After years of investigation into the function of sexually dimorphic ornamental traits, researchers are beginning to understand how bright plumage colour in birds acts as an intraspeci¢c signal. This work has focused primarily on pigment-based ornaments because they are highly variable in patch size, hue and brightness for some species. In contrast, structurally based ornaments have been little studied, in part because they do not appear to be as variable as pigment-based ornaments. We investigated a structurally based plumage ornament in a wild population of blue grosbeaks (Guiraca caerulea), a sexually dimorphic passerine. We report plumage variation that extends into the ultraviolet region of the spectrum. The pattern of covariation between four out of ¢ve elements of plumage variation suggests that structurally based ornamentation is pushed towards extreme expression of the trait as predicted by the sexual selection theory. The`bluest' birds have the highest percentage of blue feathers on the body. These ornamental feathers re£ect light maximally at the shortest wavelengths (ultraviolet), with the greatest intensity and the greatest contrast. Age may have some e¡ect on expression of blueness. In addition, plumage variables are correlated with growth bars in tail feathers (a record of nutritional condition during moult in a nonornamental trait). This suggests that the ornament is partially condition dependent. Thus, blue plumage in male grosbeaks may serve as an honest indicator of age and quality.
Many species of Neotropical migrant songbirds are experiencing population declines. Degradation of habitat on breeding grounds, particularly the fragmentation and isolation of forest habitats, has been suggested as an important factor in the decline of these populations. Using artificial ground nests containing quail and clay eggs, we compared risk of predation relative to the size of forest fragments, the proximity of nests to forest edge, and the density of nests. Because small predators rarely can damage or remove quail eggs, the loss of these eggs reveals activity by large predators. Small predators, however, often leave identifiable claw and tooth marks on artificial eggs, and we used such marks to assess the importance of small predators as nest predators. We found a negative correlation between predation rate and fragment size. Nest clustering increased predation by large predators but did not affect small predator activity. No significant edge effects on predation were apparent. The relative threat to nesting songbirds posed by small and large predators changed with forest fragmentation. Large predator activity increased as forests become more fragmented. Historically, interior forest‐dwelling birds have been subjected primarily to small predator activity. The increased large predator activity we documented, against which songbirds have no defense, could be partially responsible for recent population declines. Efectos del Tamaño de Fragmento de Bosque, Densidad de Nidos y Proximidad al Borde en el Riego de Depredación de Aves Paserinas que Anidan en el Suelo. Muchas de las especies de aves canoras neotropicales migratorias están experimentando disminuciones poblacionales. La degradación del hábitat en las áreas de reproducción, particularmente la fragmentación y el aislamiento de los hábitats boscosos han sido sugeridos como factores importantes en la disminución de estas poblaciones. Mediante el uso de nidos artificiales con huevos de codornices y barro, comparamos el riesgo de depredación relativa al tamaño del fragmento de bosque, la proximidad de los nidos al borde del mismo y la densidad de nidos. Debido a que los depredadores pequeños raramente pueden dañar o remover huevos de codorniz, la pérdida de éstos revela la actividad de depredadores grandes. Sin embargo, los depredadores pequeños dejan frecuentemente marcas de garras y dientes en huevos artificiales porloque utilizamos éstas para evaluar la importancia de estos como depredadores de nidos. Encontramos una correlación negativa entre la tasa de depredación y el tamaño del fragmento de bosque. El hacinamiento de nidos incrementó la depredación por depredadores grandes pero no afectó la actividad de los depredadores pequeños. De manera aparente, no se encontraron efectos de borde significativos con respeto a la depredación. La amenaza relativa a las aves canoras por depredadores pequeños y grandes cambia con la fragmentación del bosque. La actividad de los depredadores aumenta a medida que el bosque se encuentra más fragmentado. Históricamente, las aves que habitan en el interior del bosque han estado primordialmente sujetas a la actividad de depredadores pequeños. El incremento en la actividad de depredadores grandes en contra de aves canoras indefensas, podría ser parcialmente responsable de disminuciones poblacionales recientes.
Understanding the evolution of complex social behaviours, such as cooperative breeding, is a fundamental problem in evolutionary biology, which has attracted much theoretical and empirical interest. Variation within and between species in the frequency of helping behaviour has been typically associated with variation in direct costs and benefits due to ecological constraints, or with indirect fitness payoffs (i.e. kin selection). Here, we provide the first evidence that individual variation in cooperative behaviour within a natural population also has a heritable component. Using a seven-generation pedigree in a wild population of western bluebirds (Sialia mexicana), we show significant heritable variation for the propensity to help rather than breed, as well as for the probability of having a helper at the nest. We also document a strong positive relationship between a bird's lifespan and its prospect of receiving help when breeding, in accordance with earlier comparative studies across species. These findings provide useful insights into the possible mechanisms which have led to the evolution of cooperative breeding and other social systems.
Combinations of microstructural and pigmentary components of barbs create the colour displays of feathers. It follows that evolutionary changes in colour displays must reflect changes in the underlying production mechanisms, but rarely have the mechanisms of feather colour evolution been studied. Among bluebirds in the genus Sialia, male rump colour varies among species from dark blue to light blue while breast colour varies from blue to rusty. We use spectrometry, transmission electron microscopy and Fourier analysis to identify the morphology responsible for these divergent colour displays. The morphology of blue rump barbs is similar among the three species, with an outer keratin cortex layer surrounding a medullary 'spongy layer' and a basal row of melanin granules. A spongy layer is also present in blue breast barbs of mountain bluebirds Sialia currucoides and in rusty breast barbs of western Sialia mexicana and eastern bluebirds Sialia sialis. In blue barbs melanin is basal to the spongy layer, but is not present in the outer cortex or spongy layer, while in rusty barbs, melanin is present only in the cortex. The placement of melanin in the cortex masks expression of structural blue, creating a rusty display. Such shifts in microstructures and pigments may be widespread mechanisms for the evolutionary changes in the colours of feathers and other reflective structures across colourful organisms.
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