[7] The southwestern red squirrel (T. fremonti) was long considered conspecific with T. hudsonicus, but a 2016 phylogenetic study found it to be a distinct species.
[8][12] American red squirrels are primarily granivores, but incorporate other food items into their diets opportunistically.
[7] In The Yukon, extensive behavioral observations suggest white spruce seeds (Picea glauca) comprise more than 50% of a red squirrel's diet, but squirrels have also been observed eating spruce buds and needles, mushrooms, willow (Salix sp.)
[13] White spruce cones mature in late July and are harvested by red squirrels in August and September.
These harvested cones are stored in a central cache and provide energy and nutrients for survival over the winter and reproduction the following spring.
The fallen scales from consumed seed cones can collect in piles, called middens, up to twelve meters across.
American red squirrels eat a variety of mushroom species, including some that are deadly to humans.
Nests are also excavated from witches' broom – abnormally dense vegetative growth resulting from a rust disease – or cavities in the trunks of spruce, poplar, and walnut trees.
[21] Red squirrels are highly territorial and asocial with very few non-reproductive physical interactions (0.6% of all recorded behaviours in one 19-year study).
This somewhat rare (15% of litters) female behavior is referred to as breeding dispersal or bequeathal, and is a form of maternal investment in offspring.
[23] Observations suggest that male red squirrels have environmentally induced, alternative reproductive strategies that result in higher incidences of sexually selected infanticide in years when food is plentiful.
[25] American red squirrels experience severe early mortality (on average only 22% survive to one year of age).
[27] Chief predators include Canada lynx (Lynx canadensis), bobcat (Lynx rufus), coyote (Canis latrans), great horned owl (Bubo virginianus), American goshawk (Accipiter atricapillus), red-tailed hawk (Buteo jamaicensis), American crow (Corvus brachyrynchos), American marten (Martes americana), pacific marten (Martes caurina), red fox (Vulpes vulpes), gray fox (Urocyon cinereoargenteus), wolf (Canis lupus), and weasel (Mustela sp.).