Bill Size Differences Research Articles

The case for splitting the Square-tailed Drongo Dicrurus ludwigii: a review of the phenetic evidence

The validity of recent changes in the taxonomic treatment of the Square-tailed Drongo Dicrurus ludwigii is assessed via analyses of morphological, morphometric, acoustic and distributional evidence. In addition to confirming the characters already recognised to distinguish West and Central African sharpei from the ludwigii group, we report previously undocumented but notable differences in voice and tail morphology. The occurrence of a hitherto unrecognised population of sharpei in south-east DR Congo, inferred from recent molecular studies, is confirmed, whilst evidence of the close approach of the two taxa in north-west Angola, within c.60 km of each other, further supports their distinctiveness. The proposal to separate sharpei itself into two species—with populations west of the Niger River recognised as occidentalis—is, however, not supported by vocal data, whilst differences in bill size, the sole known physical discriminant, are here found to be much more modest than previously reported, perhaps attributable to the larger sample sizes used in this study. Thus, on phenetic evidence, occidentalis merits no more than subspecific status.

MORPHOLOGICAL VARIATION IN THE TIT-LIKE DACNIS (XENODACNIS PARINA): A CALL TO REVISE THE TAXONOMIC STATUS OF ECUADOR’S POPULATION

Xenodacnis is a specialized high Andean bird genus of which males are larger and show different feather coloration from females. However, sexual dimorphism has not been analyzed in detail yet. Distributed in Ecuador and Peru, Xenodacnis have long been considered as a single species genus, with three subspecies. Discovered in the 1980`s, the population in Ecuador has not been given a taxonomic identity so far. Thus, this study gathers morphological data from Xenodacnis throughout its distribution, and confirmed the morphological differences between sexes: males are significantly larger and heavier than females; but without consistent differences in bill size. Between populations, the research shows smaller individuals in the south, with size increasing towards the northernmost Ecuadorian populations. These results suggest that the taxonomy of the genus Xenodacnis should be revised and the population from Ecuador should have a proper taxonomic identity different from all previously described members of the genus

Sexual dimorphism in a feeding apparatus is driven by mate choice and not niche partitioning

Lay summaryFemale sparrows prefer bigger bills, causing the evolution of distinctly different bill sizes for males and females. The size and shape of a bird’s bill has historically been viewed only as an adaptation for eating, but in at least one species of sparrow, bill size differences echo female mating preferences better than diet. Older males have larger bills, so females who mate with larger billed males get the advantage of more experienced mates that have proven their ability to survive.

Correction: Heat Loss May Explain Bill Size Differences between Birds Occupying Different Habitats

The last author's name was misspelled. The correct name is: Glenn J. Tattersall. The correct citation is: Greenberg R, Cadena V, Danner RM, Tattersall GJ (2012) Heat Loss May Explain Bill Size Differences between Birds Occupying Different Habitats. PLoS ONE 7(7): e40933. doi:10.1371/journal.pone.0040933

Correction: Heat Loss May Explain Bill Size Differences between Birds Occupying Different Habitats

Correction: Heat Loss May Explain Bill Size Differences between Birds Occupying Different Habitats

THE INFLUENCE OF THE CALIFORNIA MARINE LAYER ON BILL SIZE IN A GENERALIST SONGBIRD

The hypothesis is tested that birds in hotter and drier environments may have larger bills to increase the surface area for heat dissipation. California provides a climatic gradient to test the influence of climate on bill size. Much of California experiences dry warm/hot summers and coastal areas experience cooler summers than interior localities. Based on measurements from 1488 museum skins, song sparrows showed increasing body-size-corrected bill surface area from the coast to the interior and declining in the far eastern desert. As predicted by Newton's convective heat transfer equation, relative bill size increased monotonically with temperature, and then decreased where average high temperatures exceed body temperature. Of the variables considered, distance from coast, average high summer temperature, and potential evapotranspiration showed a strong quadratic association with bill size and rainfall had a weaker negative relationship. Song sparrows on larger, warmer islands also had larger bills. A subsample of radiographed specimens showed that skeletal bill size is also correlated with temperature, demonstrating that bill size differences are not a result of variation in growth and wear of keratin. Combined with recent thermographic studies of heat loss in song sparrow bills, these results support the hypothesis that bill size in California song sparrows is selected for heat dissipation.

Heat Loss May Explain Bill Size Differences between Birds Occupying Different Habitats

BackgroundResearch on variation in bill morphology has focused on the role of diet. Bills have other functions, however, including a role in heat and water balance. The role of the bill in heat loss may be particularly important in birds where water is limiting. Song sparrows localized in coastal dunes and salt marsh edge (Melospiza melodia atlantica) are similar in size to, but have bills with a 17% greater surface area than, those that live in mesic habitats (M. m. melodia), a pattern shared with other coastal sparrows. We tested the hypotheses that sparrows can use their bills to dissipate “dry” heat, and that heat loss from the bill is higher in M. m. atlantica than M. m. melodia, which would indicate a role of heat loss and water conservation in selection for bill size.Methodology/Principal FindingsBill, tarsus, and body surface temperatures were measured using thermal imaging of sparrows exposed to temperatures from 15–37°C and combined with surface area and physical modeling to estimate the contribution of each body part to total heat loss. Song sparrow bills averaged 5–10°C hotter than ambient. The bill of M. m atlantica dissipated up to 33% more heat and 38% greater proportion of total heat than that of M. m. melodia. This could potentially reduce water loss requirements by approximately 7.7%.Conclusions/SignificanceThis >30% higher heat loss in the bill of M. m. atlantica is independent of evaporative water loss and thus could play an important role in the water balance of sparrows occupying the hot and exposed dune/salt marsh environments during the summer. Heat loss capacity and water conservation could play an important role in the selection for bill size differences between bird populations and should be considered along with trophic adaptations when studying variation in bill size.

Fitting the bill: do different winter food resources influence juniper titmouse (Baeolophus ridgwayi) bill morphology?

Divergent adaptive selection is a prominent mechanism influencing patterns of morphological diversity. We used the juniper titmouse [Baeolophus ridgwayi (Richmond, 1902)], a nonmigratory passerine that inhabits woodlands throughout western North America, to investigate variation in bill morphology in relation to diet and geography. We gathered data from museum specimens and used morphometric techniques to determine the relative strength of support for competing hypotheses using information theoretics: (1) differences in bill morphology are predicted by a key winter food resource that each regional population consumes (seeds of different juniper tree species); or (2) bill morphology scales with body size, and both increase along a latitudinal gradient. Juniper species emerged as the variable with the most support explaining variation in bill size, supporting the hypothesis that seed sizes influence bill size, independently of body size. The shape analysis revealed no distinct patterns in bill shape variation, but employed a powerful method for evaluating the strength of support for numerous candidate models. The differences in bill size of juniper titmice across their range are likely to reflect adaptive variation, because bill morphology is highly heritable in birds, juniper titmouse gene flow appears to be relatively low, and there is a clear mechanistic explanation for why bill sizes may differ among the ranges of the three juniper species. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101, 667–679.

Sexual dimorphism in bill morphology and feeding ecology in Cory's shearwater ( Calonectris diomedea)

The bill is a sexually dimorphic structure in many bird species and implicated in numerous functions. Sexual differences may arise from sexual selection or ecological divergence. Here, we examined differences in bill size and shape between males and females and explored to what extent these relate to feeding ecology of each sex in Cory's shearwater ( Calonectris diomedea). We applied linear measurements and geometric morphometric methods to examine sexual differences in bill size and shape. We investigated feeding ecology by tracking foraging movements during the breeding period and by analysing stable isotope signatures in blood during the breeding period and in feathers grown during the non-breeding period. Bill traits were all sexually dimorphic, both in absolute and relative terms, and scaled hypermetrically with body mass in several characters in males. However, males and females did not differ in their feeding areas or isotopic signatures and no significant correlation was observed between these traits and bill dimorphism. Therefore, we discard the foraging-niche divergence hypothesis, and suggest that sexual dimorphism in bill size in this species is more likely driven by sexual selection related to antagonistic interactions.

Status, ecology, behaviour and conservation of Raso Lark Alauda razae

In October 2001, the world population of Raso Lark Alauda razae, confined to the uninhabited, arid islet of Raso in the Cape Verde Islands, was estimated at between 128 and 138 birds, of which 61–66% were males. The biased sex ratio was confirmed by standardized observations of randomly selected birds. The male-dominated sex ratio may result from differences in bill morphology and feeding methods. The males spent much time digging for the bulbs of the nutsedge Cyperus bulbosus, whereas females were seen to dig far less frequently. These differences may have been due to significant sexual differences in bill size. Both sexes also took a range of invertebrate prey, particularly lepidopteran larvae. Unpaired males showed differences in behaviour to paired males, particularly with respect to song flight length. Measurement of museum skins suggested significant seasonal changes in bill size, possibly related to changes in diet or feeding methods. Bill sizes in both sexes were highly variable, but significantly more variable in males. Nest predation, almost certainly by a near-endemic gecko, was extremely high, Mayfield estimates suggesting a nest survival rate of less than 5% between the onset of laying and the end of incubation. Cats and dogs were thought to be absent, although analysis of droppings shows that both have been present in the recent past. Analysis of historical data shows a strong correlation between population size and rainfall, and numbers of birds have fallen to extremely low levels during droughts. The conservation of the species is discussed in the light of these findings.

Patterns of morphological and geographic variation in trophic bill morphs of the African finch Pyrenestes

Intra- and inter-population variability was studied in three species of tropical African estrildid finches comprising the genus Pyrenestes. Eleven characters were measured on P. ostrinus captured on a study area in Cameroon. Most of these same characters were also measured on museum specimens of this species and P. saguineus and P. minor. Data were analysed using univariate and multivariate statistical methods in order to characterize variation within and between populations and species. Variation in all three species of Pyrenestes is greatest in bill size, resulting from an exceptional non-sex-linked polymorphism. Bill size differences between morphs are as high as between congeneric species, with extremely large coefficients of variation, while other body characters show comparatively little variation. Sexual dimorphisms and differences in size due to age occur, but contribute little to overall size variation. Distributions of bill characters in each age and sex class are bimodal or greatly skewed, and in some geographical regions tend to be trimodal. Distributions of other body characters tend not to be significantly different from normal. Bill morphs differ in both shape and size and may be separated using principal component analysis. Static allometries of bill morphs differ significantly: relative to body size, bill size increases more rapidly in the large morph. Bill size and shape also vary geographically. The three species differ in mean size but show much overlap. Bill size is negatively correlated with total annual rainfall. In regions characterized by ecotonal transition zones between forest and savanna, tentative evidence suggests that a third, yet larger bill mode occurs. This third mode apparently results from the presence of a distinct larger species of hard-seeded sedge found only in these regions. The taxonomic implications of the polymorphism are discussed.

Bill size polymorphism and intraspecific niche utilization in an African finch

The African estrildid finch, Pyrenestes ostrinus, shows a non-sex-linked polymorphism of bill size. Within P. ostrinus and its congeners, P. sanguineus and P. minor, variation in bill size is extreme, bimodally distributed and not determined by sex, age, body size or geographical origin. P. ostrinus appears to mate randomly with respect to bill size. Bill morphs differ with respect to diet and feeding performance on hard and soft seeds of sedges, Scleria spp. (Cyperaceae), the bird's principal food. The novel pattern of intraspecific divergence in trophic structure contrasts sharply with patterns of morphological variation in Darwin's finches (Geospiza) and other avian species not showing bill size polymorphisms. Coefficients of variation (CVs) for bill size are extraordinarily large while CVs for body size are small and typical of other passerine species. Differences in bill size between morphs exceed those reported for several sympatric species of Darwin's finches believed to show character displacement1, and like many co-occurring congeneric species the morphs differ in diet. Discrete intrapopulation variation in the trophic structures of birds usually consists either of differences between the sexes (sexual dimorphism)2, or of morphological asymmetries, such as bill crossing3,4. Here I describe a third pattern of discrete variation previously hypothesized to occur in the hook-billed kite (Chondrohierax uncinatus)5, and found only in a few species of other vertebrates6–10, in which size differences in trophic apparatus are not determined mainly by sex. Polymorphisms of trophic apparatus have been correlated with differential resource utilization and, despite the scarcity of reports, have been postulated to be important in reducing intraspecific competition and in promoting speciation6.

The Feeding Ecology of Five Sympatric Finch Species.

Within habitats the overlap of food-size utilization of five sympatric finch species is almost 100% despite considerable differences in culmen lengths. It is suggested that the differences in bill size reflect differences in the proportion of large seeds in the set of habitats in which each species feeds and does not function to reduce competition within habitats. Although seed production was sufficient to maintain the finch populations through winter, the finches did eat at least 40% of the available seeds. Of the five finch species commonly present at least three seem to be limited by their winter food supply. The diversity of finches in the yield was not significantly correlated with either food abundance or food diversity.

Ecological Overlap and Bill-Size Differences: A Comment

Ecological Overlap and Bill-Size Differences: A Comment

Seed Selection By Pigeons

The food preferences of 59 captive Pigeons Columbia livia were tested in a series of experiments in which each bird was given a 25 g. seed mixture. This consisted of 5 g. each of Maize, Millet, Grain (Wheat, Oats and Barley), and two varieties of Field Pea (Big Peas and Small Peas). There was a wide range of individual variation. Most birds preferred Peas, and took little Maize; however, others actually preferred minority seeds such as Maize. These individual preferences were stable over short periods, though there were gradual, long-term changes in birds tested repeatedly over a period of four months. There were also individual differences in feeding technique, though these could not be correlated with the differences in seed preference. The origins of these individual variations are discussed. It was not possible to relate them to individual differences in bill size, nor to the proportions of the seeds in the birds' normal feed. When the total quantity of the experimental seed mixture was increased from 25 g. to 50 g., the birds increased their total intake while keeping constant the proportions of the different seeds in it. Since the seeds differ in nutritional composition, it is suggested that the birds were maintaining a nutritionally balanced diet. It is possible that the variations in seed preference may reflect individual differences in metabolism. Individual birds also maintained a constant ratio of Big Peas to Small Peas in their seed intake, even though these differ little, if at all, in nutritional content. It is suggested that such apparently arbitrary food choices might increase the efficiency of the birds' food-finding tactics. Some ecological implications are briefly outlined. If feeding is controlled by the quality as well as the quantity of available food, this could influence the extent to which the birds exploit their environment, and the degree to which their numbers are limited by the food supply.

THE EVOLUTION OF BILL SIZE DIFFERENCES AMONG SYMPATRIC CONGENERIC SPECIES OF BIRDS

Recently, Hutchinson (1959), Klopfer and MacArthur (1961), and Klopfer (1962) have introduced the use of the quantitative comparison of bill size differences among groups of sympatric congeneric species of birds in the study of the evolution of niche overlap and size. In his analysis of character displacement, Hutchinson (1959) has shown that the ratio of the size of the larger to smaller trophic appendages of congeneric species generally falls between 1.2 and 1.4 where they are sympatric, but is less where they are allopatric. Klopfer and MacArthur (1961), using Ridgway's data, found a much smaller ratio for certain associations of sympatric tropical birds. This paper presents the results of an analysis of bill length, in which representatives of 46 bird families inhabiting temperate, subtropical, and tropical zones are compared. Several models are proposed to explain interfamilial, regional, and intrafamilial differences. Finally, the implications of this study for the concepts of niche overlap and behavioral stereotypy are discussed.