A typical, wildly variable Michigan week at Bear Creek – heavy frost with a filmy backdrop of cold fog suspended between the treetops and then sunny, almost spring-like weather at the end of the week. Surprisingly on the very coldest mornings this week, Eastern Bluebirds chirped on the western slope while Crows caused a racket in pines at the park’s eastern edge. And then the park seemed hushed when I walked through fields and woods on the sunnier, warmer days. As winter settles in, I thought I’d learn more about creatures that inhabit the dark world under the ice that’s now forming and melting and forming again in Bear Creek’s ponds and marshes .
Bluebirds, Crows, Doves and a Flying Mammal
Spring is the time when we’re supposed “keep on looking for the bluebird and listening for his song” and bird guides insist that our bluebirds migrate south . So perhaps the reason I see them in Bear Creek most Decembers is that the Eastern Bluebirds (Sialia sialis) who spend the winter in the Canadian far north migrate here for the winter season. Or our summer bluebirds just delay their migration.
According to Stokes Nature Guides, “During cold weather, the [Bluebirds] may roost in nest boxes or other protected spots. In some cases, the birds arrange themselves side by side, with their heads pointed inward, possibly to conserve warmth.” That would be something to see, eh? On a frosty, foggy morning this week, three Bluebirds fluttered and chirped along the western slope of the park. This one was definitely fluffing its feathers for a bit of insulation!
Bluebird populations had dropped dramatically in the 20th century due partly to competition for nest holes from non-native birds like House Sparrows and Starlings, but also as nesting sites became more scarce due to development. Today there are just fewer snags (dead standing trees) and wooden fence posts for bluebird nests. Fortunately, a campaign to provide bluebirds with nest boxes has brought an upswing in the populations of these lovely little birds.
Meanwhile, Bear Creek’s neighbors to the east were subjected to a flock of gregarious American Crows (Corvus brachyrhynchos) creating quite a stir in the pines to the east of the park. I couldn’t reach them through the heavy brush and didn’t want to trespass, so here’s one on a snowy day last December who paused long enough for a photo.
Outside the nesting season, these sociable, highly intelligent birds form large flocks which separate into smaller feeding groups during the day and roost together at night. This week, these winter flocks of crows at Bear Creek were calling, chasing each other and diving through the treetops. Fun for the crows, no doubt but the poor neighbors! Here’s my recording. (You may or may not want to turn up your volume this time!)
Just before full dark on Friday, I saw two park visitors that I’d somehow missed before. Though I see lots of Mourning Doves (Zenaida macroura)at home, I seldom see them at the park. But I saw two foraging doves take off near the Snell entrance, their wings whistling as usual in the falling dark. This abundant bird fills its crop with seed, consuming 12-20% of its body weight daily! No wonder they flock under winter feeders! Here’s a closeup from home so you can see its beautiful plumage.
What I assume was a common Little Brown Bat (Myotis lucifugus) beat its leathery wings high in the air over the meadow just outside the woods at the Snell entrance to the park. Their flight is so distinctive – rapid angular wing beats to keep that heavier little body in the air. The distance and the dimness of the light allowed me only a silhouette of this small flying mammal but here’s a link to see one up close.
These little mammals go into torpor in winter, slowing down their metabolism to save body fat. But periodically they wake if the temperature rises above 50°, as it did this week, to get a drink or look for insects that might also have hatched on a warmer day. Since bats keep wasps, mosquitoes, gnats, midges and other insects under control for us and for the farmers, we can hope that this one has not contracted White Nose Syndrome, which is deadly to Little Brown Bats. Bats spend the winter in hollow trees, caves if available and of course on occasion, nice, warm human attics!
A Winter Series: Who’s Under the Ice?
This Week: The Muskrat’s Solution to Ice Overhead
I got curious about muskrats, air-breathing mammals who are active in the winter. How do they breathe and eat while trapped under the ice? Muskrats can hold their breath underwater up to 15 minutes, which is impressive but doesn’t explain how they stay under the ice for months at a time. One muskrat solution, most common in marshes, is “push-ups.” Not the exercise kind. Smaller push-ups/mounds are feeding and resting platforms built by muskrats for winter survival. They first gnaw a 4 or 5 inch hole in the ice, push plant material through the hole and then chew out the center to create a space for eating, resting and breathing. If predators, like mink, are around, the hole is sometimes plugged with plant material each night. Here’s one, probably constructed on a recent icy morning by a muskrat in the northern part of our marsh. It sits at the water surface so when the ice reforms, the muskrat should be able to break through the thinner ice again to reach the air.
Muskrats have larger family homes as well, constructions built up from the marsh bottom in layers of mud, reed, cattails and other plant material. In these lodges, three to five muskrats (adults and their second litter) snuggle together for warmth in the winter. Though they’re more common in marshes, here’s one currently in the Center Pond.
In ponds and rivers, muskrats tend to reside in holes in the bank as well. One morning a week or so ago, when thin ice had formed on the pond, I spotted what appeared to be a muskrat trail in the water. The muskrat had evidently left its hole in the northern shore of the pond to head off to the lodge on the other side, breaking ice before it dove. Perhaps it used its dexterous paws to gather plant roots or other vegetation along the bottom before emerging into the lodge for a breath of air fresher than the air in the family hole!
Muskrats have to contend with both lack of oxygen and heat loss during the long, dark months under the ice. Their dual-layered fur (sleek on the outside, soft beneath) sheds water and keeps an air layer close to their body for insulation. And amazingly, according to Bernd Heinrich’s book, Winter World, they can raise their body temperature about 35° f before they dive into icy water and also store more oxygen in the winter by increasing their number of red blood cells. He also hypothesizes that “they may exhale air bubbles that get trapped under the ice” which they later inhale. Maybe ones like these in the Center Pond?
I like thinking that this sleek little mammal, propelled by its swaying tail is awake, sitting inside its push-up, breathing peacefully and eating a meal. Nature’s provided quite ingenious survival solutions for a small mammal spending another winter under the ice!
So if the dark days are getting to you, consider the muskrat who has to live under the snow and ice for months. Or better still, look for a bluebird! That will definitely put a smile on your face on a cold morning.
*Footnote: As well as Oakland Township Stewardship Manager, Dr. Ben VanderWeide, my sources for information are as follows: Ritland, D. B., & Brower, L. P. (1991);Stokes Nature Guides: A Guide to Bird Behavior Volumes 1-3, Allaboutbirds.org, the website of the Cornell Ornithology Lab at Cornell University; Wikipedia; http://www.butterfliesandmoths.org; Herbarium of the University of Michigan at michiganflora.net.; various Michigan Field Guides by Stan Tekiela; Butterflies of Michigan Field Guide by Jaret C. Daniels; University of Wisconsin's Bug Lady at www4.uwm.edu/fieldstation/naturalhistory/bugoftheweek/ for insect info http://www.migrationresearch.org/mbo/id/rbgr.html for migration info; invaluable wildflower identification from local expert, Maryann Whitman; experienced birder Ruth Glass, bird walk leader at Stoney Creek Metro Park for bird identification;Birds of North American Online; Audubon.org, Nature in Winter by Donald Stokes, Trees in My Forest by Bernd Heinrich, Winter World by Bernd Heinrich