766 BIRDS [MIGRATION. t of the ob-
- cause
igra- tlie first sharp frost has a decided effect upon their distri bution, and a heavy fall of snow drives them towards the homesteads for the larger supply of food they find there, while should severe and long-continued hard weather follow even these birds vanish, leaving only the few which have become almost domesticated. These two species have been here chosen as illustrative cases because they are at once plentiful and familiar, and want of space only forbids us from citing others, but we shall find on inquiry that there is scarcely a Bird of either the Palsearctic or Nearctic Region, whose habits are at all well known, of which much the same may not be said, and hence we are led to the conclusion that every Bird of the northern hemisphere is to a greater or less degree migratory in some part or other of its range. Such a con clusion brings us to a still more general inference namely that Migration, instead of being the exceptional character istic it used formerly to be thought, may really be almost universal, and though the lack of observations in other, and especially tropical, countries does not allow us to declare that such is the case, it seems very probable to be so. Before proceeding however to any further conclusions it is necessary to examine another class of facts which may possibly throw some light on the matter. It must be within the experience of every one who has ever been a birds -nesting boy that the most sedentary of Birds year after year occupy the same quarters in the breeding season. 1 In some instances this may be ascribed, it is true, to the old haunt affording the sole or the most convenient site for the nest in the neighbourhood, but in so many instances such is not the case that we are led to believe in the existence of a real partiality, while there are quite enough exceptions to show that a choice is frequently exercised. The same may equally be said of the most migrant of Birds, and perhaps the strongest instance that has ever come to the knowledge of the writer refers to one of the latter. A pair of Stone-Curlews (CEdicnermis crepitans) a very migratory species, affecting almost exclusively the most open country were in the habit of breeding for many years on the same spot though its character had undergone a complete change. It had been part of an extensive and barren rabbit-warren, and was become the centre of a large and flourishing plantation. With these two sets of facts before us we may begin to try and account for the cause or causes of Migration. In some cases want of food would seem to be enough, as it is undoubtedly the most obvious cause that presents itself to our mind." The need which all animals have of finding for themselves proper and sufficient sustenance is all- powerful, and the difficulties they have to encounter in 1 Two remarkable instances of this persistency may be noticed. The nest of a Falcon ( Falco peregrinus) on Avasaxa a hill in Finland somewhat celebrated as one of the most southern points whence the midnight sun may be seen is mentioned by the French astronomer Maupeiluis as having been observed by him in the year 1736. In 1799 it was rediscovered by Skjoldebrand and Acerbi. In 1833 Wolley found it tenanted, and from enquiries he made of the neigh bours it was evident that such had yearly been the case so far as any one could remember, and so it was in 1855 as the writer can testify. In 1779 according to one account, in 1785 according to another, a pair of the Blue Titmouse (Parus ccendeus) built their nest in a large earthenware bottle placed in the branches of a tree in a garden at Oxbridge near Stockton-on-Tees. With two exceptions only, this bottle, or a second which has lately been placed close to it, has been tenanted by a pair of birds of this species from the year in which it was first occupied until 1873, v.-hen the writer saw it. See Yarrell s British Birds, 4th ed. i. pp. 58, 486. 2 Far more so than variation of the temperature, though in popular belief that probably holds the first place. But Birds generally, as compared with other Vertebrates, are but slightly affected by extremes of heat or cold, and indeed (so far as we can judge) by most climatic Influences, provided only their supply of food is not affected thereby. Cf. Max Schmidt, Zoolog. Garten, 1865, pp. 330-340.) obtaining it are so great that none can wonder that those which possess the power of removing themselves from a place of scarcity should avail themselves of it, while it is unquestionable that no Class of animals has this facility in a greater degree than Birds. 3 Even among many of those species which we commonly speak of as sedentary, it is only the adults which maintain their ground throughout the year. It has long been known that Birds-of-prey cus tomarily drive away their offspring from their own haunts so soon as the young are able to shift for themselves. The reason generally, and no doubt truly, given for this be haviour, which at first sight appears so unnatural, is the impossibility of both parents and progeny getting a liveli hood in the same vicinity. The practice, however, is not limited to the Birds-of-prey alone, but is much more uni versal. We find it to obtain with the Redbreast, and if we watch our feathered neighbours closely we shall per ceive that most of them indulge in it. The period of expulsion, it is true, is in some Birds deferred from the end of summer or the autumn, in which it is usually per formed, until the following spring, when indeed from the maturity of the young it must be regarded as much in the light of a voluntary secession on their part as in that of an act of parental compulsion, but the effect is ultimately the same. These cases, however, which make certainly the exception rather than the rule, we can account for in another manner. It is to be observed that they are con fined to species having a peculiar mode of life, the indi viduals associating in family-parties to form small bands. The members of the Titmouse-family (Paridce) offer a good instance of this peculiarity, but it requires no very abstruse reflection to perceive that the adoption of this habit is one eminently conducive to the easy attainment of their food, which is collected, as it were, into particular spots often far apart, but where it does occur occurring plentifully. Thus a single Titmouse searching alone might hunt for a whole day without meeting with a sufficiency, while if a dozen are united by the same motive it is hardly possible for the place in which the food is lodged to escape their detection, and when discovered a few call-notes from the lucky finder are enough to assemble the whole com pany to share the feast. It is impossible to watch a band of any species of Titmouse, even for a few minutes, with out arriving at this conclusion. One tree after another is visited by the active little rovers, and its branches examined : if nothing be forthcoming away goes the explorer to the next that presents itself, merely giving utte^nce to the usual twitter that serves to keep the body together. But if the object of search be found, another kind of chirp is emitted, and the next moment the several members of the band are flitting in succession to the tree and eagerly en gaged with the spoil. 4 The mode in which the want of sustenance produces Migration may best be illustrated by confining ourselves to the unquestionably migrant Birds of our own northern hemisphere. As food grows scarce towards the end of summer in the most northern limits of the range of a species, the individuals affected thereby seek it elsewhere. Thus doing, they press upon the haunt of other individuals : these in like manner upon that of yet others, and so on, 3 The only animals which approach Birds in the extent and character of their migrations are Fishes, of which there is no need here to say anything.
- The case is altogether different with those species which in winter
form themselves into large flocks, as most of the Finches (Fringillidcc) and Buntings (Emberizidcc). The discoverer of a favourite morsel perhaps by his actions betrays what he has obtained, and accordingly his fellows may repair to the place, but it Is without invitation on his part, and the only particular bond of union not entirely selfish which keeps them together is the cry of alarm with which a stranger is greeted. Banish ment ol young. Excep tional
cases.