brain subdivisions in which the vocal nuclei are located. This included relatively lower levels of all four AMPA subunits in lMAN, strikingly higher levels of the kainite subunit GluR5 in the robust nucleus of the arcopallium (RA), higher and lower levels respectively of the NMDA subunits NR2A and NR2B in most vocal nuclei and lower levels of the metabotropic group I subtypes (mGluR1 and -5) in most vocal nuclei and the group II subtype (mGluR2), showing a unique expression pattern of very low levels in RA and very high levels in HVC. The splice variants of AMPA subunits showed further differential expression in vocal nuclei. Some of the receptor subunits/subtypes also showed differential expression in hummingbird and parrot vocal nuclei. The magnitude of differential expression in vocal nuclei of all three vocal learners was unique compared with the smaller magnitude of differences found for nonvocal areas of vocal learners and vocal nonlearners. Our results suggest that evolution of vocal learning was accompanied by differential expression of a conserved gene family for synaptic transmission and plasticity in vocal nuclei. They also suggest that neural activity and signal transduction in vocal nuclei of vocal learners will be different relative to the surrounding brain areas.

Elevated testosterone reduces choosiness in female dark-eyed juncos (Junco hyemalis): evidence for a hormonal constraint on sexual selection?

Because testosterone (T) often mediates the expression of attractive displays and ornaments, in the absence of constraints sexual selection should lead to an evolutionary increase in male T levels. One candidate constraint would be a genetic correlation between the sexes that leads to a correlated response in females. If increased T in females were to have deleterious effects on mate choice, the effect of sexual selection on male T would be weakened. Using female dark-eyed juncos (Junco hyemalis), we tested whether experimentally enhancing female T would lead to a decrease in discrimination between two classes of males, one treated with T (T-males) and one control (C-males). The two female treatments (T-implanted and C-females) spent equal amounts of time with both classes of males, but T-treated females failed to show a preference for either male treatment, whereas C-females showed a significant preference, albeit in an unexpected direction (for C-males). T-females were less discriminating than C-females, irrespective of the direction of their preference. To our knowledge, this is the first study to show that circulating hormones can alter female choosiness without reducing sexual motivation. Our results suggest that hormonal correlations between the sexes have the potential to constrain sexual selection on males.

Heterogeneity in stable isotope profiles predicts coexistence of populations of barn swallows Hirundo rustica differing in morphology and reproductive performance

Population studies assume that individuals belonging to a study population are homogeneous for natal and breeding origin, although this assumption is rarely tested. We tested for heterogeneity in stable-isotope profiles (delta15N, delta13C, deltaD) of feathers grown in the African winter quarters from a Danish breeding population of adult barn swallows, Hirundo rustica. Deuterium isotope values did not provide useful information on population segregation of wintering swallows in Africa. However, both delta15N and delta13C values showed a clearly bimodal distribution with 6% belonging to one category and the remaining birds belonging to another category, resulting in this population comprising three categories of birds. Adults belonging to the two categories of delta13C isotope profiles differed weakly in morphology for several different characters. The frequency and the size of second broods differed between categories of delta13C isotope profiles. Phenotypes of nestlings from the first brood in terms of tarsus length, body mass and T-cell response differed significantly between the two delta15N isotope categories, suggesting that conditions during winter carried over to the breeding season at least as late as the first brood. Polymorphism can be maintained only if fitness is similar for birds from categories of isotope profiles. We suggest that fluctuating selection or migration-selection balance may maintain the observed polymorphism.

Testosterone, cuckoldry risk and extra-pair opportunities in the Seychelles warbler

In male birds, testosterone (T) plays an important role in aggressive and mate-attraction behaviour. In the cooperatively breeding Seychelles warbler, Acrocephalus sechellensis, extra-group copulations (EGCs) occur frequently, but are not accompanied by sexual courtship displays as in within-pair copulations. Paternity is nearly always gained by primary males. We investigated whether T levels and sperm storage capability (cloacal protuberance (CP)) in adult primary and subordinate males were related to timing of egg laying, levels of cuckoldry and extra-group paternity (EGP) opportunities. During the sexually active period before egg laying, T levels and CP were only elevated or enlarged (respectively) in primary males, and some suggestion was found that subordinate males do not invest in elevated T levels. The peak in T occurred during the fertile period of the female partner and corresponded to the peak period of male sexual displays and mate guarding, but was independent of cuckoldry risk (density of neighbouring primary males). CP was also enhanced during this period; however, CP but not T remained elevated after egg laying by their mates, and CP but not T was positively related to EGP opportunities (density of neighbouring fertile females). We conclude that T is involved in sexual courtship displays and mate guarding, but not in gaining EGCs. These findings contrast with those in other species where EGP involves elaborate sexual displays.

Negative effects of early developmental stress on yolk testosterone levels in a passerine bird

Female birds incorporate in the yolks of their eggs significant concentrations of a number of different androgens. Yolk androgen has been shown to positively affect several fitness components at the embryo, nestling and juvenile stages. Previous experiments have shown that females lay eggs with higher androgen concentrations when they are paired with highly ornamented males. This pattern suggests that yolk androgens are costly to females. In this study, we experimentally manipulated adult female condition in zebra finches Taeniopygia guttata by modifying the level of developmental stress they suffered as nestlings. This was achieved by cross-fostering nestlings to broods of varying brood size. Subsequently, we measured the yolk testosterone contents of the female offspring that resulted from the experimental manipulation. As predicted, females deposited decreasing concentrations of testosterone with increasing brood sizes experienced as nestlings: testosterone concentration (mean +/- S.E.M.) of eggs laid by females from small broods, 20.66+/-2.08 pg mg(-1); medium broods, 15.32+/-1.94 pg mg(-1); and large broods, 14.51+/-1.66 pg mg(-1). Additionally, testosterone concentration decreased with laying order, and varied with clutch size in a complex way. Differences in egg testosterone between females exposed to different brood sizes are in line with previous findings in showing that early developmental stress can affect adult reproductive performance, although our study did not detect an effect in other breeding parameters, such as latency to breed or clutch size. Furthermore, the results are consistent with the hypothesis that there is a cost associated with yolk testosterone. However, it is still unclear what the nature of this cost may be, and whether it is paid by females, offspring, or both.

Nestling immunocompetence and testosterone covary with brood size in a songbird

The social and ecological conditions that individuals experience during early development have marked effects on their developmental trajectory. In songbirds, brood size is a key environmental factor affecting development, and experimental increases in brood size have been shown to have negative effects on growth, condition and fitness. Possible causes of decreased growth in chicks from enlarged broods are nutritional stress, crowding and increased social competition, i.e. environmental factors known to affect adult steroid levels (especially of testosterone and corticosteroids) in mammals and birds. Little, however, is known about environmental effects on steroid synthesis in nestlings. We addressed this question by following the development of zebra finch (Taeniopygia guttata) chicks that were cross-fostered and raised in different brood sizes. In line with previous findings, nestling growth and cell-mediated immunocompetence were negatively affected by brood size. Moreover, nestling testosterone levels covaried with treatment: plasma testosterone increased with experimental brood size. This result provides experimental evidence that levels of circulating testosterone in nestlings can be influenced by their physiological response to environmental conditions.

European starling chicks benefit from high yolk testosterone levels during a drought year

Avian egg yolk contains androgenic hormones, such as testosterone, of maternal origin. Experimental elevation of yolk testosterone levels enhances growth of canary chicks. Success in sibling competition, due to increased begging, is presumed to underlie this growth enhancement, because canary hatchlings from testosterone-treated eggs beg longer in response to vibrational stimuli than controls. Furthermore, experimental elevation of both yolk androstenedione and testosterone increased chick growth and begging in black-headed gulls. We measured daily growth of European starling (Sturnus vulgaris) chicks hatching from testosterone-treated or vehicle-treated (control) eggs until 14 days of age, and measured begging behavior at hatching and at 5 days of age. A temporary drought caused relatively high levels of early brood reduction for this population; 2- and 3-day-old chicks were most likely to starve. We found that chicks from testosterone-treated eggs were less likely to starve than control chicks, and were heavier on the days when most brood reduction occurred. However, chicks from testosterone-treated eggs begged less than control chicks on the day of hatching, and begged similarly at 5 days of age. Thus, while yolk testosterone did increase growth during periods of (presumably) high competition, increased begging does not appear to mediate this effect. Instead, testosterone may induce more efficient energy use, for example, by decreasing ineffective begging. While our results indicate that elevated yolk testosterone enhances survival, and thus offspring and parental fitness, further evidence regarding the fitness consequences of yolk androgens are vital to understanding their role in avian life history.

Glial aromatization decreases neural injury in the zebra finch (Taeniopygia guttata): influence on apoptosis

Emerging evidence suggests a neuroprotective role for oestrogens following damage to the vertebrate brain. Aromatase (oestrogen synthase) is rapidly transcribed and translated in glial cells around areas of neural damage in several vertebrates. However, the potential neuroprotection afforded by locally up-regulated glial aromatase immediately surrounding the injury remains to be tested. Towards this end, individual birds sustained penetrating mechanical injuries via a needle that contained either vehicle or the aromatase inhibitor fadrozole into contralateral hemispheres. Seventy-two hours later, the size of neural injury (as assessed by the extent of necrotic tissue) and the number of apoptotic cells around the injuries were evaluated. The size of injury in the hemisphere injected with fadrozole was significantly larger than the injury caused by vehicle injection. Furthermore, a greater number of apoptotic nuclei were found around the fadrozole-associated lesion relative to vehicle. Finally, constitutively expressed, neuronal aromatase close to the injury site did not differ between hemispheres. We conclude that local inhibition of glial aromatase immediately around the site of injury plays a neuroprotective role in the songbird brain and this protection involves apoptotic pathways. Local up-regulation of glial aromatase may play a pivotal role in the limitation of secondary damage and/or the acceleration of restorative processes following injury to the vertebrate brain.

Metal and metalloid concentrations in the eggs of threatened Florida scrub-jays in suburban habitat from south-central Florida

Monitoring and assessing changes in contaminants in urban and suburban environments is essential to assessing ecosystem well-being in human-influenced landscapes. We analyzed metal and metalloid levels in the eggs of the threatened Florida scrub-jay (Aphelocoma coerulescens), an extremely sedentary and modestly long-lived passerine bird that is federally threatened and endemic only in Florida. Eggs that failed to hatch were collected in a suburban environment to compare with the long-term study of this species at the Archbold Biological Station, located 8 km south in a more rural part of south-central Florida. Florida scrub-jays remain in a permanent territory in scrub oak habitat where they feed mainly on insects and acorns, but in suburban habitats human-provided foods comprise 30% of their diet. From the data previously collected at Archbold, and their low position on the food chain, we expected levels of contaminants to be comparatively low and remain relatively constant over time. Except for the low mercury value, all means were within the range of mean values reported for a wide range of non-passerine species (including those at higher tropic levels), are lower than concentrations associated with abnormalities in birds, and are lower than those previously reported for scrub-jays from Archbold Biological Station. A significant increase in selenium occurred from 1996 to 2001, but did not appear associated with changes in human density.

Synaptic and Molecular Mechanisms Regulating Plasticity during Early Learning

Many behaviors are learned most easily during a discrete developmental period, and it is generally agreed that these "sensitive periods" for learning reflect the developmental regulation of molecular or synaptic properties that underlie experience-dependent changes in neural organization and function. Avian song learning provides one example of such temporally restricted learning, and several features of this behavior and its underlying neural circuitry make it a powerful model for studying how early experience sculpts neural and behavioral organization. Here we describe evidence that within the basal ganglia-thalamocortical loop implicated in vocal learning, song acquisition engages N-methyl-d-aspartate receptors (NMDARs), as well as signal transduction cascades strongly implicated in other instances of learning. Furthermore, NMDAR phenotype changes in parallel with developmental and seasonal periods for vocal plasticity. We also review recent studies in the avian song system that challenge the popular notion that sensitive periods for learning reflect developmental changes in the NMDAR that alter thresholds for synaptic plasticity.

Individual variation in plasma estradiol-17beta and androgen levels during egg formation in the European starling Sturnus vulgaris: implications for regulation of yolk steroids

While it is clear that maternal transfer of steroids to egg yolk can have significant effects on offspring phenotype, an unresolved question is whether females can facultatively adjust yolk hormone levels independently of their own plasma levels or whether yolk steroid levels are simply a direct consequence of temporal variation in the female's hormonal status. In part, this is because we lack detailed information about the day-to-day pattern of changes in plasma hormone levels during the laying cycle for non-domesticated birds. Here, we describe changes in plasma estradiol-17beta (E2) and androgens, throughout laying in relation to specific stages of ovarian follicular development in the European starling (Sturnus vulgaris). Plasma E2 levels increased rapidly from the onset of rapid yolk development (RYD) to reach maximum levels in birds with a complete follicle hierarchy (> or = 4 yolky follicles). However, levels decreased linearly throughout the later stages of follicle development returning to pre-breeding values before the final yolky follicle was ovulated. In females with > or = 4 yolky follicles there was 10-fold variation in plasma E2 levels among individual females, but this was not related to plasma levels of the main yolk precursor vitellogenin or to the total mass of yolky follicles developing at the time of blood sampling. In contrast to E2, plasma androgen levels showed only a very gradual linear decline throughout the laying cycle from pre-RYD to clutch completion. Furthermore, androgen levels showed less individual variability: 4-fold variation among females with > or = 4 yolky follicles, although this was also independent of our measures of reproductive function. Data on inter- and intra-individual variation in female hormone levels are important to set-up a priori predictions for, and interpretation of, studies of yolk hormone levels.

Digit ratio varies with sex, egg order and strength of mate preference in zebra finches

The steroid environment encountered by developing vertebrates has important organizational effects on physiology and behaviour that persist throughout an organism's lifetime. Optimal allocation of maternal steroids to zygotes may be difficult to achieve because of the sexually antagonistic effects of steroids; thus, for example, a hormone environment beneficial to a developing male may be much less beneficial to a developing female. Research into the important topic of how mothers might adaptively adjust steroid titres experienced by particular young has been constrained by the difficulty of measuring the steroid environment experienced by the embryo at critical times in development. A potential approach to this problem has been suggested by research on variation in digit ratios in humans, where the ratio of the length of the second and fourth digits reflects the steroid environment experienced by the foetus; notably, digit 4 lengthens in response to androgens. In light of the conservative nature of homeobox genes regulating early development in tetrapods, we questioned whether a sex difference in digit ratio exists in a passerine bird, the zebra finch, Taeniopygia guttata castanotis, and whether observed variation in the ratio is consistent with the previously reported pattern that androgen allocation to zebra finch egg yolk declines across laying order. We established an aviary population of outbred, wild-type zebra finches, and allowed them to breed freely. Hatchlings were marked to correspond to their egg order, and their digit ratios were measured after birds reached adulthood. We found that digit ratio increased across egg order, which is consistent with a pattern of decreasing androgen allocation. Moreover, digit ratios differed between the sexes. We also investigated whether variation in digit ratio among adult females predicted variation in their performance in mate-choice tests. Digit ratio accounted for almost 50% of the variance in strength of female preference for an attractive male trait: specifically, females with higher (presumably less 'androgenized') ratios had stronger preferences for attractive males. Digit ratio may prove to be an extremely useful tool for addressing a wide range of questions about vertebrate differentiation and behaviour.