onstantly augmenting their numbers. Some time in September the
migration call reaches the Martins, and, yielding to its spell, they at
once depart toward their winter home in tropical South America.

_The Usual Movement._--Many of our smaller birds, such as Warblers and
Vireos, do not possess a strong flocking instinct, but, nevertheless,
they may be seen associated in numbers during the season of the
northern and southern movements. Such birds migrate chiefly at night
and have been observed through telescopes at high altitudes. Such
observations are made by pointing the telescope at the disk of the full
moon on clear nights. On cloudy or foggy nights the birds fly lower,
as may be known by the clearer sounds of their calls as they pass over;
at times one may even hear the flutter of their wings. There is a {68}
good reason for their travelling at this time, as they need the
daylight for gathering food.

There appear to be certain popular pathways of migration along which
many, though by no means all, of the aerial voyageurs wing their way.
As to the distribution of these avian highways, we know at least that
the coastlines of the continents are favourite routes. Longfellow, in
the valley of the Charles, lived beneath one of these arteries of
migration, and on still autumn nights often listened to the voices of
the migrating hosts, "falling dreamily through the sky."

A small number of the species migrate by day; among these are the
Hawks, Swallows, Ducks, and Geese. The last two groups also travel by
night. The rate at which they proceed on their journey is not as great
as was formerly supposed. From twenty to thirty miles an hour is the
speed generally taken, and perhaps fifty miles an hour is the greatest
rapidity attained. Flights are usually not long sustained, a hundred
and fifty miles a day being above the {69} average. Individuals will
at times pause and remain for a few days in a favourable locality
before proceeding farther. When large bodies of water are encountered
longer flights are of course necessary, for land birds cannot rest on
the water as their feathers would soon become water-soaked and drowning
would result. Multitudes of small birds, including even the little
Ruby-throated Hummingbird, annually cross the Gulf of Mexico at a
single flight. This necessitates a continuous journey of from five
hundred to seven hundred miles. Some North American birds migrate
southward only a few hundr