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.


February scrapbook

warm and bright observations in an icy world

Winter finally arrived sometime in early February, with good snow on the ground and very cool temperatures. I’ve lived here for three decades, so I’m quite well acquainted with Juneau’s local microclimates—it’s often warmer, wetter, and windier downtown than it is in the upper Valley where I live. But I recently saw what seemed to be an extreme case: as I drove Out the Road one morning, I left my house at a temperature of minus six degrees (F), then the car thermometer registered plus thirteen, dropped quickly to minus two, and rose again to plus fourteen degrees. That’s a twenty-degree span in fewer than twenty minutes. Extraordinary.

Along the way, I passed a place where a thin blanket of white mist lay over an estuary and shallow inlet. We often see this phenomenon in cold weather and sometimes call it ‘sea smoke’ or ‘steam’. But it’s not steam…steam is hot water vapor, and it’s not really smoke, either…not full of organic particles and carbon dioxide. Whatever the right name is, the cause is well-known. Liquid fresh water cannot be colder than thirty-two degrees (or it would become ice). So the surface of the estuary was warmer than the frigid air and water was evaporating. When that rising water vapor encountered the cold air, which holds less water than warm air does, it condensed into small droplets that hung over the water surface in thin mist.

I met a friend at the Point Bridget trailhead and we set off to see what we could find. The best find was the trail of an otter, bounding and sliding over flat ground and out onto the frozen beaver pond. Even on the flat, this otter was sliding as much as eight feet before gathering itself for another bound and slide. Wouldn’t that be fun to do! Blowing snow had drifted into some other tracks, but we found those of porcupine, moose, and a deer or small moose; red squirrels had made new highways under some of the trees.

A stiff breeze was churning Lynn Canal into a froth and big waves were roaring onto the beach where we look out at Lion’s Head. By the time we got there, it was afternoon and the wind was increasing, as it often does then. So the beach log where we often perch for lunch was not very hospitable. Even behind the beach berm, the wind was making the emergent tall grasses lie almost flat on the snow. So we found a windbreak in a sunny spot for a comfortable lunch.

Home again, with temperatures a relatively balmy plus sixteen degrees. The birds were active on the feeders, among them ‘my’ pair of red-breasted nuthatches. They brought two youngsters to the feeders one day last summer but they have apparently stayed on their territory for the winter. Each pair is socially monogamous; there apparently have been no studies of extra-pair matings (which are common in many other birds). Nuthatches defend their territories from other nuthatches; the male is especially vigorous in defense when the pair is excavating a nest in a dead tree. They also defend the nest cavity from red squirrels, which are potential predators of eggs and chicks.

Nuthatches have the odd habit of putting sticky conifer resin around the opening of the nest cavity. It is thought that this helps deter predators. One study found that more resin was placed around the nest entrance right after a face-off with a squirrel. Rarely, however, this tactic backfires, and one parent gets inextricably stuck in the resin and dies.

After the nest is built, females incubate five to eight eggs and the male brings her food. Incubation takes about twelve days and chicks stay in the nest for almost three weeks. Sometimes the male joins the female in the nest during incubation and brooding very small chicks. After the chicks fledge, the parents feed them for another two weeks and then the youngsters sometimes stay with the parents for many weeks, or they may become independent and disperse. Most nuthatches probably live only a few years; the maximum known lifespan is just over seven years.

Nuthatches forage by walking up and down and around tree trunks and big branches, especially in winter, presumably because dormant arthropods lurk in the crevices. They can walk head-first down a tree trunk and even walk upside down underneath a branch. They have a very short tail, not usable for bracing again the wood as woodpeckers and creepers do. Having a relatively long hind toe helps them scamper down and sideways. Outside of the winter season, they also forage on twigs and leaves, even on the ground sometimes, and occasionally catch insects out of the air.

Photo by Gwen Baluss

Captured food is often cached in holes and crevices, sometimes covered with bits of lichen or bark. A big item is wedged into a crack and then hacked into smaller bits (unlike chickadees, which hold such items in their feet). In fact, their English name may have originally been nut-hacker. At a seed feeder, nuthatches can be very choosy, carefully selecting the largest and heaviest items.

Little appears to be known about how they manage in extremely cold weather. They do join mixed-species foraging flocks in winter, along with chickadees, kinglets, and other small birds. Presumably their insulation is quite good, but they don’t seem to roost communally or have elevated metabolic rates then (as some other birds do). More questions to be answered!