ply of insect life, so their numbers increase until the
wood becomes an important breeding station, resonant with the song of
many individuals. But slowly the growth increases; the bushes pass into
saplings and the saplings into trees, and the undergrowth then
disappears just as surely as do the migrants which can no longer find
there the conditions which they require.

Or, as an illustration of the effect produced by natural agency, let me
describe a change which has taken place in a corner of Co. Donegal. The
promontory of Horn Head is bounded on the west by extensive sand-hills,
100 ft. or more in height. On the southern side it is divided from the
mainland by a channel, which narrows down to 100 yards or so in width
where it fringes the sand-hills, and then widens out again, covering an
area of approximately 270 acres. As far as is known in the memory of
man, this area has always been tidal. But in recent years a change has
taken place, and the blown sand has silted up the channel, with the
result that this tidal area has been transformed into a brackish lake.
What has brought about the change is not easy to determine. There is
evidence, however, of a slow alteration of the level of the shore-line;
for in the midst of the sand-hills, situated 150 yards or so from the
present sea-margin, and running parallel with it, there is an
accumulation of pebbles some 3 feet high by 4 feet deep. This raised
beach is now separated from the Atlantic by sand-drifts of considerable
height, and consequently there are some grounds for believing that
secular elevation is taking place, which, if it be the case, will
account for the change in progress. Now the effect on the bird
population can be seen even now, and will doubtless become more apparent
as the years pass by. Sand-Martins used to find plenty of places to
breed amongst the sand-drifts, and moreover do so still. But their
nesting sites are constantly changing and disappearing, and the
breeding-place of one colony, that was situated in the bank of a stream
twelve years ago, is now buried 10 feet or more below the surface of the
sand. The area that was once tidal, but is now a brackish lake, is fed
by mountain streams, and as the fresh water predominates, so in course
of time will it become fringed with vegetation; and instead of the
flocks of Curlew, Dunlin, and other waders that, at low water, resorted
there to feed, Coots will fight with one another for the possession of
ter