title: Comparison of geometric morphometric outline methods in the discrimination of age-related differences in feather shape.
abstract: BACKGROUND
  Geometric morphometric methods of capturing information about curves or outlines of organismal structures may be used in conjunction with canonical variates analysis (CVA) to assign specimens to groups or populations based on their shapes. This methodological paper examines approaches to optimizing the classification of specimens based on their outlines. This study examines the performance of four approaches to the mathematical representation of outlines and two different approaches to curve measurement as applied to a collection of feather outlines. A new approach to the dimension reduction necessary to carry out a CVA on this type of outline data with modest sample sizes is also presented, and its performance is compared to two other approaches to dimension reduction.
  
  
  RESULTS
  Two semi-landmark-based methods, bending energy alignment and perpendicular projection, are shown to produce roughly equal rates of classification, as do elliptical Fourier methods and the extended eigenshape method of outline measurement. Rates of classification were not highly dependent on the number of points used to represent a curve or the manner in which those points were acquired. The new approach to dimensionality reduction, which utilizes a variable number of principal component (PC) axes, produced higher cross-validation assignment rates than either the standard approach of using a fixed number of PC axes or a partial least squares method.
  
  
  CONCLUSIONS
  Classification of specimens based on feather shape was not highly dependent of the details of the method used to capture shape information. The choice of dimensionality reduction approach was more of a factor, and the cross validation rate of assignment may be optimized using the variable number of PC axes method presented herein.
authors.affiliation: Department of Physics, Canisius College, Buffalo, NY 14208, USA. sheets@canisius.edu
authors.forename: H David
authors.initials: HD
authors.lastname: Sheets
authors.forename: Kristen M
authors.initials: KM
authors.lastname: Covino
authors.forename: Joanna M
authors.initials: JM
authors.lastname: Panasiewicz
authors.forename: Sara R
authors.initials: SR
authors.lastname: Morris