PREFACE. Xili 
of its breeding near Springfield. The casual visits of northern 
birds in winter, which we may suppose sometimes results from 
their being driven south by want of food or the severity of the 
season, are also less remarkable, it appears to me, than the 
occurrence here of southern species, as of the two Egrets, the 
Little Blue Heron (Florida Cerulea) the Gallinules and other 
aquatic species, which never, so far as known (with one excep- 
tion perhaps), breed so far north. In’the latter case they are 
generally young birds that reach us towards fall in their chance 
wanderings. 
“Tt may here be added that the cause of the migration of our 
birds still offers an interesting field for investigation. _Obser- 
vers are of late noting that in the case of some northern spe- 
cies that reach us only occasionally in their winter migrations, 
young birds only are at first seen, but if the migration contin- 
ues 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 eneucleator). '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—vwhile at other times it is not 
seen at all during some of our severest winters. The probable 
cause is more frequently, doubtless, a 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 migra- 
tion was compelled by a scarcity of food. 
‘¢ Probably fewer birds are actually permanently resident at 
a given locality than is commonly-supposed, for species seen 
the whole year at the same locality, as the Blue Jay, the Tit- 
mouse, the Brown Creeper, and the Hairy and Downy Wood- 
pecker, 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 
