White on white

They are hard to see against a backdrop of fresh snow, except for the large, dark black eyes, and the black tip at the end of their tail.

Meet Herman, the ermine, who is in the winter fur phase. Herman had just been feeding on part of a carcass left out for the overwintering birds at Sax-Zim bog, and the tip of his nose is bright red from the bloody meat. This little Short-tailed Weasel has been hanging out at the Visitor Center for several days, and shows up regularly every morning (and evening) for a snack.
It’s a cold day to be running around on the snow, and these ferocious little predators have huge appetites to keep their metabolic furnaces going. They need to consume about 30% of their body weight daily. Deer carcasses are for weasels what bird feeders are to our feathered friends — a ready source of rich nutrients in times of need.

The long, thin bodies of weasels are a great adaptation for hunting rodents in their burrows, but they are a disadvantage in the winter climate because thin cylindrical bodies lose heat much faster than more spherical-shaped ones do.

Weasels are excellent climbers, so this part of the carcass hung in a tree is no problem for Herman to get to. Time for a little fat to add to the meaty part of the meal consumed earlier.

Weasels usually retreat to a burrow, perhaps of some animal they have recently eaten, when they are not active, and they will often cache extra food there to tide them over on days when prey are scarce.

This is what Herman looked like last summer — but molting the summer hair to all white coat in the winter serves two purposes: being less conspicuous to prey they are stalking, and being less visible to predators that focus on the black tip of the tail instead of the head of the animal that is white on white against the snow.

What determines when weasels molt into their white fur? It’s thought that declining hours of daylight in the fall are the primary cue, just as it is for migratory birds to begin their preparation to fly south. Molting from brown to white fur coat takes about three weeks, but a drop in temperature can hasten the molt, and unseasonably warm fall weather can slow it. Usually the molt to the all-white ermine phase is completed just about the time of the first snowfall, so the animal is perfectly camouflaged.

Why is Fall the best of the four seasons?

Number 1 (for me) has to be the colors of Fall — they’re bright, warm, and make us (well, me, especially) want to be outside communing with Nature.

Beaver River Falls, Beaver Bay, MN

How could you resist a walk along this river admiring the fall foliage?  I think this is the last waterfall before the Beaver River empties into Lake Superior (near Beaver Bay).

Number 2 might be the temperature — warm days, cool nights, always changing, something for everyone…


Perfect temperature for canoeing down the Mississippi River


Or for studying outdoors in a warm, sunny space. If you get too warm, you can just move into more shade.

Number 3 is a little more abstract:  the effect of short daylength on our brains makes us a little lazy and lethargic (or is it the warm sunshine?); long nights give us the perfect excuse to get more sack time.  The end result is we might feel more rested and less stressed (or is it the warm, bright colors of fall that do that?).


Beginning in the Fall and continuing through the winter, extended periods of sleep are common in some human cultures. Drawing from the British Medical Journal article on Human Hibernation

Number 4 is the fact that Fall is harvest season — it’s a time of plenty, for both humans and animals.

Sometimes the "harvest" is almost too big to carry...

Sometimes the “harvest” is almost too big to carry…

Scads of berries, nuts, seeds, and fruits are available for primary consumers, along with a large supply of young, naive prey animals for the predators.


With their high sugar content (30%) these berries are a great resource for migratory birds like this Yellow-rumped (Myrtle) Warbler. Even the waxy coating on the berry can be utilized by the birds, whose digestive systems have been primed to secrete an enzyme to break down the wax esters.

At other times of year, consumers most likely face food shortages, with scanty plant and animal productivity during winter and spring, and limited food supplies during much of the summer as plants and animals are growing the next generation.

I’m sure there are many more reasons why Fall is the best time of year — or do you disagree?


Fall color along the Mississippi River. Click on the image for a larger view.

12 is the magic number

Most daytime-active animals respond to daylengths of 12 hours or more by getting ready to reproduce:  singing, courtship displays, nesting, territorial disputes, etc.  There are hundreds of studies showing this works in a huge variety of species from bacteria to bugs to birds, etc. (Actually, some plants, too.)

"I just feel like singing"

“I just feel like singing”

But actually, it’s not only the absolute length of the day that is important, but the increasing or decreasing nature of the day length.  Right now, we are gaining an additional 3 minutes of daylight each day, as the sun rises higher on its arc through the sky, a welcome change from the dreary days of December.

"I can't tell you how happy that makes me!"

“I can’t tell you how happy that makes me!”

We won’t hit the magic number of 12 hours of light from sunrise to sunset until March 18 in Minnesota, but you know birds will be singing their heads off by then.  So, is 12 really the magic number?

"What?  I have to wait until March 18 to start singing?"

“What? I have to wait until March 18 to start singing?  That just makes me grumpy!”

Factor in that beautiful light that we receive at dawn (morning civil twilight when the sun is 6 degrees below the horizon until sunrise) and at dusk (sunset through evening civil twilight when the sun is again 6 degrees below the horizon), and we will reach the 12 hours of daylight on February 26 in Minnesota.

So I propose an experiment.  Look up the dates for 12 hours of light from sunrise to sunset vs 12 hours of light from dawn (morning civil twilight) to dusk (evening civil twilight) in your area and monitor how much singing you hear before or between or after those dates.   What cues are birds really using to get them started on their “spring fling?”

I’ll check back with you in April.