How to tell if a snake is venomous
Almost always, no. Williamson County holds two venomous snakes and eighteen that are not, and the eighteen include several that are banded, blotched, brown, and perfectly willing to rear up and buzz their tails at you. Here is what separates them, and what does not.
Read the waist of the marking. A copperhead’s crossbands are wide down the sides and pinch to a narrow waist over the spine, like a row of hourglasses seen from above. Every common look-alike in this county (the juvenile ratsnake, the milksnake, the watersnake, the prairie kingsnake) has it the other way round: widest across the back, narrowing toward the belly.
What to actually look at.
The hourglass, not the color.
Tennessee’s copperhead is the only snake here with dark crossbands that are widest at the sides and narrowest over the spine. Color is the least reliable thing about any snake in this county; several species run from tan to nearly black in the same population.
The facial pit.
Both venomous snakes here are pit vipers, and both carry a heat-sensing pit on each side of the head, between the nostril and the eye and set slightly below the line between them. No harmless snake in Tennessee has one. It shows clearly in a photograph of the side of the head taken from a safe distance.
The head, plain or patterned.
A copperhead’s head is solid coppery red with no markings on it. A juvenile ratsnake has a dark line through the eye, a milksnake has a patterned crown and often a pale V, a prairie kingsnake has a backward arrowhead. Patterned crown means harmless.
The tail, on a shed or a dead snake.
Turn it over and look at the scales behind the vent. In every pit viper north of Mexico those run in a single row; in the harmless snakes here they run in a double row. It is the most reliable tell in this guide and the only one you should never get close enough to a live snake to use.
What everyone says, and why it is wrong.
A triangular head.
Watersnakes, hognose snakes, gartersnakes and ratsnakes all flatten the head into a broad triangle when threatened, and a threatened snake is exactly the one you are trying to identify. This is the single tell responsible for the most dead harmless snakes in Tennessee.
Cat-eye pupils.
True of both venomous species here, and useless. Seeing the shape of a snake’s pupil with the naked eye means standing inside its strike range. Use the camera and look at the pit instead.
It rattled at me.
Ratsnakes, racers, kingsnakes and milksnakes all vibrate the tail against dry leaves, and in leaf litter it is a convincing buzz. A rattlesnake has an actual rattle and a black tail. Sound alone identifies nothing.
Everything, to size.
Adult length from TWRA, drawn against each other. Most “baby copperheads” reported in this county are adults of a species that never gets much bigger than a pencil. A newborn copperhead is already eight to ten inches of thick-bodied, fully banded snake with a bright yellow tail tip.
Photograph the snake from a distance that does not require courage, then post it to iNaturalist or to one of the regional herpetology groups, where people identify snakes from photographs all day for nothing. An identification you needed an hour ago is still worth having tomorrow, and it is the only way the county record gets better.
Pack it. Print it. Pass it on.
Most snakes killed in Williamson County are killed by someone who could not name them. Send this to anyone who gardens, hikes, keeps a woodpile, walks a dog off the greenway, or has a teenager who reaches into things. That is most of the county.
Sources
Medical disclaimer. This field guide is general educational information, not medical advice, and is no substitute for professional diagnosis or treatment. Any bite from a copperhead or a timber rattlesnake is a medical emergency: go to an emergency department, and call the Tennessee Poison Center at 1-800-222-1222 on the way. In an emergency, call 911. Identification disclaimer. No photograph identifies a snake with certainty, and this guide will not make anyone a herpetologist. When the answer matters, keep your distance and send a photograph to someone who does this for a living.