Hormonal and environmental control of song control region growth and new neuron addition in adult male house finches, Carpodacus mexicanus

In songbirds, testosterone (T) mediates seasonal changes in the sizes and neuroanatomical characteristics of brain regions that control singing (song control regions; SCRs). One model explaining the mechanisms of the growth of one SCR, the HVC, postulates that in the spring increasing photoperiod and circulating T concentrations enhance new neuron survival, thus increasing total neuron number. However, most research investigating the effects of T on new neuron survival has been done in autumn. The present study investigated the effects of photoperiod and T treatment on SCR growth and new neuron survival in the HVC in photosensitive adult male House Finches, Carpodacus mexicanus, under simulated spring-like conditions. Birds were castrated, given T-filled or empty Silastic capsules and maintained on short days (SD; 8L:16D) or long days (LD; 16L:8D). To mark new cells, birds received bromodeoxyuridine injections 11 days after experimental manipulations began and were sacrificed 28 days later. Testosterone treatment increased the sizes of two SCRs, the HVC and Robust nucleus of the arcopallium (RA). Exposure to LD did not affect HVC volume, but did increase RA volume. Testosterone treatment increased the total number of HVC neurons, but did not affect the number of new HVC neurons. Thus, T initiates SCR growth and increases neuron survival, but effects of T on new neuron incorporation may be limited in photosensitive birds under spring-like conditions. These results provide new insight into the effects of photoperiod and T treatment on vernal SCR growth and new neuron incorporation and support current models explaining this growth.     

Geographic variation in the carotenoid plumage pigmentation of male house finches (Carpodacus mexicanus)

Geographic variation in both the colour and pattern of carotenoid plumage pigmentation displayed by males in two subspecies of house finches ( Carpodacus mexicanus frontalis and C. m. griscom ) was quantified. The extent of ventral carotenoid pigmentation (patch size) differed markedly between these two subspecies; frontalis males from the U.S. (New York, Michigan, California and Hawaii) displayed a medium patch extending from their throats to their lower bellies, while griscomi males sampled in Guerrero, Mexico displayed small patches restricted to their throats. Frontalis males sampled in Michigan and New York and griscomi males were relatively bright in colouration, while frontalis males sampled in Hawaii were relatively drab. Populations of frontalis in California showed substantial local variation in average male colouration: in two areas only 12 km apart males were as colourful and as drab as any population sampled. In aviary experiments in which they were fed either a plain seed diet or a diet supplemented with red carotenoid pigments during moult, males from all populations converged on a similar appearance, except that griscomi males attained a brighter plumage than frontalis males when their diet was supplemented with red pigments. Regardless of diet, the difference in patch size between frontalis and griscomi males persisted after moult in captivity. The author concludes that the difference in patch size between frontalis and griscomi males reflects genetic differences between these populations, but that the differences in the mean plumage colouration of males among populations reflect differences in the access that males have to carotenoid pigments during moult.     

Yolk antioxidants vary with male attractiveness and female condition in the house finch (Carpodacus mexicanus)

The manipulation of egg content is one of the few ways by which female birds can alter offspring quality before hatch. Lipid-soluble vitamins and carotenoids are potent antioxidants. Female birds deposit these antioxidants into eggs in variable amounts according to environmental and social conditions, and the quantities deposited into eggs can have effects on offspring health and immunological condition. Allocation theory posits that females will alter the distribution of resources according to mate quality, sometimes allocating resources according to the differential allocation hypothesis (DAH), investing more in offspring sired by better-quality males, and other times allocating resources according to a compensatory strategy, enhancing the quality of offspring sired by lower-quality males. It is unknown, however, whether antioxidants are deposited into eggs according to the DAH or a compensatory strategy. We examined deposition patterns of yolk antioxidants (including vitamin E and three carotenoids) in relation to laying order, mate attractiveness, female condition, and yolk androgen content in the house finch (Carpodacus mexicanus). Female house finches deposited significantly more total antioxidants into eggs sired by less attractive males. Additionally, yolk antioxidant content was significantly positively correlated with female condition, which suggests a cost associated with the deposition of antioxidants into eggs. Finally, concentrations of antioxidants in egg yolks were positively correlated with total yolk androgen content. We suggest that yolk antioxidants are deposited according to a compensatory deposition strategy, enabling females to improve the quality of young produced with less attractive males. Additionally, yolk antioxidants may act to counter some of the detrimental effects associated with high levels of yolk androgens in eggs and, thus, may exert a complementary effect to yolk androgens.     

House finches (Carpodacus mexicanus) balance investment in behavioural and immunological defences against pathogens

Infection with parasites and pathogens is costly for hosts, causing loss of nutritional resources, reproductive potential, tissue integrity and even life. In response, animals have evolved behavioural and immunological strategies to avoid infection by pathogens and infestation by parasites. Scientists generally study these strategies in isolation from each other; however, since these defences entail costs, host individuals should benefit from balancing investment in these strategies, and understanding of infectious disease dynamics would benefit from studying the relationship between them. Here, we show that Carpodacus mexicanus (house finches) avoid sick individuals. Moreover, we show that individuals investing less in behavioural defences invest more in immune defences. Such variation has important implications for the dynamics of pathogen spread through populations, and ultimately the course of epidemics. A deeper understanding of individual- and population-level disease defence strategies will improve our ability to understand, model and predict the outcomes of pathogen spread in wildlife.     

Isolation and characterization of eight microsatellite loci from the house finch (Carpodacus mexicanus)

I isolated the first set of polymorphic microsatellite markers from the house finch, Carpodacus mexicanus, a well‐studied North American bird species, as part of an effort to compare levels of genetic diversity in introduced and native populations. Here, I describe eight independently assorting microsatellite loci screened for polymorphism using 40 house finches. Polymorphism levels ranged from six to 14 alleles (mean = 10.6), making these markers a powerful tool for paternity and population level analyses of this widely distributed North American species.     

Isolation and characterization of 17 microsatellite loci for the house finch (Carpodacus mexicanus)

The house finch (Carpodacus mexicanus) has emerged recently as a model species in studies of sexual selection, reproductive physiology, population genetics, and epizootic disease ecology. Here we describe 17 highly polymorphic microsatellite loci for this species. In a sample of 36 individuals, we observed an average of 16 alleles per locus and heterozygosity ranged from 0.61 to 0.97. One locus showed significant deviation from Hardy-Weinberg proportions, but no significant gametic disequilibrium was observed among any of the loci. Amplification by polymerase chain reaction was optimized under similar parameters across loci, thereby facilitating multiplexing and rapid multilocus genotyping.     

Super-normal length song preferences of female zebra finches (Taeniopygia guttata) and a theory of the evolution of bird song

Two way choice tests show a preference of female zebra finches for male songs four standard deviations longer than normal song. Further tests show the ontogeny of this preference to parallel song learning in general as well as a preference for songs with entirely heterogeneous notes compared to songs with four note repeats. These findings are discussed in relation to a theory of the evolution of bird song from bird calls due to female preferences for longer, more complex vocalizations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Reconciling actual and inferred population histories in the house finch (Carpodacus mexicanus) by AFLP analysis

The house finch (Carpodacus mexicanus) is a native songbird of western North America that was introduced to the eastern United States and Hawaiian Islands in historic times. As such, it provides an unusually good opportunity to test the ability of molecular markers to recover recent details of a known population history. To investigate this prospect, genetic variation in 172 individuals from 16 populations in the western and eastern United States, southeastern Canada, Hawaiian Islands, and Mexico, as well as genetic variation in the closely related purple finch (Carpodacus purpureus) and Cassin's finch (Carpodacus cassinii) was studied by a semi-automated fluorescence-labeled amplified fragment length polymorphism (AFLP) marker system. A total of 363 markers were generated, of which 258 (71.2%) were polymorphic among species, 166 (61.4%) polymorphic among house finch subspecies, and 157 (60.2%) polymorphic among populations within the frontalis subspecies complex. Heterozygosities and interpopulation divergences revealed by the analysis appeared relatively low at all taxonomic levels, but there are few similar studies in avian populations with which to compare results. Whereas the known population history predicts that both eastern and Hawaiian finches should have been derived from within western populations, tree analysis using both populations and individuals as units suggests weak monophyly of eastern populations and indicates that Hawaiian populations are not clearly derived from California populations. However, the genetic distinctiveness of native and recently founded populations was disclosed by analyses of molecular variance as well as by a model-based assignment approach in which 98%, 94%, and 99% individuals from western, Hawaiian, and eastern regions, respectively, were assigned correctly to their populations without using prior information on population of origin, suggesting that these recent introductions have resulted in detectable differentiation without substantial loss of AFLP diversity. Our results indicate that AFLPs are a useful tool for population genetic and evolutionary studies of birds, particularly as a prelude to finding molecular markers linked to traits subjected to recent adaptive evolution.     

Individual variation in growth trajectories: phenotypic and genetic correlations in ontogeny of the house finch (Carpodacus mexicanus)

We studied patterns of growth in a recently established natural population of the house finch (Carpodacus mexicanus) to examine whether phenotypic and genetic covariation among age‐specific trait values is likely to constrain morphological change favoured by selection acting on adults. We found variable patterns of allometric relationships during ontogeny, and documented relatively weak covariations among ages or among traits in individual growth trajectories. Frequent compensatory growth largely cancelled out the initial differences among nestlings, potentially enabling house finches to raise offspring under diverse and unpredictable environmental conditions. Moderate levels of additive genetic variance in morphological traits throughout ontogeny, and relatively low and fluctuating phenotypic and genetic covariation among ages imply strong potential for evolutionary change in morphological traits under selection. This conclusion is consistent with the profound population‐level divergence in morphological patterns that accompanied very successful colonization of most of North America by the house finch over the last 50 years.     

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

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

Molecular evidence for a founder effect in invasive house finch (Carpodacus mexicanus) populations experiencing an emergent disease epidemic

The impact of founder events on levels of genetic variation in natural populations remains a topic of significant interest. Well-documented introductions provide a valuable opportunity to examine how founder events influence genetic diversity in invasive species. House finches (Carpodacus mexicanus) are passerine birds native to western North America, with the large eastern North American population derived from a small number of captive individuals released in the 1940s. Previous comparisons using amplified fragment length polymorphism (AFLP) markers found equivalent levels of diversity in eastern and western populations, suggesting that any genetic effects of the founder event were ameliorated by the rapid growth of the newly established population. We used an alternative marker system, 10 highly polymorphic microsatellites, to compare levels of genetic diversity between four native and five introduced house finch populations. In contrast to the AFLP comparisons, we found significantly lower allelic richness and heterozygosity in introduced populations across all loci. Three out of five introduced populations showed significant reductions in the ratio of the number of alleles to the allele size range, a within-population characteristic of recent bottlenecks. Finally, native and introduced populations showed significant pairwise differences in allele frequencies in every case, with stronger isolation by distance within the introduced than native range. Overall, our results provide compelling molecular evidence for a founder effect during the introduction of eastern house finches that reduced diversity levels at polymorphic microsatellite loci and may have contributed to the emergence of the Mycoplasma epidemic which recently swept the eastern range of this species.     

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.