Tree squirrels, common inhabitants of both dense forests and bustling urban centers, face a relentless gauntlet of dangers. While these agile rodents are adept at navigating the canopy, their position in the ecosystem makes them a frequent target, leading to high natural mortality rates, especially in their first year. These factors encompass natural threats, infectious diseases, and the ever-present dangers introduced by human development.
Natural Causes of Mortality
Predation represents one of the most immediate and consistent threats to a squirrel’s survival in the wild. Aerial hunters like various hawks, including the Goshawk, and owls frequently target squirrels as a substantial food source. Terrestrial predators, such as foxes, badgers, pine martens, and even domestic dogs and cats, will readily prey on squirrels when the opportunity arises.
Beyond being hunted, environmental stress often causes death. Starvation, particularly during harsh winters or when natural food sources like nuts and seeds (mast crops) are scarce, is a limiting factor. Young squirrels are susceptible to early mortality influenced by severe weather, such as intense cold in the spring before their insulating fur fully develops.
Accidents are also a contributor to natural mortality, despite the squirrel’s agility. Though highly skilled climbers, they can suffer fatal injuries from falls when misjudging a leap or losing their footing during territorial disputes. An animal weakened by hunger or injury is subsequently much more vulnerable to all other threats, including predation and environmental exposure.
Fatal Illnesses and Parasitic Infections
Microscopic pathogens and internal invaders can lead to systemic failure and death, often striking animals already weakened by other factors. The bacterial disease Tularemia, caused by Francisella tularensis, is one such serious, often fatal infection found in squirrels and other rodents. Infected animals may exhibit lethargy and are easily caught, and the bacterium can be transmitted by ectoparasites like fleas and ticks.
Other biological threats include viral infections, such as adenovirus, which damages the intestinal lining and prevents nutrient absorption. This damage leads to severe diarrhea and emaciation, which can be lethal, particularly in younger or stressed individuals. Protozoan parasites, like Eimeria sciurorum, can cause a severe intestinal disease known as coccidiosis, especially when a squirrel is malnourished.
Heavy parasite loads, both internal and external, further compromise a squirrel’s health. Internal worms can cause chronic weakness, and massive infestations of external parasites, such as mange mites, can lead to severe skin conditions, secondary infections, and anemia. Diseases like Tularemia carry a zoonotic potential, meaning they can be transmitted to humans and domestic pets through contact with an infected animal or through vector bites.
Accidental and Deliberate Human Intervention
Human presence introduces several high-risk factors, particularly in urban and suburban environments. Vehicle collisions are a primary cause of death in developed areas, with some studies reporting over 60% of urban squirrel deaths are due to automobiles. Squirrels often attempt to cross roadways to access resources, making them vulnerable to traffic.
Other accidental causes involve contact with human infrastructure, such as electrocution from chewing on power lines or transformers. Non-target poisoning occurs when squirrels ingest rodenticides placed for rats or mice. This often leads to a slow death from internal bleeding caused by anticoagulant chemicals.
When deliberate control is sought, methods include traps and chemicals. Snap traps, designed for small rodents, deliver blunt force trauma that instantly crushes the skull or spine. Chemical control often involves neurotoxins or anticoagulants, which cause systemic failure, such as massive internal hemorrhaging. Any attempt at lethal control carries risk to non-target wildlife, pets, and children and must be handled with extreme caution.