THE IHACK GUILLEMOT. 
303 
tiiiguished by tlie larger beak, wliicli averages an inch and a 
quarter in length ; by the darker hue of the upper surface ; l)y 
the freedom from spot of the white patch on the wing, which 
in the young is always mottled with black until the summer 
moult ; and by the brighter colour of the feet and of the inside 
of the mouth. 
The adult acquires its summer plumage earlier than the young, 
and the males of both sooner than the females. About the begin- 
ning of March, while the young birds are still unaltered in 
appearance, the old ones which have remained with them during 
the winter have already undergone a considerable change, a 
very few being black, the rest with the under part largely 
spotted ; and shortly afterwards others presenting the same 
appearance begin to arrive, the number of arrivals rapidly 
increasing as spring advances. At this season the fishermen 
observe them in small flocks a few miles off the land, seldom 
diving, but just idling awa their time.” These new arrivals 
are usually rather shy at first. Indeed, it is not unworthy of 
notice, as bearing upon the question of migration, that in late 
winter, say in February, when great numbers may be seen in 
parties of a dozen or so, the birds in black plumage are apt to 
be extremely wild. As April, which is the pairing season, 
approaches, a bird in black plumage may be seen with a mate 
still retaining many white feathers, or two in the latter state 
may pair ; but as they are late breeders, not a sj^ot of white is 
apparent some weeks before laying commences, viz., about tlie 
first week in June. The earliest hatched birds are therefore 
about a month old when the autumn migration begins ; but 
very soon learning to provide for themselves, they then no 
longer require the assistance of their parents, whicli can by that 
time safely leave them. Yet it may be asked, what becomes 
of those which are hatched three or four weeks later ? Is it not 
possible that the adults which remain behind are their parents ? 
It is difficult to see how the question is to be investigated ; but 
could this once be proved, it would be a step at least towards 
clearing up the mystery of partial migration among birds. 
