Part 13

The records obtained from trapping racers in late spring and summer in fields provided a somewhat different picture of the population from the sample obtained along the ledges in autumn. In the former sample there were 400 males to 257 females, but in the latter sample there were 355 males to 379 females. I regard the summer sex ratio as a distorted one, brought about by the greater activity of the males in the breeding season. Racers are caught most easily in May, and the fact that two or more males often were trapped with the same female, while the reciprocal combinations did not occur, demonstrates the increased activity of the males in their search for mates at this season. In autumn there is no sexual activity; both sexes probably are equally active in seeking places to hibernate when they are trapped along the hilltop outcrops. The ratio of 51.6 per cent females in my sample of 734 may indicate that in the males greater activity at other seasons results in a somewhat higher mortality. This idea is borne out by the fact that for the supposed two-, three- and four-year-olds combined, females comprise 51.2 per cent, but they comprise 55.6 per cent of those more than four years old and 61.3 per cent of those more than five years old.

By assigning to each racer caught an arbitrary age, on the basis of size according to Table 17, I calculated the population (exclusive of those snakes in their first year of life) to have the composition shown in Table 19.

Table 19. Percentages of Adult Population of Blue Racers Comprised by Each Annual Age Group

=================+=============== | Percentage of Years of Age | population -----------------+--------------- 2 | 41.5 3 | 17.8 4 | 12.6 5 | 9.5 6 | 6.1 7 | 4.3 8 | 2.7 9 | 2.4 10 | 1.2 11 or more | 1.9 -----------------+---------------

Figures are completely lacking to show the relative numbers of juveniles, until, already approaching adult size, the young are about to enter their second hibernation. My combined fall samples include 303 of these latter young, as against 142 racers about to enter their third hibernation. Thus, after having nearly attained adult size, these adolescent snakes sustained a loss of 53 per cent in a year. Losses must occur at an even more rapid rate in the younger and smaller snakes. It may be speculated that of the approximately 300 eggs produced by a population of 100 adult racers, 150 are lost before or during the period of incubation which lasts nearly two months. Of the 150 hatchlings emerging in early September, at least one-third probably are eliminated by the following breeding season in late May, leaving 100. The 100 survivors at this stage are still small juveniles, but by autumn they have attained adolescent size. By this time, if they had undergone a further reduction by 53 per cent, only 47 would remain--approximately the number to be expected if the population were stable from year to year.

A notable difference between the fall sample and the spring sample that I obtained was the higher proportion of large and old racers in the former sample. This difference can be attributed to the year-to-year changes in the population during the 14 consecutive years spanned by my field work. The fall sample of 734 racers represented the combined catch of the years 1949 through 1962, rather evenly distributed, but the spring sample included few snakes from the years 1949 through 1957; most were from the years 1958 through 1961. In 1949 when the study was begun, the Reservation was being protected for the first time, and formerly overgrazed pastures or cultivated fields were acquiring a rank growth of grass and weedy vegetation, and thus becoming favorable habitat. The abundant new habitat promoted rapid increase in the population of racers until the newly available areas were filled to their "carrying capacity." Table 21 shows the changing trends of the different age groups. Although the separate annual samples are perhaps too small to show the composition of the population accurately, it is significant that in the fall of 1949 an unusually high proportion of the racers caught were one-year-olds, hatched in September, 1948.

Table 20 shows that in a typical group of 100 subadult and adult racers (second year and older) only a little more than one-fourth are productive females. The largest females, six years old and older, making up less than ten per cent of the adult population, contribute nearly half the total complement of eggs.

The calculated number of eggs pertains to a stage before oviposition, and subsequent losses through resorption under unfavorable conditions, through inviability of embryos and through deaths of some of the gravid females, are to be expected. In the weeks of incubation further losses are sustained. Although these losses cannot be measured, they must be severe as on numerous occasions scattered and torn eggshells representing entire clutches dug out and destroyed by predators, have been found. Probably other clutches are destroyed underground by such predators as moles and egg-eating snakes, and still others by insects. Tinkle (1959:195) wrote that in a clutch of 15 eggs found under a board, four were parasitized and had small perforations. Molds destroy a high percentage of all reptilian eggs that are incubated artificially and doubtless destroy many under natural conditions also. Excessive heat or moisture, or desiccation, resulting either from climatic extremes or from poor choice of a nest site by the female, would cause further loss. In four different years, Blair (1960:108) found that losses of eggs between laying and hatching in the Texas spiny lizard (Sceloporus olivaceus) ranged from 69 per cent to 86 per cent; no other comparable study of the extent of egg losses in a species of reptile is known to me. The racer is somewhat less prolific than the spiny lizard, and potentially longer lived; the racer's eggs are larger and thicker-shelled, and they are deposited in deeper burrows. It might be expected that losses during incubation would be somewhat less in the racer than in the spiny lizard.

Table 20. Calculated Productivity in a Hypothetical Group of 100 Subadult and Adult Blue Racers

===========+==============+============+============+============+========= | Percentage | Percentage | Percentage | Eggs | Age-Group | of | of fecund | productive | per | Number (years) | population | females in | females in | productive | of eggs | in age-group | age-group | age-group | female | produced -----------+--------------+------------+------------+------------+--------- 2 | 41.5 | 51.2 | 13 | 9.2 | 26 3 | 17.8 | 51.2 | 56 | 9.9 | 51 4 | 12.6 | 51.2 | 60 | 10.8 | 42 5 | 9.5 | 55.0 | 57 | 13.0 | 39 6 and over | 18.6 | 61.3 | 80 | 15.7 | 143 -----------+--------------+------------+------------+------------+---------

Table 21. Percentages of Racers in Each Annual Age Group (Exclusive of Hatchlings) in Autumnal Samples at Different Stages of the Field Study, Showing Shift Toward Older Age-Groups in the Later Years

==============+========================================================= | Year or combination of years represented by each sample +-----------+-----------+-----------+----------+---------- | 1949 | 1950 | 1953 | 1956 | 1959 Years of Age | | 1951 | 1954 | 1957 | 1960 | | 1952 | 1955 | 1958 | 1961 | | | | | 1962 --------------+-----------+-----------+-----------+----------+---------- 1 | 54 | 44 | 39 | 39 | 41 2 | 14 | 24 | 21 | 19 | 19 3 | 18 | 11 | 23 | 16 | 13 4 | 4 | 10 | 10 | 11 | 9 5 | 6 | 6 | 3 | 8 | 8 6 | 2 | 1 | 3 | 1 | 3 7 | 2 | 4 | 4 | 2 | 3 8 | ...... | ...... | ...... | .5 | 2 9 | ...... | 1 | .4 | .5 | 2 10 | ...... | ...... | .4 | 2 | 1 Older than 10 | ...... | ...... | ...... | .5 | ..... | | | | | Number in | | | | | sample | 49 | 126 | 117 | 194 | 242 --------------+-----------+-----------+-----------+----------+----------

Although figures for the youngest age groups--one-year-olds and hatchlings--are missing, approximations of them may be furnished by extrapolation, from the information available regarding the productivity of the population. Some factors involved in productivity are that the sex ratio deviates from parity, slightly in favor of the females in the adolescents but more markedly in favor of the females among the older age groups; that some adult females apparently fail to produce eggs in the breeding season, but the percentage decreases in the older snakes; and that number of eggs per clutch increases in proportion to the size and age of the female producing them. Too few figures are available concerning most of these factors to indicate more than the trends; nevertheless the available figures have been used in Table 20 in an attempt to estimate the productivity of a hypothetical population.

Numbers

Conant (1938:178, Pl. 7) published a photograph of 106 blue racers killed in February, 1932, by farmers near Bellville, Ohio, and Pope (1944:173) mentioned that scores of blue racers aggregated in October around an oak-covered dune near Chicago. In both these instances large hibernating aggregations were involved, and the areas represented by them are unknown; nothing has been recorded regarding population densities.

The records obtained through my fall trapping, along hilltop rock outcrops, yielded no information concerning population densities, but those obtained in fields in summer did provide significant information in this regard. Even after years of trapping on the same area, the catch still consisted largely of new individuals; the method was not sufficiently effective to catch all racers present at any one time, and the total catch for a season therefore provided only a crude index of the minimum number present.

The summer trapping was carried on in three separate areas. One of these was the area of bottomland pastures and formerly cultivated fields where the Reservation headquarters are located, a block of 39 acres bounded on three sides by woodland, and on the fourth by cultivated farm land. Effective trapping in this area was carried on through the years 1955 to 1961 inclusive. A second area, of 48 acres, was one of upland fields, mostly covered with re-established prairie grasses, in the northeastern part of the Reservation. A third area, of 137 acres, also upland, was that of the Rockefeller Tract, cultivated through 1956 and sown to prairie grasses the following year, and the adjacent northwestern hilltop portion of the Reservation. Effective trapping on these two latter areas was carried on in 1958 through 1962.

For the seven years of trapping in the House Field area, the catch was as follows: 30, 33, 38, 38, 34, 24, and 20. In four years of trapping, the northeast field area yielded 42, 28, 37, 59, and 19 blue racers, and the Rockefeller Tract yielded 52, 67, 67, 126, and 106. The actual catch was hence less than one per acre in nearly all instances, but the year-to-year differences in catch are believed to be caused chiefly by differences in numbers of traps used and in trapping effort, rather than by changes in the numbers of racers present.

Best index to the number of racers actually present is provided by the number of recaptures, and their ratio to first captures. The population of course, undergoes alteration from year to year, with many racers eliminated and replaced by others.

In 1955, 26 racers were caught in the May-June-July period, in the headquarters field area. In the August-September-October period of the same year five racers were caught of which only one was a member of the original 26. The five-to-one ratio indicates that the original 26 may have represented an actual population of 130, but of course the single recapture is much too small a sample to provide a reliable ratio. Some of the racers caught in May were recaptured in June, others in July, and still others not until late summer or early autumn. Somewhat different estimates can be obtained for the population depending on how the season's records are divided. For instance, in the headquarters field area in May 1955, eleven racers were caught; in the remainder of the season 20 were caught, of which two were members of the original group of eleven. The 20 to 2 ratio indicates that the 11 caught in May represented an actual population of 110. In the period May-June, 18 racers were caught, and in July-August-September-October, 15 were caught, including four of the original 18, hence indicating a population of 67. Although obviously the population underwent some change during the course of the season, the three sets of census figures apply essentially to the same population, and the divergence in them illustrates the wide range of error arising from insufficiently small samples.

Common sources of error in the censusing of natural populations of animals by the capture-recapture method ("Lincoln Index" or "Petersen Index") arise from the fact that the composition of a local population often changes between two sampling periods, or even within them. Some of the animals marked may move elsewhere, to be replaced by unmarked immigrants, or they may die and be replaced by unmarked maturing young. First-year racers that could pass through the quarter-inch mesh of the traps in spring and early summer became too large to escape in this way in the latter half of the summer, but these young were excluded from the census computations. There was doubtless some shifting of marked individuals away from the study areas and shifting of new individuals onto these areas in the periods of weeks between successive samplings. Jackson (1939) has explained a method of correcting census computations based on capture-recapture ratios when there is a consistent trend of diminishing recaptures with increase in elapsed time. However, in my records no such trend is discernible; furthermore it has been demonstrated that individual racers tend to stay within the same home range area throughout most of their season of activity. Therefore, I conclude that shifts of individuals away from the study areas or into them, in the intervals between samplings constitute only a minor source of error.

A combination of the figures from the three samples listed above provides an intermediate "smoothed" figure that can be accepted with somewhat more confidence than any one of the separate censuses because it is based on more records. The combined ratios indicate a total of 105 racers in the headquarters field area. The figures obtained in the different sampling periods, and the census figures derived from their ratios are shown in tables 22 and 23. Differences from year-to-year in the census figures for any one area show no consistent trends and their variation is similar to that shown in different samples for the same season. Probably populations were fairly stable throughout the periods involved. If such stability is assumed, the samples from different years may be combined, and the composite figures derived from them may be accepted with more confidence. For the headquarters field area, for instance, 419 records of racers were gathered in all the preliminary sampling periods of the seven seasons involved; the records totalled 263 for all the secondary sampling periods, with 70 recaptures in secondary periods of the snakes recorded in the corresponding preliminary periods. A population of 75 racers is indicated--1.9 per acre. Corresponding figures for the northeast field area are: preliminary samples 453, secondary samples 163, recaptures 39, calculated number 135 (2.82 per acre). For the Rockefeller Tract the figures are as follows: Preliminary samples 807, secondary samples 476, recaptures 126, the ratio indicating a population of 153, or 1.11 per acre.

These figures represent the number of adults present in early summer when the population is near its annual low point. The first-year young, excluded from this census because they cannot be caught in representative numbers, perhaps approximate the number of adults, in May, so the figures obtained would need to be approximately doubled to be representative of the entire population. By late summer the adults, and especially the yearlings, have undergone substantial reduction in numbers, but in late August and early September the hatching of a new crop of young increases the population to its annual maximum. The maximum numbers probably are about three times those obtained by censusing adults in early summer. The peak population of late summer or early autumn is estimated to consist of hatchlings, comprising somewhere near 50 per cent; adults, comprising a little more than 25 per cent; and yearlings comprising a little less than 25 per cent.

Table 22. Captures Recorded and Population Calculated From Them on Hilltop Grassland Areas of Rockefeller Experimental Tract and Adjacent Reservation in Four Different Years

==========================+======+======+======+======+=========== | | | | | Four-year | 1959 | 1960 | 1961 | 1962 | average --------------------------+------+------+------+------+----------- First Census: | | | | | Early May | 0 | 8 | 21 | 32 | 61 Late May | 3 | 13 | 33 | 35 | 84 Recaptures | 0 | 1 | 2 | 7 | 10 Estimated population | | 104 | 346 | 160 | 128 | | | | | Second Census: | | | | | May | 3 | 25 | 54 | 59 | 141 June | 9 | 6 | 25 | 40 | 80 Recaptures | 0 | 3 | 6 | 24 | 33 Estimated population | | 50 | 165 | 99 | 85 | | | | | Third Census: | | | | | May-June-July | 25 | 34 | 94 | 100 | 253 Sept.-Oct | 8 | 13 | 20 | 13 | 54 Recaptures | 1 | 3 | 5 | 4 | 13 Estimated population | 200 | 182 | 376 | 324 | 263 | | | | | Fourth Census: | | | | | May-June | 31 | 30 | 73 | 76 | 210 July-Aug.-Sept.-Oct | 24 | 15 | 34 | 35 | 108 Recaptures | 3 | 2 | 14 | 12 | 31 Estimated population | 104 | 225 | 177 | 222 | 183 | | | | | Fifth Census: | | | | | May | 3 | 25 | 54 | 60 | 142 June to October | 23 | 20 | 67 | 40 | 150 Recaptures | 0 | 5 | 14 | 20 | 39 Estimated population | | 100 | 258 | 120 | 162 | | | | | Five Sets Combined: | | | | | Combined first samples | 62 | 122 | 296 | 327 | 807 Combined second samples | 67 | 67 | 179 | 163 | 476 Recaptures | 4 | 14 | 41 | 67 | 126 Estimated population | 208 | 117 | 258 | 159 | 153 --------------------------+------+------+------+------+-----------

Densities in early summer of one to three adult blue racers per acre probably are typical of the better types of habitat in the region of my study. The upland field area estimated to have 2.82 racers per acre was better habitat than the other two study areas. Prior to 1948 it had been cultivated and severely eroded. In 1949 most of it was sown to seeds of prairie grasses, and by 1958 different parts of it were dominated by different species of native perennial tall grasses interspersed with areas that supported a weedy type of vegetation, and other areas that supported dense thickets of sumac, dogwood, elm saplings, or other woody plants. The abundance and diversity of dense cover and of small animals made this area especially favorable habitat for the racer.

Table 23. Captures Recorded and Populations Estimated From Them in Headquarters Field Area of Reservation

Column headings: A: 1955 B: 1956 C: 1957 D: 1958 E: 1959 F: 1960 G: 1961 H: Seven-year average