THE MEASUREMENT OF OVERALL BODY SIZE IN BIRDS
ORNITHOLOGISTS are frequently faced with the challenge of measuring body size in birds. A measure of overall size is required to test hypotheses predicting patterns of geographic variation.
An estimate of body size in birds is also required to test hypotheses about the evolution of sexual dimorphism in body size. In addition, species must be ranked by body size to test models that predict size ratios among coexisting species in ecological communities.
In physiology, standard measures of metabolic activity are frequently expressed as a function of body size, and it is often useful to examine the relationship of structures or organs relative to overall body size.
Body size in birds, however, is difficult to measure. Perhaps the best measure of overall body size is total mass, but reliable information on mass is often difficult to obtain. Although recent compilations of data contribute much to our knowledge of the mass of birds the available data on mass are all too often unsatisfactory because of seasonal and diet-related variability.
Consequently, ornithologists commonly use a measure of wing length as an estimate of relative body size, wing length is measured easily on museum study skins and living birds; however, at least in some cases, it is a poor estimate of body size in birds when compared with other, more precise measurements.
Even discounting measurement error, many factors that are difficult to quantify affect the wing length of a bird. First, wing feathers are subject to wear. Thus, the reliability of measurements of wing length decreases as the feathers progressively become more worn. This may be especially important in studies of sexual dimorphism, because in many species behavioral differences between the sexes lead to sexual differences in rates of feather wear. Second, the wing length of an individual varies from year to year–even though the bird’s skeleton is completely ossified (and thus, in this sense, the bird is completely grown).
how to measure a bird (body size in birds)?
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 body size in birds
The length (also bill-to-tail 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.
Culmen
The upper margin of the beak or bill is referred to as the culmen and the measurement is taken using calipers with one jaw at the tip of the upper mandible and the other at base of the skull or the first feathers depending on the standard chosen. In the case of birds of prey where the tip of the mandible may form a long festoon, the length of the festoon may be measured separately as well. In birds of prey the measurement is usually from the bill tip to the ceres. In some birds the distance between the back of the skull and the tip of the beak may be more suitable and less prone to variation resulting from the difficulty of interpreting the feathered base of the mandible.
Head
In some cases, it is more reliable to measure the distance between the back of the skull and the tip of the bill. This measure is then termed as the head. This measurement is however not suitable for use with living birds that have strong neck musculature such as the cormorants.
Tarsus.
The shank of the bird is usually exposed and the length from the inner bend of the tibiotarsus articulation to the base of the toes which is often marked by a difference in the scalation is used as a standard measure. In most cases the tarsus is held bent but in some cases the measurement may be made of the length of this bone as visible on the outer side of the bend to the base of the toes.
Foot
In the case of cranes and bustards, the length of the tarsus is often measured along with the length of the longest toe to the tip of the claw
Tail
The measurement of the tail is taken from the base of the tail to the tip of the longest feathers. In the case where special structures such as racquets or streamers exist, these are separately measured. In some cases, the difference between the longest and shortest feathers, that is the depth of the fork or notch can also be of use.
Wing
The wing is usually measured from the bend of the wing to the tip of the longest primary feathers. Often the wings and feathers may be flattened so that the measure is maximized but in some cases the chord length with natural curvature is preferred. In some cases, the relative lengths of the longest primaries and the pattern of size variation among them can be important to measure.
Wingspan
Wingspan is the distance between wingtips when the wings are held outstretched. This is particularly prone to variation resulting from wing posture and is rarely used except as a rough indicator of size. Additionally, this cannot be easily and reliably measured in the field with living birds.
Weight
The weights of birds are notoriously variable and cannot be used as indication of size. They are however useful in quantifying growth in laboratory conditions and for use in clinical diagnostics as an indicator of physiological condition. Birds in captivity are often heavier than wild specimens. Migratory birds gain weight prior to the migratory period but lose weight during handling or temporary captivity. Dead birds tend to weigh less than in life. Even during the course of a day, the weight can vary by 5 to 10%. The male emperor penguin loses 40% of its weight during the course of incubation