No consistent trend of difference between the sexes in incidence of injuries to the tail was noticeable, but there was definite correlation with age. In the entire sample from the Reservation and Rockefeller Tract the percentages in each supposed age group (actually size group) lacking part of the tail were as follows: hatchlings, 2.9 per cent; one-year-olds, 9.5 per cent; two-year-olds, 15.8 per cent; three-year-olds, 14.3 per cent; four-year-olds, 15.7 per cent; five-year-olds, 23.2 per cent; six-year-olds, 30.9 per cent; seven-year-olds, 28.9 per cent; those eight years old or older, 21.1 per cent. The seeming reversal of trend in the older racers is difficult to explain, but probably results from inadequate numbers in this part of the sample.
Although the racer's most characteristic response to any disturbance is to flee at high speed, certain individuals are inclined to behave aggressively under exceptional circumstances. Records of such aggressive behavior nearly all pertain to large individuals of the northeastern black racer (C. c. constrictor) in the breeding season. Cope (1900:794) wrote: "it is courageous and will sometimes attack, moving forward with the head raised from 1 to 2 feet above the ground." Ditmars (1944:13) wrote: "Occasional specimens in the breeding season ... will actually attack ... glide toward an intruder ... striking madly at one's feet or legs." Woods (1944:257) quoted a 13-year-old amateur herpetologist, Leon Gonthier, regarding the latter's encounter with an aggressive black racer on May 7, 1944, as follows: "When I ran toward the snake about 20 feet away, it turned and came for me. As I bent over to catch it, the snake grabbed me by the shirt and hung on ... held it off with a stick. It jumped twice more at me and came clear off the ground." Finneran (1948:124) describing the species' habits in Connecticut, stated: "A farmer ... constantly warned me away from Coon Ledge during the spring, saying the snakes were breeding and would 'chase you.' This very thing happened. In 1943 a blacksnake followed me for approximately ten feet, and, in 1946, a male aggressively attacked me for a period of three minutes. There was ample opportunity for escape."
Little has been published concerning the natural enemies of the common racer. The king snake (Lampropeltis getulus) is notorious for ophiphagy, and doubtless preys upon the racer at times. Wright and Bishop (1915:169) wrote of the king snake in Okefinokee Swamp, in Georgia, that "all the smaller snakes suffer, and of the larger species, the blacksnake [racer] and spreading adder are the commonest prey." However, Clark (1949:252) examined 301 stomachs of king snakes of this species in Louisiana and found no racers, although many other kinds of snakes were represented, and, collectively, made up the greater part of the food.
A rat snake (Elaphe obsoleta) found in Leon County, Florida, on June 10, 1924, had a racer in its stomach, according to the Patuxent food habits file of the U. S. Fish and Wildlife Service. A California garter snake (Thamnophis elegans terrestris) was found to have a juvenal racer in its stomach (Fitch, 1940:96). A large alligator lizard (Gerrhonotus multicarinatus) was found swallowing a small racer that had been confined with it in a bag (Fitch, 1935:12).
A few records of predation on blue racers by other reptiles on the Reservation and Rockefeller Tract were obtained. In 25 scats of the prairie king snake (Lampropeltis calligaster), there were 29 vertebrate prey items of which one was a blue racer. On September 15, 1962, an adult male prairie king snake 491/2 inches in length was found to have a 27-inch yearling racer in its stomach. In 254 scats of the slender glass lizard (Ophisaurus attenuatus) vertebrate remains were rare, but there were scales and bones of one hatchling blue racer. Among 21 vertebrate prey items in 14 scats of the timber rattlesnake (Crotalus horridus) there were remains of one blue racer. Among 589 prey items of copperheads there were two juvenal blue racers (Fitch, 1960:200). In contrast to these scarce records of racers in the prey of other snakes, there were more frequent records of the blue racer preying on its own young. The 1008 food items from 479 racer scats contained remains of 16 racers, small young in most instances.
Raptorial birds are known to be important predators on snakes. Breckenridge (1944:118) reported finding remains of a blue racer in the pellet of a marsh hawk (Circus cyaneus). Many pellets of the marsh hawk collected on the Reservation were all found during the colder half of the year, and they contained no remains of reptiles. The same seasonal restrictions applied to the many pellets of four species of owls that were collected, and these also lacked remains of reptiles. One of the predators whose food habits have been most thoroughly investigated on the area is the red-tailed hawk (Buteo jamaicensis). Over the period 1955 through 1962, 1131 pellets of these hawks were collected, many from the Reservation, but more from localities scattered throughout the eastern one-fourth of Kansas. The pellets were those of nestlings and fledglings, nearly all collected from beneath the nests, in late May, June, or early July. Some 49 different nests were represented, and remains of 43 blue racers were found. In one nest which yielded a total of 191 food items, racer remains occurred 13 times, but no other nest yielded records of more than three racers, and some other species of reptiles, notably the black rat snake (Elaphe obsoleta) comprised much more important components of the food. Since only one meal in 26 contained remains of racers, it seems that a red-tailed hawk would destroy only a few racers in the course of a year on its territory of perhaps half a square mile, and that its predation would not be a major factor in the racer's ecology.
That the red-tailed hawk is a natural enemy to be reckoned with throughout the racer's extensive range was demonstrated by the analysis of contents of 116 stomachs from localities well scattered over the United States and Canada. Among the 152 vertebrate prey items represented there were three racers: a juvenile C. c. constrictor from Stag Lake, New Jersey, October 16, 1927; a C. c. mormon from Weiser, Idaho, April 11, 1930, and a C. c. stejnegerianus from San Roman, Cameron County, Texas, in the spring of 1938. Because of the widely scattered geographical origin of these stomachs, a remarkably large number of species of prey were represented, and no one species predominated. Small mammals of the genera Sylvilagus, Spermophilus, and Thomomys comprised the most important component of the food; the only other reptile so well represented as the racer was the common garter snake (Thamnophis sirtalis) which likewise had three occurrences.
In June and July, 1954, a nest of broad-winged hawks (Buteo platypterus) on the Reservation yielded 71 prey items of which seven were blue racers. At least 19 species of prey were represented, although specific determinations were not possible in some instances. The prairie vole with eight records was the most frequent prey, and the racer and cardinal (nestlings) each had seven, while all other species were represented by fewer occurrences. Six of the racers were first-year young but the remaining one was thought to be an undersized second-year individual. Although an adult racer, especially a large one, would probably be an adversary too powerful to be killed and eaten by a broad-winged hawk, this hawk is perhaps one of the more important natural enemies of the first-year young in the eastern United States.
Further records of predation on racers by raptors were obtained from the U. S. Fish and Wildlife Service files through the kindness of Dr. Wm. H. Stickel. There were three records of such predation by red-shouldered hawks (Buteo lineatus) from Lunenberg, Massachusetts, May 1, 1896; Portland, Maine, March 16, 1906; and 65 miles northeast of Sarasota, Florida, June 10, 1918. There were two records of predation by broad-winged hawks from Catlettsburg, Kentucky, July 26, 1910, and Portland, Connecticut, May 18, 1912. There were four records of predation by marsh hawks on racers—Peck's Island, Maine, September 18, 1903; Edgartown, Massachusetts, April 12, 1912; West [Tisbury?], Massachusetts, July 31, 1912; and Okanagan Landing, British Columbia, June 5, 1918. A sparrow hawk (Falco sparverius) from De Ranch, Wyoming, May 27, 1910, had eaten a racer, as had a barn owl (Tyto alba), from Franklin County, Kansas, November 16, 1922, and a crow (Corvus brachyrhynchos, nestling) from Onaga, Kansas, May 16, 1914.