Burrowing, the excavation of a hole or tunnel into the ground, is a widespread survival behavior practiced by an immense variety of species, from tiny insects to sizable mammals. When a hole appears, it is evidence of an animal using the earth as a resource for life, creating a space for shelter, safety, or sustenance. Understanding the characteristics of these excavations helps in identifying the creatures responsible for these underground structures.
Why Animals Burrow
Animals primarily dig burrows to secure a stable and protected habitat away from surface dangers. These underground spaces offer immediate defense against predators and provide a remarkably consistent environment. The earth insulates occupants from harsh weather conditions, such as extreme summer heat or winter cold.
The subterranean environment is also widely used for reproductive purposes, creating a secure nursery for raising vulnerable young. Many species construct specialized nesting chambers where offspring can develop without exposure to external threats. This protective measure significantly increases the survival rate of litters and clutches.
Beyond safety, burrowing serves a practical function for foraging and resource management. Certain animals create tunnels to directly access food sources, such as roots or insects, or establish chambers for caching supplies. Stored food remains safe from competitors and spoilage, ensuring a steady supply during lean seasons.
Identifying Burrows by Structure and Location
The most reliable way to identify a burrow’s maker is by examining the architecture of the entrance and the surrounding soil. The presence or absence of a spoil pile is a major clue, as is the shape of the excavated material. Some animals push dirt straight up from a central point, creating a volcano-shaped mound where the entrance is often plugged or hidden.
Other species throw the excavated soil out behind them to form a fan-shaped or crescent-shaped pile next to an open entrance. The size of the opening is also a defining characteristic, with entrances ranging from pencil-sized holes, indicating a small rodent or insect, to openings the diameter of a basketball, suggesting a much larger mammal. The angle of entry can vary, with some animals preferring a shallow, sloping tunnel while others dig a nearly vertical shaft to a deep chamber.
Burrow location is another strong identifier, as certain animals prefer specific terrains or proximity to structures. Holes found near water sources may suggest semi-aquatic inhabitants. Burrows dug against a building foundation or under a deck often indicate a creature seeking structural support and hidden access. Open grassy areas are frequently targeted by species that feed on roots and insects just below the surface.
Common Mammalian Burrowers
Mammals are responsible for the most noticeable burrows due to their size and complex tunnel systems. Moles are primary subterranean residents, creating extensive networks of feeding tunnels just beneath the soil surface, which appear as raised ridges in a lawn. Their deeper, permanent tunnels are marked by distinct volcano-shaped mounds of finely sifted dirt, with the entrance often sealed from below.
Groundhogs, also known as woodchucks, are larger burrowers that create obvious and destructive dens. Their entrances are typically 8 to 12 inches in diameter and feature a large, fan-shaped apron of excavated soil directly in front of the opening. These burrows can be 5 to 6 feet deep and include multiple chambers for nesting and storage, often located near a fence line or beneath a shed.
Smaller mammals like voles create dime-sized holes and shallow, exposed pathways called “runways” through the grass. These mouse-like rodents are herbivores that feed on plant stems and roots, and their surface activity is frequently seen in areas with thick ground cover. Rabbits also create small burrows or use shallow depressions called scrapes, usually 4 to 6 inches wide, characterized by a sloping entrance and the presence of scattered fur or droppings nearby.
Non-Mammalian Burrowers
Burrowing behavior is not exclusive to mammals; many insects, reptiles, and birds utilize the earth for different life stages. Solitary wasps, such as the cicada killer, excavate perfect, round holes about one-half to one inch in diameter, often in sandy or well-drained soil. A small, U-shaped pile of loose soil sometimes surrounds the entrance, which the female uses to lay eggs and provision a paralyzed cicada for her larva.
Certain spiders also create burrows, notably the funnel-web spiders, which line their vertical shafts with silk and build trip-lines radiating from the entrance to sense prey. These holes are usually small and camouflaged, with the silk lining helping to stabilize the surrounding soil. Invertebrate burrows are generally much smaller than those made by mammals, making them easy to differentiate.
Some birds, like bank swallows and kingfishers, excavate nesting tunnels into vertical dirt cliffs or sand banks near water. These burrows are nearly horizontal and can extend several feet into the bank, providing a safe, elevated site for their eggs and young. Reptiles like turtles dig temporary holes for egg-laying, while snakes and lizards may occupy abandoned rodent burrows but rarely dig their own.