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Geoffrey E Hill

Publications and source records attributed to Geoffrey E Hill.

9 recordsLinked to original sources
PubMed2003

Nanostructure predicts intraspecific variation in ultraviolet-blue plumage colour.

Evidence suggests that structural plumage colour can be an honest signal of individual quality, but the mechanisms responsible for the variation in expression of structural coloration within a species have not been identified. We used full-spectrum spectrometry and transmission electron microscopy to investigate the effect of variation in the nanostructure of the spongy layer on expression of structural ultraviolet (UV)-blue coloration in eastern bluebird (Sialia sialis) feathers. Fourier analysis revealed that feather nanostructure was highly organized but did not accurately predict variation in hue. Within the spongy layer of feather barbs, the number of circular keratin rods significantly predicted UV-violet chroma, whereas the standard error of the diameter of these rods significantly predicted spectral saturation. These observations show that the precision of nanostructural arrangement determines some colour variation in feathers.

Animals↗
PubMed2003

Interaction between maternal effects: onset of incubation and offspring sex in two populations of a passerine bird.

Maternal phenotype and maternal environment can profoundly affect the phenotype and fitness of offspring. Yet the causes of variation in such maternal effects are rarely known. Embryos in avian eggs cannot develop without being incubated and this creates an opportunity for maternal control of duration and onset of offspring development. However, females might adjust the start of incubation (e.g., coincident with the first egg or delayed until after egg-laying) in response to environmental conditions that they experience at the time of breeding. We studied two populations of the house finch ( Carpodacus mexicanus) that breed at the climatic extremes of the species' geographical range (Montana and Alabama) and found that in both populations, the timing of incubation onset was closely associated with the bias in the sequence in which male and female eggs were laid within a clutch. When females started incubation with the first egg, they produced sons and daughters in highly biased sequence, when females delayed the onset of incubation until after the egg-laying, the sequence of sons and daughters was not biased. Because in both populations, onset of incubation was associated with the ambient temperature, these results emphasize that maternal effects on offspring can be influenced by ecological conditions experienced by parental generation.

Animals↗
PubMed2003

The evolution of sexual size dimorphism in the house finch. V. Maternal effects.

The phenotype of a mother and the environment that she provides might differentially affect the phenotypes of her sons and daughters, leading to change in sexual size dimorphism. Whereas these maternal effects should evolve to accommodate the adaptations of both the maternal and offspring generations, the mechanisms by which this is accomplished are rarely known. In birds, females adjust the onset of incubation (coincident with the first egg or after all eggs are laid) in response to the environment during breeding, and thus, indirectly, determine the duration of offspring growth. In the two house finch (Carpodacus mexicanus) populations that breed at the extremes of the species' distribution (Montana and Alabama), females experience highly distinct climatic conditions during nesting. We show that in close association with these conditions, females adjusted jointly the onset of incubation and the sequence in which they produced male and female eggs and consequently modified the growth of sons and daughters. The onset of incubation in newly breeding females closely tracked ambient temperature in a pattern consistent with the maintenance of egg viability. Because of the very different climates in Montana and Alabama, females in these populations showed the opposite patterns of seasonal change in incubation onset and the opposite sex bias in egg-laying order. In females with breeding experience, incubation onset and sex bias in laying order were closely linked regardless of the climatic variation. In nests in which incubation began with the onset of egg laying, the first-laid eggs were mostly females in Montana, but mostly males in Alabama. Because in both populations, male, but not female, embryos grew faster when exposed to longer incubation, the sex-bias produced highly divergent sizes of male and female juveniles between the populations. Overall, the compensatory interaction between the onset of incubation and the sex-biased laying order achieved a compromise between maternal and offspring adaptations and contributed to rapid morphological divergence in sexual dimorphism between populations of the house finch breeding at the climatic extremes of the species range.

Animals↗
PubMed2002

Dietary carotenoids predict plumage coloration in wild house finches.

Carotenoid pigments are a widespread source of ornamental coloration in vertebrates and expression of carotenoid-based colour displays has been shown to serve as an important criterion in female mate choice in birds and fishes. Unlike other integumentary pigments, carotenoids cannot be synthesized; they must be ingested. Carotenoid-based coloration is condition-dependent and has been shown to be affected by both parasites and nutritional condition. A controversial hypothesis is that the expression of carotenoid-based coloration in wild vertebrates is also affected by the amount and types of carotenoid pigments that are ingested. We tested this carotenoid-limitation hypothesis by sampling the gut contents of moulting house finches and comparing the concentration of carotenoid pigments in their gut contents with the colour of growing feathers. We found a positive association: males that ingested food with a higher concentration of carotenoid pigments grew brighter ornamental plumage. We also compared the concentration of carotenoids in the gut contents of males from two subspecies of house finches with small and large patches of carotenoid-based coloration. Consistent with the hypothesis that carotenoid access drives the evolution of carotenoid-based colour displays, males from the population with limited ornamentation had much lower concentrations of carotenoids in their gut contents than males from the population with extensive ornamentation. These observations support the idea that carotenoid intake plays a part in determining the plumage brightness of male house finches.

Animals↗
PubMed2002

Sex-biased hatching order and adaptive population divergence in a passerine bird.

Most species of birds can lay only one egg per day until a clutch is complete, and the order in which eggs are laid often has strong and sex-specific effects on offspring growth and survival. In two recently established populations of the house finch (Carpodacus mexicanus) in Montana and Alabama, breeding females simultaneously adjusted the sex and growth of offspring in relation to their position in the laying order, thereby reducing the mortality of sons and daughters by 10 to 20% in both environments. We show experimentally that the reduction in mortality is produced by persistent and sex-specific maternal effects on the growth and morphology of offspring. These strong parental effects may have facilitated the rapid adaptive divergence among populations of house finches.

Adaptation, Physiological↗
PubMed2002

Species divergence in sexually selected traits: increase in song elaboration is related to decrease in plumage ornamentation in finches.

Elaborate plumage and complex songs of male birds are two of the best-known examples of sexually selected traits, yet the interaction between these traits is poorly understood. Theory suggests that among a suite of potential displays, animals will emphasize traits that are most conspicuous, least costly, or best signal condition and reduce display of other traits. Here we examined the relationship between song and plumage elaborations in cardueline finches, songbirds that are highly variable in plumage displays and songs, but that share a similar mating system. We statistically controlled for body mass, habitat characteristics, and phylogenetic relationships and found that across species song complexity was strongly negatively related to elaboration of plumage ornamentation. When plumage coloration was partitioned into carotenoid-based and melanin-based components, song complexity was negatively related to elaboration of male carotenoid-based coloration but unrelated to elaboration of melanin-based coloration. These observations support the idea that, for condition-dependent traits, animal species trade off trait expression in response to changes in the costs or the information content of these traits. We discuss several alternative explanations for the observed pattern.

Animals↗
PubMed2002

Susceptibility of a naïve population of house finches to Mycoplasma gallisepticum.

Since 1994 an epidemic of mycoplasmal conjunctivitis has spread throughout the eastern house finch (Carpodacus mexicanus) population leading to a significant decline in this population. The infection has not yet been reported from house finch populations west of the Great Plains. We hypothesized that the western population, like the eastern population, is susceptible to infection, and we tested this hypothesis by experimentally infecting house finches from Missoula, Montana (USA) with the house finch strain of Mycoplasma gallisepticum (MG). We compared the response of finches from Montana infected with MG to that of finches from Auburn, Alabama (USA) (October 1999-February 2000). Fifteen house finches from Montana were shipped to Auburn and quarantined for 6 wk at the Auburn University aviary. All birds were negative for MG antibodies when tested by serum plate agglutination assay and MG could not be detected in any bird by polymerase chain reaction. We tested two methods of inoculation, ocular inoculation and contact exposure to an infected finch. Seven house finches from Montana and four house finches from Alabama were infected by bilateral ocular inoculation with 20 microliters of a culture containing 1 x 10(6) color changing units of the house finch strain of MG. The remaining eight house finches from Montana were co-housed with a house finch from Alabama exhibiting mycoplasmal conjunctivitis. After exposure to the pathogen, all house finches became infected, regardless of origin or method of exposure, and all developed conjunctivitis. All birds seroconverted, and evidence of infection could be detected in every bird at some point during the course of disease. Our results suggest that house finches from the western United States are highly susceptible to infection with the house finch strain of MG.

Alabama↗