In the 1950-51 season, small young of the summer brood seemed unusually numerous. In the 1951-52 period, young of both age classes were relatively scarce and old adults made up an unusually high proportion of the population. Excluding the 14 marked pouch young that were later recaptured, there were only four of the total of 106 that were trapped in each of two seasons. One young less than a quarter grown, that was accidentally caught in a live-trap set for woodrats, was recaptured as a breeding adult the following winter. An adult male and two adult females each caught in the 1949-50 season were each recaptured repeatedly in the 1950-51 season. Ninety-five per cent replacement of the breeding population by the following breeding season is indicated by our figures. Only 3 (or 5 per cent) of the individuals of the population trapped and marked in the season of 1949-50, were recaptured among the 62 opossums recorded in the two subsequent seasons. Various mortality factors including predation, disease, and accidents account for some 70 per cent. These are replaced by first-year young which make up the greater part of the breeding population. The remaining 25 per cent presumably shift their ranges sufficiently in the course of a year to have moved beyond the limits of an area of the size encompassed by the present study.
No precise measurement of the population density on the study area was obtained. It was not practical to capture every individual present there, and rapid population turnover, due to mortality and wandering, obscured the trends. The information obtained concerning movements of opossums suggest that one may habitually forage as much as 900 feet from its home base. Assuming that 900 feet is the typical cruising radius, the areas drawn upon by the trap lines in the three different seasons were approximately as follows: 1949-50—400 acres; 1950-51—350 acres; 1951-52—220 acres. In these same three seasons the numbers of opossums caught were, respectively, 46, 37, and 30. If these figures represent the numbers actually present, densities of one to 8.7 acres, one to 9.5 acres, and one to 7.3 acres are indicated. However, some opossums using the area probably were missed; and on the other hand, not all those caught in the course of a season were present there simultaneously. Many of those present early in the season would have moved away a few months later, and others would have moved in, replacing them. The number present at any one time could scarcely have been more than half the number caught in the entire season.
Census With Half-monthly Sampling Periods
| Sampling period | Number of individuals taken in period | Number of individuals taken in following period | Number of recatures in following period | Computed population for sampling period |
| Early November 1949 | 3 | 7 | 1 | 21 |
| Late November 1949 | 7 | 8 | 3 | 18.7 |
| Early December 1949 | 8 | 11 | 3 | 29.3 |
| Late December 1949 | 11 | 7 | 4 | 19.2 |
| Early January 1950 | 7 | 3 | 1 | 21 |
| Early March 1950 | 5 | 8 | 2 | 20 |
| Late March 1950 | 8 | 6 | 3 | 16 |
| Early April 1950 | 6 | 3 | 1 | 18 |
| Late April 1950 | 3 | 6 | 2 | 9 |
| Early May 1950 | 6 | 3 | 2 | 9 |
| Early November 1950 | 1 | 3 | 1 | 3 |
| Late December 1950 | 3 | 6 | 1 | 18 |
| Early February 1951 | 4 | 13 | 3 | 17.3 |
| Late February 1951 | 13 | 6 | 3 | 26 |
| Early March 1951 | 6 | 4 | 3 | 8 |
| Late March 1951 | 4 | 5 | 2 | 10 |
| Early April 1951 | 5 | 1 | 1 | 5 |
| Late April 1951 | 1 | 5 | 1 | 5 |
| Early May 1951 | 5 | 3 | 2 | 7.5 |
| Early February 1952 | 9 | 4 | 2 | 18 |
| Late February 1952 | 4 | 9 | 1 | 36 |
| Early March 1952 | 9 | 6 | 2 | 27 |
| Late March 1952 | 6 | 5 | 2 | 15 |
Census with Monthly Sampling Periods
| Sampling period | Number of individuals taken in period | Number of individuals taken in following period | Number of recatures in following period | Computed population for sampling period |
| November 1949 | 9 | 16 | 7 | 21 |
| December 1949 | 16 | 9 | 3 | 48 |
| March 1950 | 11 | 9 | 3 | 33 |
| April 1950 | 9 | 7 | 2 | 32 |
| October 1950 | 9 | 3 | 3 | 9 |
| November 1950 | 3 | 3 | 1 | 9 |
| December 1950 | 3 | 7 | 3 | 7 |
| January 1951 | 7 | 14 | 3 | 33 |
| February 1951 | 14 | 7 | 4 | 25 |
| March 1951 | 7 | 5 | 3 | 12 |
| April 1951 | 5 | 6 | 3 | 10 |
| November 1951 | 3 | 6 | 1 | 18 |
| December 1951 | 6 | 5 | 1 | 30 |
| January 1952 | 5 | 11 | 3 | 18 |
| February 1952 | 11 | 13 | 4 | 36 |
| March 1952 | 13 | 9 | 5 | 23 |
| April 1952 | 9 | 3 | 1 | 27 |
Crude census-figures were obtained by utilizing the Lincoln Index and computing the total on the basis of the ratio of marked (and recognizable) individuals to others caught in a sampling period. A large number of census figures were obtained over the three-year period of the study. Each separate census, however, was based on an inadequate sample as the number of marked individuals taken at each sampling, as recaptures from the previous sampling period, varied from one to five. While little confidence can be placed in any one census computation, the trends of figures from series of such computations reveal the approximate number of opossums on the area if due allowance is made for certain distorting factors. Presumably the differences in figures obtained at different samplings result chiefly from the margin of error in the data, although it is true that there is rapid change in the actual number of opossums.
The number of active opossums in the region of the study reaches a peak in late summer and early fall, when second litters of young have grown large enough to become independent. At this season the population contains a high proportion of young of the year. During the ensuing months of fall and winter there is a steady decrease in numbers, through various mortality factors, with no replacement until young are born about the first week of March. These young do not become independent until late May or early June, and during the intervening months there is a further reduction of the adults and yearlings, so that the active population reaches its annual low point in late spring. At that time of year most opossums are in poor physical condition.