Description:
About bird bill ratios
Biometrics is the quantitative analysis of ontogenetic ("of or relating to life cycle or development") parameters such as height, weight, shape, morphology, age, etc. Biometrics can be applied to groups of organisms (such as a population of Douglas fir trees, or all butterflies, or all mammals) to gain insights into fundamental principles of growth or behavior in those groups.
Schoener (1965) assembled data (from previously published sources) on bill size ratios for populations of several hundred different species of birds. For each bird sampled from a population, the length of the culmen (the upper ridge of the upper bill) was measured. Many bird bills are curved, but the length of the culmen is often measured as the "chord" from the tip of the beak to where the bill meets the fleshy tissue or "cere".
The bill ratio is the ratio of the largest bill to the smallest bill measured in each population. For example, one population of Cooper's hawks (Accipiter cooperi) showed a mean culmen length of 15.2 mm with a ratio of 1.41; the largest bill measured was 1.41 times that of the smallest measured. A ratio of 1 indicates all the birds in that sample had the same bill size.
The bill-size ratio is essentially a measure of bill variability within the population. Not surprisingly, populations with radically varying bill sizes are few, whereas populations with all one bill size are common, as can be seen in the histogram. What conditions could lead to variability in bill sizes in a population? Large ratios are found among highly specialized feeders (woodpeckers) whose prey is not abundant, either because the prey's environment is not abundant (rotten logs) or the prey itself is not abundant (certain grub). Large ratios are found among birds whose body size is large in proportion to the total food abundance; certain hawks, for example. In these and other cases, there is a "forcing" in bill size variability among the species by external pressures (such as food availability). External forcing can lead, in the long term, to evolution of a species.
The data show a good fit to either a power law or exponential relationship between bill size ratio and frequency.
Data sieved from a large number of independent publications often have unique problems. For example, the sample size varies by an unknown amount for each bird species. Is the bill ratio dependent upon sample size? If so, are the data reliable?
