I watched a lone Caspian Tern hunt for fish in one of the small percolation ponds near Los Gatos creek in San Jose, CA, and was impressed with its success in grabbing small fish from the shallow water, as it hovered over a patch of still water and then suddenly dove head first into it.
American Robins rank third in numbers behind Red-winged Blackbirds and introduced Starlings as the most common bird in North America. To what do they owe their great success, compared to Cardinal and Bluejays, for example?
Note added after posting: Valerie Cunningham who writes a bird column for the Minneapolis Star Tribune uses a more reliable source for estimates of bird populations in the U.S. and Canada. According to the Partners in Flight Database, the American Robin is THE most numerous bird in North America, at an estimated 370,000,000 birds, far outpacing the Red-winged Blackbirds (160,000,000) and Starlings (86,000,000). So that makes what I have said below even more impressive!
One strategy for being prolific: breeding early and often — producing as many as three clutches of chicks during a breeding season lasting from April to July. Robins are one of the earliest to nest, and continue to raise broods until the flush of insects has diminished in late summer.
Another strategy for being versatile is their flexibility in changing diets as the seasons progress. We think of Robins as being primarily fruit eaters, and they do consume a lot of fruit in fall and winter — indeed, as much as 60% of their diet over the course of a year may be fruit.
This dietary switch from eating mostly animal prey to consuming mostly fruit is not trivial. There are major changes in gut anatomy, changes in types of enzymes synthesized for digestion, and amount of food to be consumed and the rate it is moved along the digestive tract daily that must take place during a short transition time of about two weeks. Ask any vegetarian what happens when they try to eat meat and you get a sense of what Robins must deal with twice a year as they switch food sources. So, we must credit their dietary versatility for their ability to survive and become one of the most common birds in North America.
But here’s a new wrinkle in the Robin’s key to success in surviving the food desert of the late winter landscape in northern latitudes — fishing! Recently someone posted photos (on Facebook’s Minnesota Birding site) of Robins fishing for minnows near the edge of a pond free of snow. One or more Robins poked a hole in thin ice, big enough for minnows to find as a place to gulp some oxygen in their severely anoxic swampy pond, and the patient Robin simply pulled them up for a meal.
This is not one isolated instance of American Robins eating fish. There are reports in the scientific literature as early as 1954 of Robins feeding on dead shiners, as well as newspaper articles documenting Robins hanging around bait shops for the dead minnows being thrown out.
American Robins are truly versatile and adaptable — and as a result are very successful in populating North America.
Today is two months after the winter solstice (Dec 21), and we now have two more hours of daylight each day (almost 11 hours). More importantly, the sun rises each day 13 degrees higher than it did on the winter solstice (35 vs 22 degrees above the horizon), and it is now more than half way to its maximum altitude in our summer sky on June 21 (68 degrees).
What does this mean for us winter-weary Minnesotans — spring is ever near! Cardinals and Chickadees are singing up a storm on sunny mornings when the radiant heat of the sun can actually be felt through the chilly (20 F) air. The polar vortex is history, and it’s time to get out and enjoy the end of winter, — like taking a morning walk along the Sucker Lake creek.
Every now and then, a red fox runs through the far back of the backyard, usually too quickly to get a photo. But today a red fox trotted right in front of my porch window where I was sitting admiring the snowfall in the backyard…and my camera was close by.
We have been in the grip of a prolonged vortex of cold air from our northern neighbors since February 4 with daytime highs in the negative digits (F) and nighttime lows dipping well below -10 F (e.g. last night was -21 F). Just for something to inspire me mentally (?), I added up the last 10 nights of low temperatures and came up with a grand total sum of -95 degrees. Now that’s arctic! Needless to say it’s difficult for my fingers to work camera buttons at these temperatures, let alone get outdoors for a walk in the backyard.
But, in the spirit of Valentine’s Day and to commemorate a time when I was braver about venturing out in -15F weather, here are a few photos of the Trumpeter Swans on the Mississippi River at Monticello, MN, engaging in courtship displays to cement their pair bond — love is in the air for these swans, most of which mate for life.
Thinking of warmer days ahead, I wish you a Happy Valentine’s Day, 2021.
It wouldn’t be winter in Minnesota without a week or two of sub-zero (in F degrees) weather (-2F this morning and that’s -19 degrees C). And the little birds have been hitting the feeders pretty hard lately, especially the peanuts.
Now here’s the problem with being a small bird trying to survive in this winter climate. Heat will be lost from a body surface when the body is warmer than the environment, and in the case of the chickadee, whose body temperature is normally about 107 F, that is a 109 degree difference between its internal core and the air temperature hitting its feathers this morning.
In addition to this huge thermal gradient for heat loss, small birds like Chickadees have a very high ratio of surface area to their heat-producing body volume — thus accelerating the rate of heat loss. [If you’re interested in why this is, we can talk about the mathematical basis of surface to volume ratios in the comments…]
Chickadees are metabolic marvels, being able to harvest enough energy from their foraging efforts all day to last them overnight and part way into the next morning, before they can visit their stash of seeds or a bird feeder. BUT…they are economical with their overnight energy expenditures (because you never know what the next morning’s weather will be like), and they make their fat reserves last by lowering their body temperature as much as 20 degrees F (from daytime highs of 107 F to night-time lows of 86 F).*
Hypothermia makes it possible for Black-capped Chickadees and most other titmice members of the Paridae family to survive winter cold in northern latitudes. Not all birds can or do utilize this strategy. Some species, like Common Redpolls, eat a more fat-rich diet to have a larger overnight energy reserve. Some species, like the Common Bushtits that weigh half as much as a Chickadee, share body heat with each other by roosting together communally, packed tightly side by side on a branch. And some species, like Eastern Bluebirds do a little of both the Chickadee and the Bushtit strategy to make ends meet energetically.
Multiple birds in the same box together would be ideal for conserving some body heat on a cold day, in the same way we use the shelters at the bus stop and (used to) stand close to friends and family to retain heat.
Groups of six to eight Eastern Bluebirds in Indiana were observed to roost together in a nest box overnight, forming a circle with their heads toward the center of the box and pointed downward below their bodies so the heat from their exhaled breaths was directed toward the other birds.** It’s possible they might be using this heat conservation strategy during the daytime as well here in Minnesota, when multiple birds enter the same box.
In addition, Eastern Bluebirds forage in small flocks during the winter, using the strategy of more eyes to find food. And when they take a break from foraging intensively, they often huddle tightly together on a branch close to the trunk of a large tree that provides some protection from heat loss from the wind.
It must be tough to be an Eastern Bluebird that depends on a fruit and insect diet to find enough to sustain itself overnight in Minnesota’s subzero climate. What do they do if they can’t find sufficient food to last them overnight? Bluebirds in Indiana were lethargic and immobile when researchers opened the nest box after a cold night; it’s possible they might also be lowering their body temperatures overnight to conserve energy like the Chickadees do. But there are no published data on this — at least that I can find.
*The data on hypothermia in Chickadees overnight was part of my Ph.D. thesis at Cornell University in 1973.
**The data on communal roosts of Eastern Bluebirds in Indiana was published by Frazier and Nolan in 1959 in Bird Banding.
As in blue sky (for a change) and Bluebirds (that shouldn’t be here at this time of year) on a sunny day with an unseasonably high temperature hovering near freezing (30 F). For a change of pace I went walking on the local golf course, curious to see whether I could find the reputed Mountain Bluebird that has taken up residence here along with aforementioned Eastern Bluebirds.
But I found only the eastern cousins, near two Bluebird boxes, which the birds were sitting on and inside of during the middle of the day.
Although Eastern Bluebirds breed here in the summer, they usually retreat south in the winter to places that have less snow cover and milder temperatures where they feed on a wide variety of shrub and tree berries and mealworms provided by friendly birdwatchers. What a treat to see their cheerful bright blue and rusty brown colors on a landscape that is mostly white, gray, and brown this time of year.
There are not one, but two mammalian predators looking for the squirrels in my backyard. One of the red foxes stopped by the other day, furtively sneaking along the fence line between backyards, pausing under some evergreens for a look at the bird feeders, and then hiding next to a shed in my neighbor’s yard.
But the next day, a coyote lingered in the backyard, hunting along the edge between the grass and the forest for unsuspecting squirrels.
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.
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 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.
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.
The Boreal Forest (also known as the taiga) is a harsh place, dominated by snowy landscapes, primarily coniferous trees, extremely cold and long winters and short-lasting, cool summers, but it is the largest land biome circling the high northern latitudes of the globe. In North America that includes most of inland Canada and Alaska.
Over 300 species of birds breed in the boreal biome during its short summers with almost continuous daylight and abundant supply of insects, but only about 30 bird species stay for the rest of the year. In years when food resources are scarce during the winter, some boreal-dwelling bird species “irrupt” and make short migrations south to the northern coniferous forests in Minnesota and other parts of the U.S. These irruptions are irregular both in timing and location, but we can usually find a few boreal species in the spruce-tamarack forest of Sax-Zim bog.
Other notable “irruptive” species one might find in the northern coniferous forests of MN like the White-winged or Red Crossbills, Bohemian Waxwings, Red-breasted Nuthatches, Purple Finches, Common or Hoary Redpolls, Pine Siskins, as well as Snowy Owls and Great Gray Owls, all make winter excursions “up north” an exciting adventure in bird watching.