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Cavity Nests
shelter index

Tree cavity nests and refugia (other names include dens, hollows, nest holes, roost holes, tree holes, woodpecker holes) play an integral role in the life of the North American flying squirrel.

Flying squirrels are secondary cavity nesters - they do not create cavities, but rely upon either primary cavity nesters who have abandoned the cavity they excavated, or naturally-occurring cavities for sleeping, resting, eating, rearing and over-wintering.
snag
The tree cavity nest is, generally, the most common type of nest employed by flying squirrels, however, in some geographic areas during summer months this is not necessarily the case. For example, in the Pacific Northwest and Atlantic Southeast, outside nests made from found plant materials are commonly employed during warmer months, and to a lesser extent throughout the rest of North America. These outside nests are called dreys.


Cavity Openings

A cavity openings' diameter will be one determining factor in a cavity's suitabilty for occupation by a flying squirrel. Flying squirrels will normally be found only in cavities with small to medium diameter openings. Generally, squirrels will seek out nests that possess cavity entrance diameters that preclude predator access (arboreal snakes excepted).
  • Class 1 cavity opening -  1 to 2 inches in diameter (2.5 to 5.5 cm)
  • Class 2 cavity opening - 2 to 4 inches in diameter (5.5 to 10 cm)
  • Class 3 cavity opening - 4 to 5 inches in diameter (10 to 12.5 cm)
  • Class 4 cavity opening - 6 inches in diameter (15 cm)

Standing Dead Trees

Cavity nests are usually found in standing dead trees, or "snags" (other names for a standing dead tree are hollow tree, roost tree, den tree, senescent tree, stub, wolf tree, cavity tree, chicot, cull tree).

Standing dead trees are segregated into two types - deciduous and coniferous. Coniferous snags generally stand longer than deciduous snags. Each group is further defined by a decay class, judged from a scale of 1 to 5. Generally, standing dead trees possess cavities that have been created by woodpeckers or are naturally occurring.

  • Decay Class 1 - tree top intact; tree death recent; fine branches present, bark mostly intact
    • prime excavator - pileated woodpecker
  • Decay Class 2 - tree top intact; fine branches lost; less than half of large branches gone; bark loosening
    • excavators include - pileated woodpecker, northern flicker
  • Decay Class 3 - tree top intact; more than half of large branches gone; bark falling off
    • excavators include - hairy, black-backed, three-toed & downy woodpecker, flicker, yellow-bellied sapsucker
  • Decay Class 4 - top broken off; all large branches gone; most bark gone
    • excavators include - hairy, black-backed, three-toed & downy woodpecker, yellow-bellied sapsucker, northern flicker
  • Decay Class 5 - top broken to stub less than 6 metres high; bark and wood deteriorating, often "punky"
    • excavators include downy woodpecker, black-capped and boreal chickadee, red-breasted nuthatch, yellow-bellied sapsucker

Live Treesnatural cavity

Cavities found in live, healthy trees are often a result of broken limbs at the trunk, thereby exposing heartrot. At right is such an instance -  an eastern white cedar trunk lost a branch, which exposed its heartrot and a potential home for a flying squirrel. Additionally, many live trees have dead or dying large branches, or broken/dead tops that can be utilized by many woodpeckers. Pileated woodpeckers, very strong excavators, often create cavities in live trees.


Heartrot

Heartrot is caused by various species of fungus and attacks trees whose heartwood is susceptible to decay. Heartwood is "dead wood", in that it does not carry nutrients. Heartrot that occurs close to the ground is often called buttrot. Many flying squirrels owe their livelihoods to heartrot, as without it, there would be far fewer suitable tree cavities in which to live.