This covering of the eggs by the parent is not unique in the bird world. The pied-billed grebe of North America also does this. When disturbed at the nest the incubating bird has been seen to use quick pecking motions to draw material from the edge of the nest over the eggs. Instead of leaving the eggs exposed the nest simply looks like a heap of trash and may thus escape the attention of a predator. It used to be thought that this grebe used to incubate only at night, leaving the eggs covered during the day to be incubated by the heat from the sun and from the decaying vegetation of the nest. However, recent studies have shown this is not the case, and protection by concealment seems to be the main advantage of this behavior.
Yet another species of quite a different group, the eider duck, covers its eggs on leaving them. The eider's nest is characterized by a blanket of down, plucked from the breast of the bird, and when the female has time, when she leaves the nest she pulls the edges of the down blanket over the eggs, perhaps for concealment, perhaps for the sake of the down's insulating properties, keeping the eggs warm in a northern climate during the parent's absence.
Here we have covering of eggs for what seems to be very different purposes: to keep the eggs cool; to keep them warm; and to hide them from view.
THE SNOWY OWL AS A TRADE INDEX [Ref]
Angus Gavin was a fur trader at the Perry River post of the Hudson's Bay Company on the edge of the Arctic Ocean. White foxes were the chief fur brought in, and the Eskimos were the trappers. Sometimes it was necessary to advance credit to an Eskimo, against the expectation of a coming season's catch out of which the advance was to be repaid. Gavin, who was a keen naturalist as well as trader, writes, "I used my observation on Snowy Owl abundance to govern extension of credit...." When snowy owls were abundant he could extend liberal credit to the Eskimo with every assurance the white-fox catch would be good and that the Eskimo would be able to liquidate his debt. When snowy owls were scarce little credit would be extended, for the white-fox catch would be small.
In general we've accepted the value of birds to man, and are appreciative of the complicated web of life in which one animal affects many others. But this use of snowy-owl abundance as a guide in granting credit strikes me as novel. Actually, of course, it is quite sound, for it uses one part of the chain that links such diverse items as owls, lemmings, foxes, Eskimo, fur trader, and finally of course milady in her white-fox furs.
LEMMINGS IMPORTANT First of the factors involved is, of course, the vegetation; the grasses, herbs, and tiny dwarf shrubs of the Arctic barrens. The next are the lemmings, mouselike creatures of the Far North that eat the vegetation. They are the first step in turning grass into flesh and fur and feathers. One of the striking facts of lemming biology is the fluctuation in their numbers. Some years they swarm, lemmings are everywhere, and in places they erupt in vast emigration, the tundra and the sea ice being covered with masses of moving lemmings. We know this best from the accounts written about the lemmings of Norway, but the same thing occurs in the American Arctic. At other times they're scarce and it is difficult to find even one. Strangely there's a periodicity in this, and periods of abundance and scarcity tend to recur every four years. What happens or what causes it we don't know.
The Arctic fox, staple fur bearer of the Far North, and the snowy owl both prey on lemmings. Lemmings are so important to them that when lemmings are abundant the foxes and the owls prosper and multiply; when the lemmings are scarce the foxes and the owls starve or migrate, in any case where there are few lemmings there are few foxes or owls.
Thus we see how it is that an abundance of snowy owls can indicate that the Eskimo will make a good fox catch and the trader will do good business.