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Bird measurement
Bird measurement or bird biometrics are approaches to quantify the size of birds in scientific studies. The measurements of the lengths of specific parts and the weights of birds varies between species, populations within species, between the sexes and depending on age and condition. In order for measurements to be useful, they need to be well defined so that measurements taken are consistent and comparable with those taken by others or at other points of time. Measurements can be useful to study growth, variation between geographically separated forms, identify differences between the sexes, age or otherwise characterize individuals birds. While certain measurements are regularly taken in the field to study living birds some others are applicable only to specimens in the museum or measurable only in a laboratory. The conventions used for measurement can vary widely between authors and works, making comparisons of sizes a matter that needs considerable care.
All measurement is prone to error, both systematic and random. The measurement of certain bird characteristics can further vary greatly depending on the method used. The total length of a bird is sometimes measured by putting a dead bird on its back and gently pressing the head so that the bill point to the tail tip can be measured. This can however vary with the handling and can depend on the age and state of shrinkage in the case of measurements taken from preserved skins in bird collections. The wing length usually defined as the distance between the bend of the wing and the longest primary can also vary widely in some large birds which have a curved wing surface as well as curved primaries. The measurement can additionally vary depending on whether a flexible tape measure is used over the curve or if measured with a rigid ruler. The definition of the length of the tail can vary when some of them have elongations, forking or other modifications.〔 The weights of birds are even more prone to variability with their feeding and health condition and in the case of migratory species differ quite widely across seasons even for a single individual.
Despite the variations, measurements are routinely taken in the process of bird ringing and for other studies. Several of the measurements are considered quite constant and well defined, at least in the vast majority of birds. Although field measurements are usually univariate, laboratory techniques can often make use of multivariate measurements derived from an analysis of variation and correlations of these univariate measures. These can often indicate variations more reliably.
==Length==
The length of a bird is usually measured from dead specimens prior to their being skinned for preservation. The measurement is made by laying the bird on its back and flattening out the head and neck gently and measuring between the tip of the bill and the tip of the tail. This measurement is however extremely prone to error and is rarely ever used for any comparative or other scientific study.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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