Xiv PREFACE. 
are generally cognizant of a migration in a given species only 
when the great ‘‘ bird wave” sweeps entirely past us either to 
the north or south. Some species, however, seem actually 
fixed at all seasons, and are really essentially non-migratory, 
as the Spruce Partridge, and Quail (Ortyx Virginianus) are in 
New England. But only a small proportion, doubtless, of the 
so-called non-migratory birds at any given locality are really 
so.* : 
‘In connection with this topic of migration, the fact that some 
of the young or immature individuals of our marine birds, as 
the Herring Gull (Larus argentatus) and other species of that 
family, and several of the Tringz, linger on our coast during ~ 
summer, while the adult all retire northward, is one of some 
interest. Mature and strong birds only, in species that breed . 
far to the north, evidently seek very high latitudes. Birds of 
the first year also appear to roam less widely than the older. 
In different species of the Gull family it is generally only the 
mature birds that in winter are seen far out at sea, though in 
the same latitudes the young may be numerous along the coast. 
All observant collectors are well aware of the fact that those 
birds that first reach us in the spring, of whatever species, are 
generally not only very appreciably larger, but brighter plum- 
aged and in every way evidently more perfect birds than those 
that arrive later; and that in those species that go entirely to 
the north of us there is a much larger proportion of paler col- 
ored and immature birds, especially among the Sylvicolidee, or 
warblers, towards the close of the migrating season than ear- 
lier. Hence the presence here of a few individuals in summer 
of species that usually go farther north is not always sufficient 
evidence that the species breeds with us.” 11 
Good illustrations are of great assistance to young students, 
teaching them, better than words, characteristic details of 
*““Tn respect to the proof whereon this proposition rests, see my remarks on 
this point in the Memoirs of the Boston Society of Natural History, Vol. i, Pt. iv, 
p. 488 (foot note).” 
11Several passages or sentences have, for convenience, been omitted in this 
extract. 
