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Tree Cavities Make Warm Winter Homes

January 25, 2010

Tree cavities can mean the difference between life and death for birds and small mammals during a cold New England night. With a large surface to volume ratio they can lose heat very quickly. Add wind to mix and life becomes even more difficult for small animals.

But not all tree cavities are created equal. The mean temperature differential between the cavity and the outside temperature can be as high as 9 degrees F. But it increases with higher day-to-night fluctuations in the ambient temperature, a smaller cavity entrance and better cavity tree health.

Squirrels den in tree cavities during winter, especially during high winds. One occupied cavity was 47 F when the outside temperature fell to a low of -4 F. The temperature in an occupied Raccoon den immediately begins to rise and remains more stable than outside air. A drop in outside temperature can be delayed by as much as two hours inside the cavity. In one den the temperature only changed 2 F while the outside temperature fell 23 F in an hour.

Because not all cavities are created equal, animals must choose the best site to roost at night. There is probably some level of competition between them. The predictors of microclimate – tree health and entrance size – may provide indirect cues to prospecting animals.

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