wich birds were
all tree swallows,--white-breasted martins,--and might fairly be
supposed to have come together from a comparatively limited extent of
country. But beside tree swallows there are purple martins, barn
swallows, sand martins, cliff swallows, and chimney swifts, all of which
breed to the northward of us in incalculable numbers. All of them go
south between the middle of July and the first of October. But who in
New England has ever seen any grand army of them actually on the wing?
Do they straggle along so loosely as to escape particular notice? If so,
what mean congregations like that in the Ipswich dunes? Or are their
grand concerted flights taken at such an altitude as to be invisible?

On several afternoons of last September, this time in an inland country,
I observed what might fairly be called a steady stream of tree swallows
flying south. Twice, while gazing up at the loose procession, I suddenly
became aware of a close bunch of birds at a prodigious height, barely
visible, circling about in a way to put a count out of the question, but
evidently some hundreds in number. On both occasions the flock vanished
almost immediately, and, as I believed, by soaring out of sight. The
second time I meant to assure myself upon this point, but my attention
was distracted by the sudden appearance of several large hawks within
the field of my glass, and when I looked again for the swallows they
were nowhere to be seen. Were the stragglers which I had for some time
been watching, flying high, but well within easy ken, and these dense,
hardly discernible clusters--hirundine nebulae, as it were--were all
these but parts of one innumerable host, the main body of which was
passing far above me altogether unseen? The conjecture was one to
gratify the imagination. It pleased me even to think that it _might_ be
true. But it was only a conjecture, and meantime another question
presented itself.

When this daily procession had been noticed for two or three afternoons,
it came to me as something remarkable that I saw it always in the same
place, or rather on the same north and south line, while no matter where
else I walked, east or west, not a swallow was visible. Had I stumbled
upon a regular route of swallow migration? It looked so, surely; but I
made little account of the matter till a month afterward, when, in
exactly the same place, I observed robins and bluebirds following the
same course. The robins were seen Octobe