northern forests! In both regions only a small number of 
the sites available for nesting are utilized. If the reason for nest¬ 
ing in the north was for increasecf safety it would be expected that 
those birds which do remain behind to contend with the perils of 
tropical nesting, would develop greater skill in building nests in¬ 
accessible to enemies than those going north who would presuma¬ 
bly be exposed to fewer perils. Such, however, is by no means 
the case. Troj^cal nests cannot be distinguished from northern 
nests by any such criterion of efficiency against enemies. Some of 
our best nest builders, the Baltimore Oriole for example, are also 
notable migrants. In the case of both of these latter theories it 
would seem as if Nature, who always works along the lines of 
least resistance, would have found it easier to adapt migrating 
birds to a different sort of nestling food or to perfect in them the 
skill necessary to build securer nests in the tropics before evolving 
the intricate machinery incident to annual migration. 
The Vacuum A theory proposed by Allen seems more rea- 
Theory sonable. It rests upon the idea that “Nature 
abhors a vacuum” and, therefore, any accessible territory from 
which animals have been temporarily excluded will not long re¬ 
main unpopulated after the cause of temporary banishment has 
been removed. During the winter birds are forced to abandon 
the northern latitudes for the tropics because of cold and the con¬ 
sequent shortage of food. When spring comes this entire vacated 
area is again thrown open for habitation at the very time when 
the birds, temporarily crowded into the tropics, are beginning to 
seek nesting places. It is quite as inconceivable to imagine that 
birds, with their active powers of flight, should fail to reInvade 
the territory, from which they had been temporarily driven by 
winter, as soon as it is again available for habitation, as that an 
expansible gas should remain in a flask after the stopper which 
confined it there had been removed. This theory, then, explains 
spring migration as a logical expansion consequent upon the com¬ 
pression into the tropics during winter of a large per cent, of the 
bird population of all latitudes. 
The Over-popula- Another factor has been emphasized by Tav- 
tion Theory emer. This may be called the over popu¬ 
lation theory depending as it does upon the circumstance that 
whenever the breeding season opens there is suddenly a 
great increase in population within a given feeding area. 
Such a condition must result in a keener competition for food 
and those birds who are stronger or who are the earliest to 
mate and produce young drive out the weaker and tardier ones 
into the surrounding region. This dispersion would not be to¬ 
wards the south, neither toward the east nor the west, because in 
