Finally, irruptives

 

A few days ago, I failed to find the Pine Grosbeaks seen by others, but have now seen Bohemian Waxwings. Both of these species are irruptive: They are birds that occasionally visit when they irrupt from their normal feeding grounds in the north. Presumably, the rather cold weather of this last week brought them south.

Bohemian Waxwings were seen feeding on crab apples in pairs…

…and singly.

 

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Anchor ice

 

It has been a week of modest cold. The average temperature has been perhaps -8 °C. It has been cold enough to produce anchor ice in local creeks.

In the accompanying pictures there is snow; in addition there is ice that has formed in two different ways. The snow is obvious: It is the whitish portions of the picture. The snow is resting on greyish border ice. Furthermore, on the floor of the creek channel there is some greenish anchor ice.

The flow of the water along the creek’s edges is gentle and the water there is stratified. That means that the densest water (T = +4 °C) has sunk to the bottom of the creek, and the least dense water (T = 0 °C) has risen to the surface, which is where the border ice forms. The snow rests upon the border ice.

In the central channel of the creek, the flow is not stratified, but is well mixed by the turbulence. As the temperature drops, it does so uniformly through the depth of the creek and when it reaches 0 °C, ice forms as crystals throughout the water. These ice crystals collide with the stream bottom, stick, and coagulate to produce the anchor ice.

The result is that regions of ice form on both the bottom and top of the stream.

The edges of the creek are covered with border ice, which in turn is covered with snow. Along the floor of the creek channel can be seen the greenish anchor ice.

Another view shows anchor ice on the floor of the main channel of the creek.

From the dipper’s point of view, the trouble with anchor ice is that it restricts access to the creek floor, a place where it usually finds its food.

 

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Diving mallard

 

Mallards are generalist foragers and will eat a wide variety of food. They don’t dive, but dabble to feed, tipping forward in the water to eat seeds and aquatic vegetation.     Cornell’s All about birds 

Anatidae (ducks, geese, swans) are generally divided into dabblers and divers — and never the twain shall meet. For years I have watched Mallards dabble in the shallows (they tip up and scouring the bottom with their bills) and wondered why they seemed incapable of learning to dive from the mergansers they see around them. 

Today (with no mergansers present) I watched Mallards dive. Now, they might have been driven to desperation by the snow covered ground, yet they did dive. (What they retrieved is unclear.) I did find some sources that allowed that mallards will occasionally dive for food, but others that said they did not do so.

A Mallard drake starts its dive.

What the Mallard retrieved was unclear, yet it did dive for comestibles.

 

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Also watching

 

In Nelson, these events start with the ornamental fruit trees.

These fruit trees seem to have been planted purposely in yards and along berms. Among their delicacies are crabapples, elderberries, and rowan berries (mountain ash). However, when snow covers the ground, these berries become particularly appealing to birds, and in particular to irruptives. 

A male Pine Grosbeak feasts on rowan berries.

Irruptive birds are irregular winter visitors. They irrupt from the north and turn up here only now and then. One can go for years and see none, but suddenly they are all over the place. An irruption is a dramatic, irregular migration of large numbers of such birds to areas where they aren’t typically found.

In this last week, there have been reports of irruptive Pine Grosbeaks feeding here on rowan berries. I went looking for them. Alas, I did not find any so include a picture of one from a previous irruptive year.

Now, we are in the realm of unintended consequences: a city plants fruit trees; the fruit trees attract birds; the birds attract predators. Previously, I mentioned that Pygmy Owls fly in to feast on irruptives. But, yesterday, I unexpectedly saw a Merlin in Nelson also watching small birds (in this case robins) feeding in fruit trees.

One of our two rarely seen resident falcons, a Merlin, was beside a school watching small birds feasting on berries. The Merlin had the clear intention of feasting upon them.

 

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Desperate pigeons

 

Rock Pigeons live in cities where they typically eat seeds.

Pigeons inhabit cities because the buildings in cities mimic the pigeon’s original habitat of cliffs. So some pigeons began to adapt to urban habitats and found that cities also provided safety and food. In cities, pigeons live mainly on bread crumbs, popcorn, or other junk food provided by humans. Away from cities, pigeons feed on waste grain, and seeds, but turn to berries when times are desperate. 

The local ground is now covered in deep snow and the temperature is low. Seeds and and food are in sort supply. What is an urban pigeon to do?

Pigeons eat, what for them is a food of last resort, elderberries.

 

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Waterfront supplement

 

The varied water birds seen at Nelson’s winter waterfront continue to impress. This season I have shown swanscoots, goldeneyes and scaups. Here are three more species recorded today.

Mr. and Mrs. Hooded Merganser swim past.

This is one of a number of Redhead Ducks seen during a snow shower.

Finally, an adult Great Blue Heron was seen resting on the broken pilings of the old C.P.R. dock.

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Whiteness on trees

 

Blossoms: This posting is just about white on trees resulting from water. Blossoms and seeds are another matter.

Following the reaction to my posting about a rime band, I thought it might be worthwhile to offer a guide to some sources of whiteness on trees.

Of course, the most obvious source of whiteness on trees is snow. When close at hand, snow is fairly distinctive (Jan. 1, 2012). However, when seen from a distance the assessment that the whiteness results from snow becomes less obvious — there are other possibilities.

Snow that falls on a mountainside will often fall right through the melting level and turn to rain below (Dec. 10, 2015). However all the snow does not melt at the same rate.

As the snow falls into air that has a temperature above 0 °C, the smallest crystals melt first, while the larger ones survive longer to fall farther. The result is that the bottom of the white is diffuse. The whiteness gradually tapers from extensive to nothing, without providing a sharp boundary in between (Dec. 13, 2015). So, the clue that we are seeing snow on the distant trees when the valley bottom is above 0 °C, is that the base of the white is tapered. 

What then is one to make of the sight of a sharp lower boundary? This obviously did not result from snow falling through the melting level. This is the result of rime. A cloud of supercooled water droplets (T < 0 °C) had been resting against the mountain. As the droplets collided with the branches, they froze to cover the trees with rime. The sharp base results from the transition of the droplets from above the melting level to those below it (Nov. 26, 2005).

The sharp lower boundary of the whiteness in this picture of rime probably did not result from the bottom of the cloud, but rather from the presence of the melting level. But, this rime shows a sharp upper boundary also, and it will have resulted from the cloud top. The result is a band of rime across the mountain (Nov. 29, 2013). 

Often when the temperature is low, there will be steam fog rising from the open water of local creeks. Then the trees alongside the creek can be covered with rime as the supercooled fog droplets collide and collect on them.

Now for something completely different. This is a picture of the sylvanshine, and although it looks to be a picture of snow-covered trees in the winter, it is actually retroreflection from dew-covered trees in the summertime when seen with a spotlight or the headlights of a car. Only a few species of trees can do this — those which have leaves (needles) that have a rather large contact angle with water. The effect is similar to that of retroreflective glass-beaded highway signs.

 

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Rime band

 

In this season, snow is seen that falls on the mountainside. Yet, all the white on distant trees need not be snow — sometimes what is seen is rime.

When snow falls from a cloud above the mountain, it spreads from the mountain top down to the melting level where it gradually diminishes. It thus presents a white mountaintop with a somewhat indistinct base.

What is one to make of a isolated white line across the mountainside? This is a result of rime from a shallow cloud. 

Normally a cloud is filled with liquid droplets even when the temperature is well below 0 °C: the droplets are supercooled. They remain liquid at sub-zero temperatures. But, when the drops collide with objects, such as trees, they instantly freeze to produce rime on those trees.

A thin cloud of super-cooled water droplets had rested against a mountainside. As the droplets collided with the trees, they froze to produce a band of rime across the slope.

As the cloud extended along the whole mountain range, so did the band of rime.

 

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December goulash

 

This is a collection of images from this December, none of which has had a posting of its own.

A Coyote hunts for voles in a field.

Each winter, Nelson’s waterfront plays host to a variety of interesting water birds. Already having received postings have been coots and Barrow’s Goldeneyes. Here are four Greater Scaups. The males have black heads, the females, brown.

This is a view of border ice as seen from the bank of a creek. The circular patches are methane bubbles that have formed as a consequence of dead organic material having first settled on the stream bottom. Then bacterial decomposition released methane, which rose as bubbles, but became trapped under the ice.

A bighorn ram curls his upper lip back in the flehmen response. This gives him an enhanced ability to detect the pheromones of ewes in oestrus. Apparently, he hopes that the rut is still on. 

For a while centred on 2017, this region experienced many hares. Hare populations are notoriously cyclic and by 2019 the visibility of hares had plummeted. However, all the hares have apparently not vanished, as is evident when a fresh snowfall reveals their tracks.

A dipper flies underwater as it forages for Kokanee eggs and aquatic arthropods on the creek bed.

Nicknamed for being leucistic, Lucy (Goosey) was spotted earlier this month for the first time in a half-dozen years. Seen again at the month’s end, she is apparently now hanging about nearby.

The final picture is here merely for the fun of it. It is a Common Merganser that has just lifted its head after scanning underwater for fish. That it is a juvenile is evident by the yellow eye and white patch between bill and eye.

 

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Four swans a-swimming

 

This has been a good winter for seeing swans. However, by Christmas Eve, I had not seen any for nearly a month. Then four swans came a-visiting.

A family of Trumpeter Swans appeared at Kokanee Creek Park. The grey ones are the juveniles.

On Christmas morning, they were feeding at the mouth of the creek.

Then there were four swans a-swimming.

 

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