Photo: Bonnie Ott/Flickr/CC by 2.0
The mysterious disappearance of the eggs, baby birds, and even an adult from an active bird nest is relatively common. Studies of nesting birds found that predation by snakes is the leading cause of nest failure, and only half of all bird nests are successful.
This article reviews the interactions between snakes and bird nests, identify snake species known to eat bird eggs and baby birds, and proposes ways to stop snakes from raiding nesting boxes.
How does one know what eats the eggs or baby birds from a nest?
When eggs, babies, and perhaps one adult go missing from a nest, it generally means that a predator raided it. Because predators move typically at night, the actual raid is seldom witnessed.
Telltale signs left at the nest site often reveal the identity of the culprit.
- If the nest or nesting box appears intact without any signs of nesting material torn apart or hanging and all or some eggs, all chicks, and maybe the female goes missing, the culprit was likely a snake. Nearly always, one or both parents are still around days after the snake raid happens.
- If a cup nest is tipped over or appears dislodged and torn apart or nesting material hangs from a nesting box’s entrance hole, then the most likely culprit is a raccoon.
- When the eggs are still inside the nest with holes poked in them or on the ground below the nest, the most likely culprit is a house wren or a house sparrow.
- When a bear is a culprit, the pole and nesting box are knocked over and torn apart, and the eggs and baby birds are gone.
I know snakes raid birds’ nests, but how bad is the problem?
Based on a review of 53 studies on nest predation conducted in North America, ornithologists found that snakes predated 26% of nests on average. Bear in mind, though, that this is an average number.
In some studies, snakes did not predate on any of the nests, while in others, up to 90% of the nests were predated by snakes. This range of results suggests that the loss of eggs, baby birds, and adults lost to snakes is highly variable and depends on many factors.
An interesting revelation was that nest predation by snakes is more frequent at lower (or southern) latitudes and warmer states than in colder northern states.
Ornithologists and bird enthusiasts have identified nine snake species specialized in finding bird nests to eat the eggs and baby birds.
Ornithologists found that 70% of snake raids on bird nests were made by:
- Rat snakes (Elaphe obsolete)
- Corn snakes (Elaphe guttata), and
- Fox snakes (Elaphe vulpinus)
These three snakes of the genus Elaphe and others of the genus Pituophis (pine snakes) are specialized in feeding on mice, rats, roosting birds, and bird nests.
These snakes are excellent climbers that regularly explore the forest canopy and human-made structures in search of prey.
Rat snakes are the leading bird nest predator.
Rat snakes include several subspecies of the genus Elaphe, which show different color patterns.
Studies that used surveillance cameras on bird nests determined that rat snakes are the single most important predator of bird nests.
Studies also found that Rat snakes were more likely to prey on bird nests during the nestling stage than during the egg incubating period. Moreover, Rat and Corn snake raids on the nest were predominantly during the night.
Rat and Corn snakes are common and widespread in North America.
How do rat snakes locate bird nests?
Most snakes use smell and visual cues to locate their prey. Rat snakes are good at using signals even when potential prey is located high in the forest canopy.
An enclosure study aimed at learning how rat snakes find their prey used nests placed at different heights in actual vegetation. The researchers then used a model bird that visited specific nests but not others.
Rat snakes in the enclosure were observed tracking the model birds’ movements with their eyes and movements of the head during the day.
Snakes waited for the night to climb to the nests, where they saw the activity and ate the content. The nests that did not have visits by the model bird were largely ignored.
Field observations appear to agree with this behavior.
Casual observers watched an active bird nest and noticed a rat snake watching that same nest and saw that the snake’s head movements tracked the adult birds’ movement as they visited and left the nest.
Greater activity around a nest as parents go in and out feeding the young gives snakes cues about the presence of a bird’s nest with potential food.
Most bird eggs and baby birds disappear overnight.
Field studies that monitored actual bird nests and boxes found that snake predation was much higher in nests where baby birds were being fed than at nests where eggs were being incubated.
Rat snake behavior is consistent with the fact that most snake raids on bird nests occur at night.
It is commonly accepted that rat snakes are mostly active during the day and appear to be much less active during the night. However, a study showed that most raids (80%) on bird nests by rat snakes took place during the night.
This finding supports the idea that rat snakes locate the nests during the day using visual cues but wait until dark to raid the nest.
Night raids are safer for the snake because it does expose it to snake predators. Also, birds do not defend their nests aggressively during the night.
How to tell if snakes are around?
Snakes that are known to raid bird nests and nesting boxes are habitat generalists. They thrive in just about any vegetation, barns, abandoned buildings, near and under woodpiles, or any structures where they can hide or where their prey hide.
Snakes do well in suburban areas where human activities attract mice, which are snakes’ preferred food items.
Snakes may be difficult to see because they are naturally secretive. They may be more active during the warmer months and hide during the cold months. Also, they do not usually leave damage or dead mice and lizards that may alert us of their presence.
However, some clues may alert us of their presence.
- Whole or pieces of snakeskin: Snakes spend a good deal of time hiding under covers. However, they shed their skin every three weeks to 2 months, and they have to come out to do so. They leave most of the scaly old skin or pieces somewhere outside a hideout.
- Droppings: As with shedding the skin, snakes have to leave their dens to defecate. Snake droppings look like bird droppings that vary in color from pale to brown or dark. Their droppings may include hair.
- Snake Tracks: this is more difficult and may only be detected if there is smooth sand or a similar surface where a snake leaves a tract.
- Distinctive Odor: Snakes often use barns and sheds where they establish residence. If you notice a peculiar smell that you cannot relate to anything you know, it may be a sign that snakes live there.
Snakes that are most likely to raid bird nests.
Snakes most likely to raid bird nests include rat snakes, corn snakes, and fox snakes.
1. Black Rat Snake (Elaphe obsoleta)
Unlike black racers, rat snakes show a white throat, neck, and belly. Photo: Andy-Reago-&-Chrissy-McClarren/Flickr/CC by 2.0
Description: The appearance of rat snakes varies throughout the geographic range. Yellow rat snakes are greenish, yellow, or orange, with four dark stripes along the body’s length. Black rat snakes are typically black on top with a faint hint of white between some of the scales. Black rat snakes are more northern in distribution. The juveniles of all subspecies resemble the gray rat snake.
Adult rat snakes are typically 3-5 ft (91-152 cm), but large individuals may be more than 6 ft (183 cm) long.
Habits: Rat snakes are habitat generalists and are found just about anywhere. They are often the most common large snake in suburban areas and abandoned buildings and barns.
2. Corn snake (Elaphe [Pantherophis] guttata)
Photo: Noah-K.-Fields/Flickr/CC by 2.0
Description: The Corn Snake has a rather attractive color pattern. The body is a warm brown with large reddish-brown roundish or squarish blotches. In some individuals, the blotches have black edges. A distinguishing field mark is an arrow-like marking on top of the head. The belly is white with large black spots giving the appearance of a checkered pattern.
The corn snake can reach lengths of up to 76-122 cm (30 – 48 in).
Habits: Corn Snakes are somewhat shy, spending a good deal of time hiding undercover. They are diurnal and nocturnal but seem to become primarily nocturnal during the warm months of the year. They are excellent climbers and feed on just about anything they can catch and can swallow.
3. Fox snake (Elaphe vulpine)
Photo: Dawn Scranton/Flickr/CC by 2.0
Description: The Fox snake is tan-brown on the sides, grading to dusky on top of the body. It has large black blotches on top of the back, some of which project onto the sides, giving the impression of shapeless bands. The head in adults is generally reddish-brown with fewer markings.
The fox snake has a rather restricted range limited to the Northeast, Midwest, and adjacent Canada.
The corn snake’s length ranges from 3 to 5 feet (91 to 152 cm). Males are larger than females.
Habits: As with other genus members, the fox snake is mostly active during the day but can be active at night during the warmer days of the summer. They are good climbers.
Fox snakes are habitat generalists and can be found in woodlands, hedgerows, forest edges, fallow fields, and abandoned buildings.
Snakes more likely to raid bird nests
Snakes that are more likely to raid bird nests include the black racer, pine snake, and the speckled king snake.
4. Black Racer (Coluber constrictor)
Unlike black rat snakes, black racers show white only on the chin. Photo: Steve 1828/Flickr/CC by 2.0
Description: Black racers are solid black, have large eyes, and often have some white coloration under their chin. The belly is generally uniformly dark gray or black. Adult black racers can be mistaken for black rat snakes and other dark or black snakes. But, black racers usually are more slender and uniformly black than those species.
Young black racers are generally tan or grayish with a series of brown or reddish blotches running down the center of the back.
Black racers can get as long as 60 in (152 cm).
Habits: Racers are habitat generalists. However, they are most abundant in edge habitats such as forest edges, old fields, and wetland edges. They are also often found in moderately disturbed or agricultural habitats. Black racers are only active during the daytime and are most active in warm weather.
5. Pine Snake (Pituophis melanoleucus)
Photo: Florida Fish & Wildlife Conservation Commission/Flickr/CC by 2.0
Description: The Pine snake has a variable color pattern ranging from tan to brown with dark saddles on its back. It shows reddish spots near the tail. The belly or underside is white or whitish with dark spots on the sides. Pine snakes can be identified by the presence of a triangular frontal (facial) scale. The head itself is relatively small.
Pine snakes average 48-66 inches (122-168 cm).
Habits: Pine snakes favor semi-open woodland, preferably flat. They use stands of longleaf pine and also other types of semi-open woods, and abandoned fields. Pine snakes dig dens for the summer and winter and spend a good deal of time underground.
6. Speckled Kingsnake (Lampropeltis getula)
Photo: Peter Paplanus/Flickr/CC by 2.0
Description: The Speckled King Snake is overall dark with yellowish specks all over the snake’s upper half. The lower half and belly are yellowish.
The speckled kingsnake usually grows up to 48 inches (120 cm).
Habits: The Speckled king snake is often found in areas close to water. They favor hardwood hammocks, pinelands, bottomlands, farmland, and suburban areas.
The speckled kingsnake frequently hides under woodpiles and any other type of cover. Although they are active exclusively during the day, they can be very secretive and hard to spot.
Snakes that occasionally raid bird nests.
This group of snakes includes the coach-whip snake, scarlet king snake, and garter snake.
7. Coach-whip (Masticophis flagellum)
Photo: Peter Paplanus/Flickr/CC by 2.0
Description: An adult Coach-whip has a bicolored body. The head and roughly the front half of the body are dark and even black, grading toward tan or brown toward the back half. The name derives from the fact that the tip of the tail shows scales that appear to be braided. Juvenile Coach-whips are all tan with brown bands.
Coach-whip snakes reach over 8 feet (244 cm) in length.
Habits: Coach-whips favor open and semi-open habitats such as old fields, scrub vegetation, hedgerows, and forest edges. They are exclusively diurnal snake that uses visual cues to locate and often actively pursue their prey.
8. Scarlet King snake (Lampropeltis triangulum)
Photo: Peter Paplanus/Flickr/CC by 2.0
Description: Milk and Scarlet King Snakes encompass several subspecies. The ornate color pattern is similar in all forms and consists of bright black, red, and yellow bands. The red bands are the widest. This color pattern resembles that of the venomous eastern coral snake (Micrurus fulvius).
Scarlet king snakes reach lengths of 14 – 20 in (36-51 cm).
Habits: Milk and Scarlet kingsnakes are highly secretive and are seldom seen during the day. They seem to spend most of the time hiding undercover, coming out to look for prey during the night.
They favor dry habitat types with plenty of covers and prey. They often concentrate in and around places such as barns, livestock pens, and similar areas.
9. Garter Snake (Thamnophis sirtalis)
Photo: Larry Reis/Flickr/CC by 2.0
Description: Garter snakes show some variability in their color patterns but are readily identified by the presence of three longitudinal yellowish bands. Some garter snakes have brown, gray, blackish, and reddish bodies.
Garter snakes are similar to Ribbon snakes (Thamnophis sauritus), but the latter lack the vertical black lines on the mandible and lips and are more slender.
Ribbon snakes typically measure 18 – 26 in (45.7 – 66 cm) but occasionally reach lengths up to 49 in (124 cm).
Habits: Garter snakes are active during the warmer months and warm days of the cold months. Their diet is generally composed of frogs, toads, salamanders, and warms.
How to deter snakes from places where birds are nesting?
- Keep tall vegetation, Grass, or brush around nesting boxes mowed. As indicated above, rat and corn snakes track bird activity from concealed places and wait for the night to raid the nest. Low vegetation would not allow snakes to be close enough to hide during the day and have a direct view of the nesting box.
- Remove piles of debris, woodpiles, or structures that may serve as hiding places for mice and lizards that attract snakes. Snakes can use these structures as hiding places as well.
- Make sure that you do not leave food for livestock or birdseed exposed that attracts mice. Wherever there is a concentration of prey, snakes will be attracted to and stay.
How to keep snakes from reaching a nesting box?
These are the most popular devices aimed at stopping snakes from reaching a nesting box mounted on a pole. Other methods may be lethal for snakes, and I do not advocate them.
Let’s keep in mind that snake predation on birds is a natural process. We are somehow interfering by putting extra nesting sites for the birds we enjoy, but this should not result in snakes’ death.
1) Use a small-diameter, smooth pole to mount the nesting box. A half-inch metal conduit (used for plumbing) is narrow and has a smooth surface preventing snakes from climbing to reach the nesting box.
Wooden poles and larger diameter pipes to mount the nesting box are climbed by snakes.
Do it yourself.
- From the hardware store, get a 10 feet long rebar (steel reinforcement rod) and a five-foot-long half-inch metal electrical conduit.
- Hammer (bury) half the rebar’s length into the ground, leaving five feet above the ground.
- Next, slip the five-foot-long half-inch metal electrical conduit over the rebar.
- Next, attach the nesting box to the conduit with a pair of two-hole, half-inch conduit clamps.
2) Greased Pole. Some folks utilize grease to a narrow diameter pole to further prevent snakes from climbing. This method works but needs maintenance.
The pole needs to be cleaned up and re-greased. If grease is left to collect dust and dry, it hardens and becomes a ladder for the snakes.
3) The Stove Pipe Guard (stovepipe baffle) is the most effective snake guard. Not only prevent climbing snakes but also climbing rats, raccoons, and other mammals. It is the standard for predator protection for many bluebird societies.
Do it Yourself.
You will need:
• A galvanized stovepipe of 8” diameter x 24-26”.
• ½ hardware cloth, 8” circle
• ¾ galvanized pipe for mounting, 7’ long.
• Hanger irons strips (2), 8” long.
• Hardware, (2) #8-32 x 3/4” machine screws and nuts.
- Cut the hardware cloth into an 8” diameter circle. Make a small circular hole in the center of the circular hardware cloth. This hole should be of the size and shape of the pole you will be using.
- Shape the edges of the hardware cloth circle to fit into the stovepipe.
- Make four tab-like cuts around the top of the stovepipe, as shown in the drawing. Bend the tabs inward, over the hardware cloth.
- Bolt hanger iron straps around mounting pole using #8-32 x 32 x 3/4” machine screws and nuts. Bend the free ends outwards, as shown in the drawing, to support the hardware cloth. A few wraps of electrical tape below the strap attachment will keep it from slipping down the pole.
- Once the stovepipe is assembled, slide it over the top of the mounting pole to rest on the hanger straps.
- Now you are ready to attach the nest box. Make sure you leave 6 inches between the bottom of the nesting box and the baffle. To prevent ants from climbing onto the nesting box, smear heavy-duty wax. Carnauba wax is recommended.
Other less used methods:
Folks that keep nesting boxes have used electric fences around the nesting pole with success. Electric fences need a source of power nearby. The problem is that pets and children may touch the hot fence and get sapped and maybe hurt. The electric fence can be expensive.
Use of chemicals.
Folks have used sulfur and naphthalene ropes around nesting boxes. Some users claim that these chemicals prevent snakes from approaching nesting boxes. But the majority indicate that sulfur and naphthalene do nothing to deter snakes from coming and raiding nesting boxes.
Nearly half of birds’ nesting attempts will fail due to natural causes. Snakes have typically been involved in losing most eggs, baby birds, and even incubating adults.
While nest predation on bird nests is a natural process, we can lend nesting birds a little hand by identifying the snakes that may be raiding their nest and taking measures to prevent further damage.
Even when all protective measures are adopted, many nests still will be lost to snakes.
As mentioned above, predation on bird nests by snakes is a natural process. Hence, I would not advocate any protection measures that may result in the killing of snakes.
- Bluebirds Forever. By Connie Toops, 1994.
- Snakes of North America.
- Snake predation on North American bird nests: culprits, patterns and future directions. Brett A. DeGregorio Scott J. Chiavacci Patrick J. Weatherhead John D. Willson Thomas J. Benson Jinelle H. Sperry. Journal of Avian Biology, Volume 45, Issue 4.
- Wait Until Dark? Daily Activity Patterns and Nest Predation by Snakes Brett A. DeGregorio, Jinelle H. Sperry, Michael P. Ward & Patrick J. Weatherhead. International Journal of Behavioral Biology. Ethology 121 (2015) 1–10.
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