Lessons from Travels: Rodent Cycles

by Carl Strang

One of the phenomena of wildlife in the far North is the dramatic cycling of small rodent populations. I had the opportunity to witness this when I was doing my graduate research in western Alaska. I was studying glaucous gulls rather than small mammals, but there was some relevance because the gulls feed heavily on tundra voles early in the season, when the thaw floods the voles into exposed positions.

Tundra vole, enjoying a snack provided by a colleague at the tent frame.

Lemmings were present in small numbers as well, but the only rodent that we saw undergoing violent population fluctuations was the vole. At the low point in the cycle one was hard pressed to find an active runway, and sightings of the voles themselves were few and far between. At the high point the pingos (ice-elevated rounded hills) were riddled with runs.

The voles dug into the soil, chewed a dense maze of runways, and seemed to be everywhere.

I was there in four consecutive summers, and saw one high-density year.

In the peak year they invaded my home.

Now I want to refer back to my recent literature review on food web stability. Species diversity is relatively low in the North, and in general there is a gradient of diminishing diversity from tropics to tundra. Low species diversity is associated with lower stability, and stability clearly is lacking in the vole population. It is not, however, simply a matter of few predators available to exert top-down control. In addition to glaucous gulls there were mew gulls, parasitic and long-tailed jaegers, less common predators like short-eared owls, and foxes. The last were represented by two species.

Red foxes were larger. This one got muddy.

Arctic foxes were the smaller species. This one, which appears to have a tundra vole in its mouth, hasn’t yet molted to its summer pelage.

Furthermore, all of these predators have broad diets and so can switch to focus on the most abundant prey (birds and their eggs being the chief alternative for most of them). Switching, however, isn’t stabilizing the voles. I haven’t followed the literature on this, so I don’t know where the current consensus is, but I think it’s important to point out that for much of the year the voles are protected by a deep layer of snow, and the avian predators all are gone outside the relatively short breeding season. The long winters are depauperate of species indeed, voles can breed in every month, and that surely plays a role in this food web.

While modest cycling of small rodents occurs in Illinois, it doesn’t come close to matching what we saw in Alaska. We have many more kinds of plants, rodents and predators here, and the rodents are vulnerable year round. This seems to be enough to account for the difference in food web stability of the two places.

Lessons from Travels: Migrants Elsewhere

by Carl Strang

When we are at home in Illinois, we categorize our birds with respect to their status when we see them here. They may be year-round residents, breeders that migrate south for the winter, winter residents, or migrants that breed north of us and pass through in the spring and fall. Those categories do not define the birds from their own perspective, however, and we can get some sense of this when we see them perfectly at home in other places. When we think of yellow-throated warblers, for instance, we typically associate them with sycamores, not with palm trees.

Yellow-throated warbler in Belize.

Still, there is a consistency in the open canopies of sycamores and palms that makes sense from the bird’s perspective. Though we commonly think of our breeding birds as being northern animals that head south to escape the winter, it might be better to regard them instead as tropical birds that travel north to take advantage of high summer productivity and fewer predators.

Travel also allows us to broaden our perspective on migrants, when we see them on their breeding grounds. This was one of the side benefits of the summers I spent in western Alaska. On the rare occasions when we see long-tailed ducks in northeast Illinois they are quiet, placid, unobtrusive. They are quite the opposite on their breeding grounds.

Male long-tailed duck, Kokechik Bay study area.

When courtship commences they become very noisy with un-duck-like tenor voices, chasing each other at rocket speeds and coming very close, apparently using people as picks. The females incubate large clutches of eggs, producing tiny dark ducklings.

In those days we called them oldsquaws. Here a mother and ducklings share a pond with red-necked phalaropes, which then were known as northern phalaropes.

Tundra swans have extraordinary courtship and territorial displays, and make huge nest mounds. Biologists can count eggs from the air. The young are placid.

Nonbreeders gather into flocks of 30 or more.

In the treeless tundra, dunlins advertise by hovering 10 feet above the ground, trilling a song that is almost identical to that of American toads. They have well camouflaged ground nests with 4 eggs.

When we see them as migrants in Illinois, dunlins are traveling in small flocks and behaving as shorebirds.

Jaegers are rarely encountered seabirds in Illinois, sought along the edge of Lake Michigan especially during the fall migration. On the breeding grounds they are predators.

Long-tailed jaegers are beautiful and graceful, hovering like kestrels in their hunt for tundra voles and bird eggs.

Two species nested there, the other being the parasitic jaeger.

Parasitic jaegers are larger than long-taileds. Once I saw one chase down and swallow whole an adult red-necked phalarope.

Such experiences sit in my mind, reminding me to think of these animals in terms of their entire lives rather than the more limited glimpses we see in Illinois.

%d bloggers like this: