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
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
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?
Schoener, T. W. (1965), The evolution of bill size differences among sympatric congeneric species of birds;
Evolution, vol. 19, pp. 189-213.