Just as the daylight was fading in the backyard, I spied a young fox chasing a very immature, small rabbit around the backyard. This young fox wasn’t interested in eating the rabbit, but certainly seemed to enjoy the chase. All the better to hone its predatory skills. The rabbit did what prey instinctively do when threatened — sat as still as a stone, until the fox turned its head, at which point the rabbit tried to escape. It was quite comical to watch!
When I went out to pick raspberries this morning, I found something much more delightful than a bunch of mating Japanese beetles (the scourge of the berry patch!) — a couple of 1-inch Gray Treefrogs hiding in plain sight on the green leaves of the raspberries.
Although this species is named the Gray Treefrog, because they are quite gray with a dark blotchy pattern sometimes, in bright sunlight on a green background, they are well camouflaged as they match their background. In fact, this frog even matches the particular shade of green of the raspberry leaf on which it rests.
Frog skin contains a stack of color-producing cells called chromatophores, and many frog species like the Gray Treefrog, have 3 sets of them: a deep layer called melanophores that contain a black/brown pigment called melanin, an intermediate layer called iridophores that lack pigment but contain particles that can reflect blue light, and an upper (most superficial) layer called xanthophores that contain yellow pigment.
Now, it should be more obvious how a Gray Treefrog can transform quickly from its gray color that is produced by the dispersion of deep-lying melanin pigment to a bright green color, produced by the interaction of blue-reflected light from the iridophores passing through the yellow pigment of the xanthiphores (i.e., blue plus yellow equals green to our eyes).
The dispersion of pigment in frog skin is controlled by nerves and hormones, which act on the chromatophores to aggregate (condense) or disperse pigment. Physiologically, in a matter of seconds, when melanophores aggregate their pigment to uncover the iridophores and xanthophores disperse their pigment, a gray frog turns green!
In the Minnesota backyard, some of the summer blooms are in their full glory, particularly the purple coneflower. Butterflies and bees are drawn to these flowers…
We’re on the road again, taking our time traveling west to see how many bird species we can find and photograph along the way. Today as we drove through the farm fields of north-central Iowa’s Lake District, we encountered a large flock of Red-winged Blackbirds doing an imitation of a murmuration of starlings, bunched tightly together as they burst upward from the fields, then turning and spreading apart before they landed again, and repeating the pattern over and over. Their numbers were impressive!
This blog has been cataloging the rebirth of Spring each year since 2012, so I thought it might be fun to look back at what I have posted on Easter each year. Since the dates of Easter vary so widely each year, and because we traveled to distant places some years, the landscapes of Easter scenes from the blog vary markedly.
A year later, the corona virus is still with us, but we look forward to Spring and the seasons beyond with more hopefulness and expectations than this time last year. I hope you feel the same, dear Reader.
We made an unusual sighting today — a pair of Canada Geese defending their chosen nest site high in a canopy tree from passing Bald Eagles! What makes it unusual is that the geese had staked out a former eagle nest as their own, and were prepared to fight for it with a couple of immature and one adult Bald Eagle that flew by (the latter carrying a stick to add to its nest).
Fellow photographer Debbie was shocked that Canada Geese would nest anywhere but a raised hummock in or near a lake, and what were they doing so high up in this tree?
But I had seen this behavior from the geese before:
So what is up with these geese nesting in what we think of as un-goose-like nest sites?
Going back a couple of centuries, during the late 1800s and into Depression Era 1900s, the “giant” race of Canada Geese (the big ones we mostly see today) were almost completely extirpated from North America by unregulated hunting, egg collecting, and habitat destruction. They might have disappeared altogether if it weren’t for a tiny population discovered in Rochester, Minnesota, in the 1950s and some captive birds being bred in Boone County, Missouri. Some of these geese were used in a captive breeding program in the 1960s, and by 1981, 6,000 geese were released to the wild at 63 sites to repopulate the “giant” race of Canada Geese throughout North America. It’s hard to believe today with the huge numbers of these geese that roam fields, parks, and wetlands that the species was almost an ornithological footnote.
Now what does this history have to do with geese nesting in eagle nests and on osprey platforms? Observations of Canada Geese nesting high on bluffs above the Missouri River were recorded as early as the Lewis and Clark expedition of the river. But I think natural selection, especially the selection that resulted in culling their numbers almost to the point of extinction, could have played a part in explaining the flexibility of Canada Goose nesting behavior. The geese that survived the pogrom of overhunting were probably the ones that sought out remote places that were hard for hunters to get to, like cliff faces or raptor nests in tall trees. Survivors were probably the birds that used unpredictable and untraditional nesting places, not only to avoid being found, but because their traditional nesting sites had disappeared with human activity there. Today, Canada Geese nest in a variety of habitats, usually in wide open spaces with open fields of view: islands in rivers, the tops of beaver lodges and muskrat house, cliff faces, high nest platforms, and yes, raptor nests — even those of Eagles!
A taste of spring hit the backyard as temperatures soared into the 60s the other day, and major amounts of snow melted. When I walked into the wetland beyond the backyard I was greeted with signs of life awakening after the long winter — like this tiny garden on a rotting log.
One of the fascinating things about bird migration is the patterns in which the birds fly and maneuver in a group as they move along. We are all familiar with the classic V-formation of bird flight, which we have been told is the most aerodynamic way for large groups of birds to fly together. But how exactly does the V-shape work, and how are the birds using it?
I happened to be sorting through a bunch of images of large birds flying in groups together and noticed that there didn’t seem to be a consistent pattern in their wing movements from bird to bird. In fact, it looked disorganized rather than the synchrony I had expected.
In a unique study of imprinted young Bald Ibis that were being trained to fly from their breeding area in Austria to a wintering area in Italy, transmitters were fitted on the birds to provide data on their flight mechanics during V-formation flying.*
Bottom line: what matters is how close and where (left, right, or center) a bird is relative to the bird in front of it. As the lead bird flaps down, it pushes that air up and over its wingtips; the bird behind can take advantage of that updraft (as lift) if it positions itself a certain distance behind and just to the right or left of the lead bird. Therefore, it doesn’t need to flap downward as hard in order to stay aloft. And that is the energetic savings of following rather than leading. How simple! But proximity behind the lead bird is critical, because the updraft from the wing tips is spatially limited.
If you think of the pressure wave of the downward wing flap of the lead bird as a sine curve, the best lift is achieved if the following bird stays in the same place as the lead bird on that curve. It’s similar to the “push” you get by drafting at an angle off a bicyclist just ahead of you. Since the lead bird is continually flapping, the following bird must continue to flap in exactly the same phase in order to get the benefit.
However, if following birds are too close or too far from the lead bird or directly behind the lead bird, synchrony is actually less efficient because instead of catching the upwash air, it might be catching the downwash instead — which would necessitate flapping harder and expending more energy.
A fascinating summary of this unique study on the Bald Ibis appeared in Nature News, with a video that more clearly explains what I have tried to describe above.
*S. J. Portugal, et al. Upwash exploitation and downwash avoidance by flap phasing in ibis formation flight. Nature, 16 Jan 2014.
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.