RARER BIRDS OF MASSACHUSETTS. 511 
(Florida cerulea) the Gallinules and other aquatic species, 
which never, so far as known (with one exception perhaps), 
breed so far north. In the latter case they are generally 
young birds that reach us towards fall in their chance wan- 
derings. 
It may here be added that the cause of the migration of 
our birds still offers an interesting field for investigation. 
Observers are of late noting that in the case of some north- 
ern species that reach us only occasionally in their winter 
migrations, young birds only are at first seen, but if the 
migration continues the older birds appear at a later date. 
But sometimes young birds only are seen. This frequently 
happens in the case of the Pine Grosbeak (Pinicola eneucle- 
ator). The cause of their visits is not always, it is evident, 
severe weather; the last named species appearing sometimes 
in November, — weeks before severe cold sets in— while at 
other times it is not seen at all during some of our severest 
winters. The probable cause is more frequently, doubtless, 
à short supply of food, as last winter was remarkable in this 
state for its mildness and for the great number of northern 
birds that then visited us. It has repeatedly been observed 
that on their first arrival these unusual visitors are generally 
very lean, but that they soon fatten; an argument in favor 
of the theory that their migration was compelled by a scarc- 
ity of food. - 
Probably fewer birds are actually permanently resident at 
à given locality than is commonly supposed, for species seen 
the whole year at the same locality, as the Blue Jay, the 
Titmouse, the Brown Creeper, and the Hairy and Downy 
Woodpecker, etc., in Massachusetts, are represented, not by 
the same, but by different sets of individuals, those seen 
here in summer being not those seen in winter, the species 
migrating north and south, en masse, with the change of 
Season. We 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, how- 


