On the Nature of the Relationship Existing among Land Birds during Sustained Aerial Migration

Abstract

THE following article is in part manifestly speculative, but sufficient data are in hand to seem to warrant its publication for the purpose of calling attention to the sort of observations which can be made to test the hypothesis advanced. My attention and that of Mrs. Whittle was once directed for a period of four hours to the manner Myrtle Warblers (Dendroica coronata) were migrating.' During this period many detached flocks of ten to twenty or more individuals flew across a narrow strait between two islands, the Isle of Palms and Sullivan Island, lying easterly of Charleston, South Carolina, flying northeasterly up the coast. The total birds of this species moving northward during these four hours was not less than 24,000. These birds were unaccompanied by any other species, or at least negligibly so for no other species was seen, the date (March 4, 1920) being too early to expect mixed migration. A short distance to the southwest at a place on Sullivan Island which forms the first land on the northerly side of the entrance to Charleston Harbor, this mighty host of Warblers, having crossed the entrance to Charleston Harbor, was traveling northeast on the land in a very scattered and disorganized manner, but these same birds when they again took themselves to the air, shortly afterwards, assumed the grouping already described. A natural question to ask oneself on witnessing this great movement of small groups of a single species of Warbler is: would these groups still remain intact if other species of Warblers in numbers, and, say Vireos, were migrating with them? The above facts, and other observations made prior to and since that date, to be described, have led to the formation of the hypothesis that had there been another species or several species of Warblers, or species belonging to other families, migrating