In the case of certain island-inhabiting birds, however, some of these questions may be answered with a fair degree of certainty. Thus Ipswich Sparrows are known to nest only on Sable Island, off the Nova Scotia coast, and we are warranted in believing that the same birds, fate permitting, return to their sandy home year after year. Gannets (Sula bassana) nest in the western hemisphere only on three islets in the Gulf of St. Lawrence, and it is probable that the surviving individuals return each year to their former breeding grounds. The Terns of Muskeget and Penikese, forming the only two large colonies of these birds remaining on the Atlantic coast, return to their island retreats every spring; and actuated by this same love of home, the Brown Pelicans of the Indian River region of eastern Florida annually repair to a certain small island for the purpose of rearing their young. Many similar cases might be cited in confirmation of the belief—supported also by isolated observations on the mainland—that birds nest in the same locality throughout their lives, and, on occasion, may even occupy their previous season’s nest.
As regards the manner in which these island-inhabiting birds arrive at the nesting grounds, as far as our recorded information goes, it seems that without relation to latitude they appear each spring with remarkable regularity, not straggling back a few at a time, but sending on an advance guard, which usually remains only a short time and is followed, a few days later, by apparently the entire colony.
Thus, Mackay writes of the Terns of Penikese: “In 1893 the Terns arrived on May 10th, in the night, an advance guard of several hundred being noted early the following morning at daylight; these all left before noon of the 11th, and on the morning of the 12th, before daylight, immense numbers had again arrived.... In 1896 the Terns commenced to arrive during the night of May 9th; they were in evidence at daylight on the 10th, and continued to arrive all day, and on the morning of the 11th the usual colony had taken possession of the island.” (Auk, xiv, 1897, p. 284.)
The migration of the island-nesting Terns in the tropics is apparently no less regular. Scott states that the Noddy arrived in the Tortugas “on April 20th in large numbers, but remained only two days; after inspecting their breeding grounds, all departed to return about a week later in greatly increased numbers, when breeding was almost at once commenced.” (Auk, vii, 1890, p. 306.)
These insular colonies, however, not only throw much light on certain existing phases of bird migration, but they also furnish us with a clew to the origin of migration itself. This is especially true of those species whose lives are passed in the tropics or subtropics, and which we are accustomed to class as nonmigratory or as “permanent residents,” but which are as regularly migratory, in the real meaning of the word, as if they summered within the arctic circle and wintered south of the equator.
Their movements are apparently in no way influenced by climate nor, at this season, are they governed by the food supply, but prompted solely by the annually recurring physiological change which fits both sexes for reproduction, they repair to a certain islet, perhaps in the heart of their range, with the one object of finding a suitable nesting site in which their eggs may be laid and young reared in safety; and this object accomplished, they desert the locality, where they may be unknown until the following spring.
Divested, therefore, of the complications which ensue when in studying the migration of birds the questions of food and climate must be considered, we have here the problem reduced to its simplest terms; and in the desire for seclusion during the breeding season which induces birds to conceal their nests, if possible perhaps near by, but if necessary after a journey of varying length undertaken especially for the purpose, we have a good and sufficient cause for the origin of bird migration.
An attempt to explain the present manifestation of the migratory movement involves a study of the climatic changes to which our globe has been subjected. No doubt many birds controlled by “heredity of habit” make semiannual journeys which at one time were necessary, but under existing circumstances are no longer required. Why, for example, should the Bobolink winter south of the Amazon, while its ally, the Red-winged Blackbird (Agelaius phœniceus), does not leave the eastern United States? I have, however, no intention of writing an essay on bird migration, and these thoughts are presented merely as preliminary to a study of the life of Pelican Island, of a visit to which they are in part the outcome.
Pelican Island is situated midway between the northern and southern extremities of Indian River, near the eastern shore of a key which here makes the river about three miles wide. It is triangular in shape and contains about three acres of ground, on which grow a few black mangroves, a cabbage palm or two, and great patches of grass; but at least one fourth of its surface is bare ground.
On one of the islands of the near-by Narrows a few pairs of Brown Pelicans are said to have nested, but, with this exception, Pelican Island doubtless forms the nesting ground of all the Pelicans of Indian River.