Date of this Version
I have assembled figures for the 43-year period from 1938 through 1980, and have subjected them to various kinds of analysis. During this period, 220 young and 1,437 postjuvenile birds have appeared on the wintering grounds. This works out to an annual recruitment rate of 13.3 percent. (This is not exactly the same as the birthrate, of course, since the loss of eggs, chicks, and juveniles prior to arrival at Aransas is excluded.) The recruitment rate provides an estimate of the rate of maximum annual population increase. One hundred fifty-four birds that had been counted in any given year failed to arrive the following fall and can be considered mortalities. The deaths of these postjuvenile birds, when considered relative to the collective population of surviving postjuveniles, indicate an overall relative mortality rate of 9.7 percent for the 43-year period. Subtracting the annual mortality rate from the annual recruitment rate provides the actual annual rate of population increase: 3.6 percent. Such a rate of increase, which is comparable to that of many human populations, results in a theoretical population doubling time of 19.2 years. Indeed, the wild population has essentially doubled twice in the nearly fifty years since the establishment of the Aransas refuge.