72 MR. J. H. GURNEY ON MIGRATION OF CROSSBILLS. 
of a very tall Spruce-fir, but this was the only nest he heard 
of in spite of constant search in that neighbourhood. 
At Keswick a pair were seen treading, and an unidentified 
nest was found by Bates near the extremity of a Scotch fir 
branch, but it was never completed, and a Squirrel or a Jay 
demolished it. 
Mr. Whitaker, who has studied this great Crossbill 
invasion, is of opinion that lack of ripe fir-cones in 
the area of their natural range — which is Northern and 
Central Europe — is the cause which has led to this general 
movement southwards and westwards. To this must be 
added, considering their vast numbers, the probability that 
the breeding season of 1909 was a prolific one with them. 
In any case the migration must have had its origin throughout 
an enormous tract of country, or it could never have supplied 
so many birds. 
The number of fir-cones which Crossbills pull off and throw 
to the ground, must be enormous, certainly one bird must 
often dispose of a hundred or more in a day. While thus 
engaged, the)’’ do not mind being watched, and it is pretty 
to see them fly from one branch to another holding a cone in 
their mandibles, until they find a standing place where they 
can hack at it to their liking. 
The usual appearance of a Scotch-fir cone, which has been 
dealt with by a Crossbill, is well shown in the annexed cut, 
done by Mr. G. Wilson, from one picked up after it had been 
manipulated by one of these birds. 
I noticed on different occasions, as others must have done, 
