Dear Sir,--I have now, past dispute, made out three distinct species of
the willow-wrens (_motacillae trochili_) which constantly and invariably
use distinct notes. But at the same time I am obliged to confess that I
know nothing of your willow-lark. In my letter of April 18th, I had told
you peremptorily that I knew your willow-lark, but had not seen it then;
but when I came to procure it, it proved in all respects a very
_motacilla trochilus_, only that it is a size larger than the two other,
and the yellow-green of the whole upper part of the body is more vivid,
and the belly of a clearer white. I have specimens of the three sorts
now lying before me, and can discern that there are three gradations of
sizes, and that the least has black legs, and the other two
flesh-coloured ones. The yellowest bird is considerably the largest, and
has its quill-feathers and secondary feathers tipped with white, which
the others have not. This last haunts only the tops of trees in high
beechen woods, and makes a sibilous grasshopper-like noise, now and then,
at short intervals, shivering a little with its wings when it sings; and
is, I make no doubt now, the _regulus non cristatus_ of Ray, which he
says "_cantat voce stridula locustae_." Yet this great ornithologist
never suspected that there were three species.