NATURAL SCIENCES OF PHILADELnilA. 41 



off in November, and few are seen after this month. I am not 

 sure that amj remain all winter. 



Chroecocephalus Philadelphia. 



Extremely abundant, but only in spring and fall, being strictly 



migrants here. The}' appear about the first week in April with 



the last species, generally remain through most of May, and 



return in September, remaining until the end of November. But 



this general statement may be somewhat qualified in detail b}' 



some observations that have interesting bearing, unless I wrongly 



interpret them. In 1869, from the beginning of April until the 



22d, the birds were here in great numbers, and with a marked 



preponderance of old full-plumaged individuals. Without any 



change in the weather, or other assignable cause, they suddenly'' 



disappeared, presumablj^ having moved northward. For a week 



or ten days scarcely one was to be seen; when they became more 



numerous, if possible, than before, and so continued through the 



greater part of May. This last lot was almost entirely composed 



of birds of the previous year, as shown b}^ the plumage, there 



being hardly a black-headed one among them. I should judge, 



therefore, that the old birds migrate in advance of the young, and 



also that they move the more hurriedly of the two. It is probable, 



moreover, that only these adults were going to breed that year, 



although it is pretty certain that some gulls, at least, reproduce 



before gaining their perfect plumage. In the fall few black-headed 



ones are ever seen, the hood being lost before they reach this 



latitude; but even at this season the old and young are readily 



distiuguished by other marks. Man}' return in September, and 



then the earliest ones to come are the young. I verified the same 



o-eneral observations through four migrations that I witnessed. 



This year (1810) there were few — almost no — birds in part of 



October, after the September young appeared to have passed on ; 



but as I write (Nov. 18) the harbor is covered with thousands 



of old ones in their winter dress. They appear at a little distance 



almost pure white, and are beautifully conspicuous among the 



other gulls and the terns with which they associate. 



Audubon is certainly mistaken in representing the female of 

 this species with a brown hood. The sexes are not distinguish- 

 able by any outward marks. If there is a United States gull with 

 the head of the color shown in the plate just alluded to, it must 

 18*11.] PART I. — 4 



