The general pattern of this migration, as manifested particularly about the northwestern portion of the lake, was explained by Charles Schweder. As a rule, practically all the local Caribou spend the winter in the wooded country to the southward. When the northward movement starts in the spring, the does precede the bucks; they migrate through this area mostly in May (and apparently more commonly in the latter half of the month), but to some extent in April or even earlier. During June the majority of the animals passing by are bucks. According to Fred Schweder, Jr., the spring migration here is more regular, less erratic, than the fall migration. When I arrived at Nueltin Lake on May 31, the spring migration was in full swing; it continued through June in gradually lessening force, and the last northbound band was seen on July 1. In the meantime many interesting details of caribou behavior came to light. While flying from Churchill to Duck Lake Post on May 31, at a height of about 500 feet, we detected three bands on the frozen surface of Seal River, where they were evidently taking a noonday rest. They numbered approximately 40, 20, and 6 individuals. Within 20 minutes after resuming our journey, from Duck Lake Post to Nueltin Lake, we saw four more bands, numbering from two to a dozen individuals. Finally, just before landing on Windy Bay, we noticed a band of perhaps 20 crossing the bay a mile from its head. Observations during the following 10 days showed that this was a very definite crossing-place for the Caribou, as if some invisible barrier on each side kept them to a certain line of march. Moreover, Charles Schweder informed me that they followed this identical route year after year. An examination of the local topography (map 1) soon revealed the reason. The rugged south side of Windy Bay rises steeply for some 500 feet to the summit of the Windy Hills, and over a considerable distance there are precipices and talus slopes barring the passage of such animals as Caribou. But the mile-long South Bay, meeting Windy Bay at right angles, affords a convenient break in the hills; and the slopes thereabouts are gradual enough to be negotiated readily by the Caribou. So here they converge from the neighboring heights, making long, slanting trails through the snow that are visible for miles; they pass out of the narrow mouth of South Bay as through a funnel, then follow the beaten path of their predecessors across the ice of Windy Bay toward the farther shore, until they are lost to view among a cluster of islands. It may be further remarked that their course northeastward from the mouth of South Bay is less obstructed by islands than almost any near-by portion of Windy Bay.