Showing papers by "Allan J. Baker published in 1984"

Morphometric Variation in Introduced Populations of the Common Myna (Acridotheres tristis): An Application of the Jackknife to Principal Component Analysis

Abstract: -Morphometric variation in samples of common mynas from 11 localities in Australia, New Zealand, Fiji and Hawaii was analyzed with principal component analysis. We attempt to avoid two shortcomings in many previous applications of principal component analysis in morphometrics by analyzing separately variation within and among populations, and by applying the jackknife procedure to reduce subjectivity in interpretation of the principal components. Within populations, only principal component I appears to have a stable orientation and this orientation is common to all localities. Component II may be a simple vector, differing across localities, but similarity in the second and third eigenvalues warns that the associated components could be largely arbitrary. The variance along component I does differ across localities. Among populations, again only the first component displays convincing stability, although component II may be a stable but extremely simple vector. Both within and among populations, component I appears to be a general factor representing size and size-related shape variation. The apparently simple patterns of covariation displayed by the introduced populations may be attributable to bottlenecks in small founding populations and the short time since the introductions were made. Future studies should incorporate some form of testing to confirm putative patterns of character covariation, and doing so probably will require sample sizes much larger than has been the custom. [Skeletal morphometrics; principal component analysis; jackknife; population variation; mynas.] A general justification for a morphometric approach to evolutionary studies arises from the common belief that morphology is a primary and direct means by which organisms interact with their environment; variation in size and shape can have physiological and mechanical consequences (Gould, 1966; Alexander, 1968, 1971; McMahon, 1973, 1975; Pedley, 1977). Support for this belief tends to come from large-scale phenomena. Morphological differences among species, for example, are related convincingly to functional differences (e.g., Lack, 1947; Bock, 1970; Liem, 1973; Abbott et al., 1977). It then is assumed generally that such correspondences apply continuously through finer scales of variation. Thus, we arrive at an assumption of morphometricians that measured differences among individuals have functional consequences to the organisms involved, though empirical support for this assumption is rather sparse (but see Grant et al. [1976] and references therein; Herrera, 1978). By further extension, morphological differences among geographically separated populations are believed not only to affect function but also to correlate with environmental differences, leading to generalizations such as the ecogeographic rules. Finally, it is assumed that both withinand amongpopulation variation is adaptive and, therefore, has been crafted by natural selection. Correlation of these two sources of variation is a central prediction of the synthetic theory of evolution (Sokal, 1978). Our goal herein is to learn how skeletal variation is organized within populations of birds, and whether the pattern of this variation is involved in among-population differentiation. Additionally, we wish to describe the pattern of among-population variation and to determine its relationship to within-population structure. Principal component analysis is a common analytical approach here, but many applications have incorporated two methodological

Morphometric variation in introduced populations of the common myna (acridotheres tristis): an application of the jackknife to

Abstract: Morphometric variation in samples of common mynas from 11 localities in Austra- lia, New Zealand, Fiji and Hawaii was analyzed with principal component analysis. We attempt to avoid two shortcomings in many previous applications of principal component analysis in morphometrics by analyzing separately variation within and among populations, and by ap- plying the jackknife procedure to reduce subjectivity in interpretation of the principal compo- nents. Within populations, only principal component I appears to have a stable orientation and this orientation is common to all localities. Component II may be a simple vector, differing across localities, but similarity in the second and third eigenvalues warns that the associated components could be largely arbitrary. The variance along component I does differ across lo- calities. Among populations, again only the first component displays convincing stability, al- though component II may be a stable but extremely simple vector. Both within and among populations, component I appears to be a general factor representing size and size-related shape variation. The apparently simple patterns of covariation displayed by the introduced popula- tions may be attributable to bottlenecks in small founding populations and the short time since the introductions were made. Future studies should incorporate some form of testing to confirm putative patterns of character covariation, and doing so probably will require sample sizes much larger than has been the custom. (Skeletal morphometrics; principal component analysis; jack- knife; population variation; mynas.) A general justification for a morpho- metric approach to evolutionary studies arises from the common belief that mor- phology is a primary and direct means by which organisms interact with their en- vironment; variation in size and shape can

Clutch initiation dates, clutch size, and egg size of the American oystercatcher in Virginia

Abstract: -The timing of egg laying, clutch size, and egg size of the American Oystercatcher (Haematopus palliatus) were studied over six consecutive breeding seasons in Virginia. Synchrony of laying dates occurred in each of five localities of the study area in at least one year. Mean clutch size was 2.8 eggs (mode = 3) in first clutches and 2.4 eggs (mode = 2) in replacement clutches. Individual females laid replacement clutches of the same size and laid eggs of similar average volume in all years. A change in mate had little effect on the date on which females initiated their first clutches in successive years. The average egg size in a clutch was correlated with the size of the laying female. Egg-size ordering occurred within clutches, the first-laid egg being smaller than the second egg and about equal in volume to the third. We propose that the second egg is largest because it has the highest probability of hatching, and the resulting sibling hierarchy reduces the frequency of sibling competition. Received 19 October 1983, accepted 19 April 1984. SYNCHRONY of clutch-initiation dates, uniform egg size, and four-egg clutches are assumed to be adaptations of many shorebirds (Charadrii and Scolopaci) for living within the constraints of high-latitude breeding seasons. Clutch-initiation dates occur over a very brief period (Holmes 1971), and replacement clutches are uncommon (Pitelka et al. 1974), because the period of abundant food resources is brief and the season is telescoped (Holmes 1972, Nettleship 1973, Pitelka et al. 1974). A definitive clutch of four eggs of similar size and shape apparently forms the optimal configuration for minimizing the rate of heat loss when the clutch is uncovered (Norton 1970). Uniform egg size within a clutch also implies no differential allocation of parental care to the chicks (Miller 1979). Uniform chick size may be as important a consequence of four similarly sized eggs as that provided by the energetic advantage during incubation, particularly because a definitive clutch of four is also found in many shorebirds nesting in the temperate zone (Maclean 1972). Oystercatchers (Haematopodidae) are one of the few families of shorebirds whose young depend almost exclusively on the adults for food. 3Present address: 364 Waterloo Avenue, Guelph, Ontario NlM 3K2, Canada. Clutch size in this group is usually less than four eggs, and this may be related to the extensive parental care (Maclean 1972). Egg-size ordering within a clutch is undocumented in oystercatchers, but it might be expected to occur in them rather than in shorebirds that do not feed their young. This is because, through preferential feeding, oystercatcher parents have a greater potential for "control" over which of their chicks survive (Alexander 1974). This potential control might be extended to the eggs, particularly if food is limiting or variable (Howe 1976), if preferential feeding is impractical, and if the probability of raising all chicks is dependent on the food supply. The purpose of this paper is to document variations in clutch size and egg size in American Oystercatchers (Haematopus palliatus) and to elucidate seasonal and annual patterns in the timing of egg laying. By comparing individually known females over several breeding seasons, we have attempted to gauge repeatability measures (Falconer 1981) of these characteristics, as well as the effect of female size.