Populations_Natural History of the Racer Coluber constrictor

_Composition_

Since there is a brief annual breeding season, any local population of

racers consists of a series of discrete annual age groups. The

population reaches its annual maximum in early September, after

undergoing sudden increase by the addition of the annual crop of

hatchlings. Throughout the remainder of the year numbers of racers

undergo gradual reduction as a result of the many combined mortality

factors that affect them. This mortality is distributed among all the

age classes, but the heaviest losses, both percentage-wise and in

actual numbers, are sustained by the first-year young. Being by far

the most numerous group, these young suffer more mortality than all

the other age classes combined. Presumably much of this mortality is

concentrated in the early weeks of life, while the young are still

near their minimum size; the rate of loss is gradually reduced as

larger size is attained and some of the early hazards are outgrown. In

the adult age classes also, the larger and older snakes live in

greater security, and the rate of mortality is higher in the smaller

and younger snakes. Even before hatching, the eggs are subject to

heavy losses from predators, and probably from drying, flooding, and

other unfavorable climatic factors. Unfortunately it was not possible

to obtain definite figures on any of these losses since the eggs were

never found under natural conditions and the hatchlings were seen only

in relatively small numbers.

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.

[Illustration: Fig. 20. Histograms comparing snout-vent length in

random samples of _Coluber c. flaviventris_ (those captured on the

Reservation and Rockefeller Tract in 1960 and 1961) and _C. c. mormon_

(specimens in the Museum of Vertebrate Zoology, University of

California). In the museum sample, collected by conventional methods,

the first-year young constitute a prominent and fairly distinct size

group, whereas in the sample of racers from the Reservation and

Rockefeller Tract, mostly caught in wire traps of quarter-inch mesh,

relatively few young were secured. It is demonstrated that hatchlings

are approximately the same length in both populations, but

_flaviventris_ grows much longer, and that the differences in length

between the sexes is approximately the same in each population.]

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

===========+==============+============+============+============+=========

-----------+--------------+------------+------------+------------+---------

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 | .....

| | | | |

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

==========================+======+======+======+======+===========

| 1959 | 1960 | 1961 | 1962 | average

--------------------------+------+------+------+------+-----------

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

| | | | |

May | 3 | 25 | 54 | 59 | 141

June | 9 | 6 | 25 | 40 | 80

Recaptures | 0 | 3 | 6 | 24 | 33

Estimated population | | 50 | 165 | 99 | 85

| | | | |

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

| | | | |

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

| | | | |

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

| | | | |

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

==========================+=====+=====+=====+=====+=====+=====+=====+====

| A | B | C | D | E | F | G | H

--------------------------+-----+-----+-----+-----+-----+-----+-----+----

May | 11 | 21 | 7 | 24 | 17 | 13 | 6 | 99

June-July-Aug. | 20 | 11 | 25 | 20 | 22 | 12 | 14 | 124

Recaptures | 2 | 1 | 4 | 5 | 5 | 2 | 0 | 19

Estimated population | 110 | 231 | 44 | 96 | 75 | 78 | ... | 92

| | | | | | | |

May-June | 18 | 25 | 17 | 28 | 27 | 15 | 13 | 143

July-Aug.-Sept.-Oct | 15 | 8 | 17 | 15 | 15 | 11 | 8 | 89

Recaptures | 4 | 1 | 7 | 5 | 9 | 3 | 1 | 30

Estimated population | 68 | 200 | 41 | 84 | 45 | 55 | 104 | 61

| | | | | | | |

May-June-July | 26 | 28 | 22 | 31 | 31 | 20 | 19 | 177

Aug.-Sept.-Oct | 5 | 7 | 13 | 12 | 6 | 5 | 2 | 50

Recaptures | 1 | 2 | 7 | 5 | 3 | 1 | 1 | 20

Estimated population | 130 | 128 | 47 | 87 | 63 | 75 | 148 | 63

| | | | | | | |

Combined first samples | 55 | 74 | 46 | 83 | 75 | 48 | 38 | 419

Combined second samples | 40 | 26 | 55 | 47 | 43 | 28 | 24 | 263

Combined recaptures | 7 | 4 | 18 | 15 | 17 | 6 | 3 | 70

Estimated population | 105 | 160 | 47 | 87 | 63 | 75 | 101 | 75

--------------------------+-----+-----+-----+-----+-----+-----+-----+----

[Illustration: PLATE 19

Fig. 1. Head of hatchling blue racer, dorsal view, September 1962,

approximately × 3.

Fig. 2. Head of hatchling blue racer, lateral view, September 1962,

approximately × 3.

Fig. 3. Head of yearling male blue racer, lateral view, August 1,

1961, a little less than twice natural size.

Fig. 4. Head of adult male blue racer, lateral view, July 16, 1961,

approximately × 2. All three snakes from Rockefeller Experimental

Tract, Jefferson County, Kansas.]

[Illustration: PLATE 20

Fig. 1. Abandoned limestone quarry on a hilltop of southward exposure

on The University of Kansas Natural History Reservation, in late

autumn of 1951. The crevices along the base of the ledge provided

favorite hibernating sites for blue racers.

Fig. 2. Hatchling blue racer and eggshell from which it had recently

emerged, in early September, 1962; × approximately 1-2/3.]

[Illustration: PLATE 21

Fig. 1. Wire funnel trap set at base of hilltop limestone outcrop in a

spot strategically located for interception of blue racers searching

for deep crevices in which to hibernate, October 15, 1949.

Fig. 2. Large clutch of 21 blue racer eggs, recently plowed out, at

Harold Brune farm, Jefferson County, Kansas, July 10, 1962.]

[Illustration: PLATE 22

Fig. 1. Habitat of blue racer, blue-stem prairie on Botany Bluff at

northwest corner of the University of Kansas Natural History

Reservation, looking south. Trees and brush in background are along

limestone outcrop at top of slope. Mowed area in foreground is

southwest corner of Rockefeller Experimental Tract, a privately owned

farm at the time this photograph was taken in the summer of 1951.

Fig. 2. Habitat of blue racer, blue-stem prairie on south slope of

Botany Bluff, looking north along west edge of the Reservation, summer

of 1951. By 1962, with exclusion of fire, and protection from mowing,

prairie vegetation had largely disappeared from this slope, and had

been replaced by trees and brush. As a result of these successional

changes racers no longer found this slope a suitable habitat in

summer, but they continued to resort to the hilltop rock outcrop to

hibernate in autumn.]

Some local areas probably support higher populations of racers than do

areas where censuses were made, but under modern conditions,

situations that offer near optimum habitat are not likely to be

extensive or to persist long. On land that is capable of producing a

good crop of vegetation, the crop is usually harvested either by

grazing of livestock or by using the land for cultivation, with the

result that the racers are, at least in some seasons, forced into

marginal situations. More than 50 years ago in Missouri, Hurter

(1911:170) wrote that the racer "was quite common 20 years ago in

pastures, meadows and fields but as cultivation has advanced it is

becoming quite rare." In 1962 the widespread and adaptable blue racer

is still common in many parts of its range, including Missouri, but in

most places its population densities probably are lower than formerly.

Reduction since 1911 has probably been far more drastic than the

reduction that had occurred up to that time. Schmidt and Necker

(1935:69), writing of the racer in the Chicago region, noted "the

snakes which raise their heads and face mowing machinery tend to be

exterminated in agricultural areas." They stated that in the Chicago

region the racer had been exterminated by the advance of agriculture

except in two extensive sand dune areas. In July 1962, Mr. V. B.

Howell, a progressive farmer of the Great Bend area in central Kansas,

told me that the kinds of snakes inhabiting cultivated land--blue

racer, bull snakes, prairie king snakes, hog-nosed snakes, and

others--had undergone great reduction in numbers during the period of

his farming. He estimated that in a forty-year period the numbers had

declined to perhaps five per cent of their level in the area most

familiar to him, centering at his farm 11 miles northwest of Great

Bend, Barton County. In accounting for this change in population

density Howell pointed out the relative destructiveness to small

animals of modern farm machinery as contrasted with horse-drawn

equipment or that used with tractors of earlier models. Modern

tractors move forward so rapidly that there is little opportunity for

snakes or other small animals to avoid them, and the plows and disks

cut wide swaths penetrating more deeply into the soil than did older

types. On July 10, 1962, in searching the furrows of a freshly plowed

small field on the Harold Brune farm, for turned-up nests of the

snakes, I found two adult blue racers that had been struck and killed

by the plow, possibly while they were underground. In fields that are

plowed or cultivated between the times of egg-laying and hatching, the

eggs are destroyed. Because of its rapid movements and alertness, the

racer is more likely to escape farm machines than are most other

kinds of snakes. Nevertheless, it is vulnerable and survives in

cultivated areas only when they are interspersed with pastures,

woodlots, or streamside thickets where at least part of the population

may find refuge.