MIGRATION. 291 
all feathered alike, he at first imagined that they 
were the young of both sexes not perfected in their 
colours; but by opening several scores, as they 
were prepared for the spit, he found them to be all 
females; and, after repeated searches, he was never 
able to find one cockbird at that time of the year. 
But in the spring of the year, another transient visit 
was paid, and then the different sexes were plainly 
distinguishable. 
A remarkable example of this kind of migration 
is afforded by the passenger-pigeon of America, the 
history of which is given by Audubon, in his Orni- 
thological Biography. ‘The most important facts,” 
he says, “connected with its habits, relate to its 
migrations. These are entirely owing to the ne- 
cessity of procuring food, and are not performed 
with the view of escaping the severity of a northern 
latitude, or of seeking a southern one for the pur- 
pose of breeding. They consequently do not take 
place at any fixed period or season of the year; in- 
deed, it sometimes happens, that a continuance of a 
sufficient supply of food in one district will keep 
these birds absent from another for years. I know, 
at least, to a certainty, that in Kentucky they re- 
mained for several years constantly, and were no- 
where else to be found. They all suddenly disap- 
peared one season when the maize was exhausted, 
and did not return for a long period. Similar facts 
have been observed in other states. 
“In the autumn of 1813, I left my house at Hen- 
derson, on the banks of the Ohio, on my way to 
Louisville. In passing over the Barrens, a few 
miles beyond Hardenbsurgh, I observed the pigeons 
flying from northeast to southwest, in greater num- 
bers than I thought I had ever seen them before; 
and feeling an inclination to count the flocks that 
might pass within the reach of my eye in one hour, 
I dismounted, seated myself on an eminence, and 
began to mark with my pencil, making a dot for 
