EXAMINATION OF REDWINGED BLACKBIRD NESTLING GROWTH RATES USING THE LOGISTIC MODEL: A CASE FOR r AND k SELECTION?

One of the few processes of an avian population that presents the opportunity to collect sensitive information about the performance of that population is growth rates of young throughout the brooding period. Growth rate data are sensitive to many conditions of the breeding cycle: proximate influences, such as food availability and weather (Fran-cis, et al., in prep.), and ultimate factors, such as species-specific characteristics, (Ricklefs, 1968). Additionally, the measurements themselves can be obtained with precision. The ability to make such detailed observations is extremely useful and is not always possible for other population parameters, such as determining life table data. Ricklefs (1967) gave considerable attention to this subject and elaborated techniques for making comparisons of growth rates of young of various bird species. From these, he and others (Fretwell and Bowen, 1974) have gone on to make several predictions about the status of various Two parameters emerged from Ricklefs' work that are quite important for examining these processes: (1) the rate of growth (g) 2 of the young throughout the nesting period and (2) fledging weight (w) of the individual. Ricklefs (1967) presented three different curves which are useful to fit nestling growth data: the logistic, Gompertz, and von Bertalanffy equations. To determine the growth function parameters, Ricklefs (1967) described a four-step graphical method of fitting the logistic equation to empirical weight data. Initially the asymptote of the growth curve is estimated; then the the growth data are recalculated as percentages of the estimated asymptote. For the third a conversion factor [= ¼ log (w/1-w) where w = percentage of the asymptote for each data point, see Ricklefs, 1967] is calculated and plotted as a function of time. In the final step the relationship resulting from step 3 is checked: if it follows a straight line, the growth rate (g) is calculated directly from the slope (g = 4 x the slope). However, if the estimated growth rate (g) does not follow a straight line, a new asymptote is determined and steps 1-4 are reiterated (see Ricklefs, 1967). We were interested in investigating the hypotheses about intraspecific variability put forward by Ricklefs (1968) and in further determining the growth performances of the two major "populations" or "ecological races" (Mayr, 1963) of the Redwinged Blackbird (Agelaius phoeniceus), viz. marshland or wetland inhabitants and upland inhabitants. There have been reports about the possibilities of their distinctness, and we wanted to test whether …