Small vertebrates in winter

surviving the challenges of being small in the cold

On the last day of November, on the way to the end of the dike trail, I saw a warbler fossicking about on a mossy area near the spruce trees. It was probably finding small moribund insects and spiders. I got a good look at it: a male Wilson’s warbler. That was a surprise! I didn’t expect to see any warbler at that time of year, especially since we were having a series of cold night with temperatures in the teens. I could only hope that he would find enough food to fuel a flight to somewhat warmer places.

Photo by Gus Van Vliet

I began to wonder if this sighting was unusual. So I explored the information recorded in a little and found that, over the years, this warbler has been spotted in northern Southeast Alaska uncommonly in November and December and even, but very rarely, in the depths of winter. Then I wanted to learn what other warblers (of those that commonly breed around here) are seen at this time of year. In particular, I thought of orange-crowned warblers and yellow-rumped warblers, which I often see in early spring (March-April), so they seemed like candidates for appearing in early winter too. Although yellow-rumps are recorded quite frequently in November-December and rarely in deep winter, orange-crowns are rare in northern Southeast in November-December. Then I found out that early arrival is not necessarily a good predictor of late fall sightings; Townsend’s warbler comes early but is seldom seen in late fall.

Do the warblers that sometimes stay into late fall have anything in common that might explain their presence? All our warblers feed chiefly on arthropods. Wilson’s, orange-crowns, and yellow-rumps are also known to eat berries and other small fruits at times, but so do some other warblers (but perhaps less often). And, in any case, our region does not offer many small fruits suitable for small birds. Maybe those three just strayed from a relatively nearby wintering ground? But that could not be the case for Wilson’s, which winters in Mexico and the Gulf coast. So neither what we know of diet nor proximity to wintering grounds goes very far to accounting for the three late-stayers. Perhaps they just misread a cue or get delayed by some unknown matter.

Other small birds characteristically spend the winter in Alaska: Pacific wrens, black-capped chickadees in the Interior and the closely related chestnut –backed species here, red-breasted nuthatches, and brown creepers. All of these species usually weigh about the same as the warblers discussed above: in the range of 8-12 g (YRWA at the top of the range). Being small means that they cannot store large quantities of fat to sustain themselves overnight or for several days— their metabolism is quite high and they are so small that there is no place to store a lot of fat on the body as large animals (such as bears and beavers) can do. And they don’t hibernate—they stay active all winter. Some of them (chickadees, nuthatches, occasionally creepers) include seeds in the diet, which are available in winter and which the warblers don’t eat. Chickadees, creepers, and nuthatches often cache their food—in effect, storing their energy outside the body, and black-capped chickadees (possibly also the others) have a temporary increase in brain size, with increased spatial memory during winter.

Red-breasted nuthatch. Photo by Bob Armstrong

In addition, chickadees (the black-capped species has been studied, but other species may do this also) can reduce their metabolism at night and let body temperature decrease; this saves energy, although in extremely cold conditions, it may be impractical, because body temperatures can’t drop too far (being ‘warm-blooded’). The over-wintering species have several tricks that are apparently not used by the warblers. Roosting in cavities, away from the winds, can increase the effective temperature by 25% or more, saving energy, and any sheltered site would be helpful to some degree. Moreover, roosting with companions would also help save energy. Both sheltering and companionship are used by these four species upon occasion. Wrens sometimes roost in cavities, sometimes communally. Chickadees sometimes roost in pairs, sometimes in cavities but more usually in dense foliage. Creepers sometimes roost in small groups, often in sheltered spots. Red-breasted nuthatches may sometimes use cavities, and if seed crops fail, they travel southward in search of better foraging. Apparently none of these methods (except for travelling south) is used by the late-staying warblers (as far as is known).

I can’t resist adding one more bird species: the common redpoll, which is slightly larger than those birds already mentioned, averaging about 13 to 14 g. They eat lots of seeds, especially in winter. And they have the intriguing habit of using snow blankets, dropping down to the snow and making a tunnel with a chamber at the end, 6-11 cm below the snow surface.

Shrews are very small, short-lived mammals that stay active during the winter. They can’t store sufficient body fat, so they have to keep eating every day—twice or three times their body weight in bugs, worms, and other inverts, to maintain their high metabolic rate. European common shrews (Sorex araneus) , weighing less than twelve grams, undergo a marked autumnal reduction in body size, including spine, internal organs, skull, and (!!) brain, as they enter their first winter. Home ranges are smaller and cognitive function related to spatial explorations seems to be diminished in winter. But they regain body mass and re-grow these parts in spring, almost to the original size, ready for the mating season. Researchers suggest that those winter reductions may be a way for saving energy. I have not found comparable information about Sorex species in Alaska, but similar seasonal changes might occur. I wonder what the Alaska tiny shrew does, living in the Interior and weighing less that two grams. Interestingly, the pygmy shrew (in a different genus) does not show these seasonal patterns, leaving open a number of intriguing questions.

European studies of least weasels and stoats (or ermine) have also revealed seasonal changes in depth of the braincase. (Both stoats and least weasels are considerably larger than shrews: stoats weigh up to about 330 g (esp. males), and the weasels weigh up to about 190g, esp. males. However, the long, narrow body shape means heat conservation is difficult, and the metabolic rates are high). Again, brain size reduction may be a way of saving energy. Juveniles decrease braincase depth during their first winter but regain it the next summer. Adults also lose braincase volume in winter and regain it the following summer, but males regain more than females (perhaps related to female’s energy expenditure on rearing offspring and less need to range widely).

Thanks to Gus van Vliet for helpful consultation.


Redpolls and siskins

Flocks of winter seed-eaters

In early February, on a damp and drizzly day, a friend and I went out the Boy Scout trail to the beach. The lichens were fresh-looking and happy in the humidity, but nothing much seemed to be happening in the bird world. Then suddenly a flock of small birds came fluttering in from somewhere and settled in the grasses of the bid meadow. They were redpolls, dozens of them. They fossicked about in the bent-over grasses, searching for seeds and probably anything else that might be edible. I watched one demolish a dark, flat seed-head (probably yarrow) completely, seed by seed. A report from Gustavus noted that redpolls were eating lots of yarrow seeds, sometimes riding the seed stalk to the ground, and lying on their sides on the snow to consume the seeds.

A little later, we perched on a low ridge with the trees as a windbreak and watched another group of redpolls work the grassy berm above the beach, occasionally dropping down to the beach itself. I don’t think of redpolls as ‘beach birds’, but on a different day a friendly birdwatcher reported them foraging in the tidal wrack at Auke Rec. They are versatile foragers, often swarming over alder trees, probing the cones and sending down a scattering of fallen seeds onto the snow.

Common redpoll. Photo by Bob Armstrong

Redpolls eat many kinds of seeds and must snatch up bugs opportunistically. They breed in the far north, but irrupt in large numbers every two years or so, when the seed crops fail up north. Then they appear in more southern regions. Redpolls are well equipped to deal with cold weather: their plumage is heavier in winter, they can store seeds overnight in an esophageal sac, and at times they tunnel under the snow and roost for the night under the white blanket.

They sometimes come to bird feeders too, but they may have to share the bounty. Pine siskins, which sometimes follow a two-year cycle but often irrupt irregularly, eat many kinds of seeds, as well as bugs. A Gustavian friend recently reported huge siskin flocks at a feeder, not much disturbed by the human nearby.When they weren’t busy gobbling up sunflower seeds, they bullied any other birds that came to the feeder, driving them off. Siskins are known to be feisty and aggressive, even challenging larger birds.  

Pine siskins. Photo by Bob Armstrong

Out of curiosity, this friend reached a finger out toward one siskin that was perched on the feeder, driving others away and methodically dragging seeds out. It didn’t leave, even when its tail was touched; it kept grabbing seeds. The friend then stroked its back and touched its feet. But this siskin could not be interrupted in its seed-gobbling. Then it stepped up on my friend’s finger and still kept grabbing seeds. We call them little piggies…and I have another name for their feistiness that rhymes with their acronym PISI.

Fallen sunflower seeds accumulate below seed-feeders, and the bird feed on them there too. Enter another actor: a red squirrel. My Gustavian friend watched a squirrel dash out to a crowd of siskins busily eating seeds on the snow, scattering the birds in all directions. The siskins came back quickly, only to have the squirrel spook them off again. And again, and again. The squirrel didn’t seem interested in the seeds; it was apparently more interested in mischief!

Siskins and redpolls don’t really look very much alike, seen close-up, although they are similar in size. Siskins show flashes of yellow feathers in wing and tail and have heavy brown streaks on the chest. Redpolls have fewer brown streaks in front, a black chin bib, and the reddish crown that gives them their name; males usually show a wash of reddish on the chest. Despite these differences, and being classified in different genera, redpolls and siskins occasionally hybridize!

Three little stories

Otters on the ice, fish at an upwelling, and an unusual feeder visitor

Here are three small stories, two from the field and one from home, two that were simply fun and one that leaves some questions.

One day in early February, I put on snowshoes with the intention of poking around in some meadows out the road. There was not a lot of snow, but the ‘shoes sometimes make it easier to walk over snow-covered grassy humps and bumps. And off I went, seeing a few tracks of mink, shrew, mouse, and weasel. A porcupine had wandered from the meadow down to the ice-covered creek and up the other side. Its trail intersected another trail, made by a river otter that had come upstream on the ice. In fact, there were two and possibly three otters, weaving in and out over the ice and finally coming up the bank. They went over a short stretch of meadow, under some trees, and down to a tiny, frozen slough. I decided to follow this trail as it went along the slough. The frozen channel got gradually wider, but I found one place where the ice was broken, perhaps by the otters. The edge of the opening was packed flat by otter feet, so it was clear that they had spent some time by this hole in the ice. What could they have found there? The trail continued down the icy slough, eventually joining the main creek again where there was some open water and a good chance of finding small fish. The total journey from creek-leaving to creek-joining was maybe half a mile. I think that these hunters knew where they were going and were just checking out another part of their home range.

I was having so much fun that I didn’t pay attention to my feet. When I finally happened to look down at my feet, I was surprised to see that I’d thrown a ‘shoe. So I back-tracked to retrieve that lost one, almost all the way back to where I’d picked up the otters’ trail. Having fun seems to be distracting!

One day in the middle of February, I wandered out into the Dredge lakes area, following a tip from a friend. After threading my way through a noisy passel of school kids, I went straight to one corner of Moose Lake. This is where we often see migrating trumpeter swans in fall, but this time I was looking for some patches of open water. I found them, under some snow-laden alder branches. The surface of the water was roiling periodically, so I knew something was going on in there. Peering closely into the dark water, I saw them: several big Dolly Varden moving slowly about in the shallows. I could pick out the white borders of their pectoral and pelvic fins, which are a good field mark. There was another big fish in the same bit of open water, a fish with no white on the fins and black speckles all over the body…probably a cutthroat trout. Dollies and cutthroats are known to overwinter in Mendenhall Lake and some of the accessible ponds in this area, where they hang out but feed little in the cold water. Why would they be in this spot? This area also sometimes hosts spawning coho in fall, so there is something special about it—and that’s the upwellings, where ground water burbles up through the sediments, bringing oxygen with it. Those fish are probably there to take advantage of the oxygenated water, which can be in short supply in some of these small ponds. As I watched, a dipper zipped out from just under the snowy bank and disappeared under the arching branches.

Dolly Varden. Photo by Bob Armstrong

On the way back to the trailhead, I started to duck under some low-hanging alder branches and saw something flit to the side. So I quickly looked up and saw a redpoll, perched not eighteen inches from my face. On the branch I had ducked under was a cluster of alder cones, a favorite food of redpolls. This bird scolded me roundly, so I apologized and moved on, while the bird went straight back to ‘his’ cones. I only saw one redpoll just then, but shortly later and not far away, I saw a whole flock of them at the seed feeders on my deck, cleaning up the millet seeds. Redpolls seem to show up around here sometime in February, most every year, making me wonder what they were doing in the earlier part of winter.

Common redpoll on alder cones. Photo by Bob Armstrong

My suet feeder at home normally attracts chickadees and juncos. But sometime in January I noticed other visitors. There were two very small birds that spent several minutes clinging to the wire-mesh suet holder and pecking at the suet. I was astonished to see that these were golden-crowned kinglets, birds that customarily feed on dormant bugs and spider eggs among the conifer needles. This was not a ‘one-off’ visit; they returned several times over the next few weeks (maybe more often than I noticed, given that I don’t spend the whole day watching from my windows).

Photo by Mark Schwann

This observation seems so unusual to me that I asked one of Juneau’s ace birders about it. I was told that, indeed, golden-crowned kinglets had occasionally visited their suet feeder too. Further inquiry via the internet revealed that this feeding behavior by golden-crowned kinglets has been reported, but very rarely, in Michigan and Tennessee, for instance.

Golden-crowned kinglets are so small (about six grams) that they lose heat rapidly (having a high surface-to-volume ratio), but they can’t store much fat. So they have to eat a lot each day, moving continuously through the foliage in search of tiny, sparse prey. They may save some energy at night by lowering their metabolism a bit and by huddling up together in sheltered spots, but the risk of going hungry and perhaps starving is significant. Given the high demand for energy, why don’t they visit suet feeders more often? And what got these few birds started on suet-feeding in the first place?