PASSERINE BIRDS OF NE\V YORK 77 



old bird from a young one. The plumage is, of course, a guide in 

 many species, when we know which is which, but it is surprising 

 how little is actually known of autumnal plumages, especially of 

 adults. Fortunately until a young bird is five or six months 

 old, immaturity may be recognized among Passerine species by 

 a very simple osteological character, and one requiring no mi- 

 croscope for its demonstration. I have made constant use of it 

 for a dozen years past and doubtless others have done the same, 

 but as yet I have never seen any explanation of it. It is simpl)' 

 this, — the prominent frontal bones of the young bird are thin and 

 transparent showing the brain beneath, while those of the adult are 

 thicker and flecked with little whitish dots, which show even better 

 as black dots, when, with the brain removed, the skull is held 

 up to the light. As the skull of the young bird ossifies, with 

 the advance of the season, it assumes the adult characters, the 

 dotted area of ossification creeping irregularh' from behind for- 

 ward and from the sides upward, until perhaps a couple of trans- 

 parent spots anteriorly may be all that is left to show immaturity. 

 When these disappear this valuable diagnostic feature is, of course, 

 lost. The dots mark the ends of slender branching columns of 

 bone that partly fill the open space between the two tables ot 

 the mature skull, and bind them together. Mutilation, or the in- 

 filtration of blood or fluid from the brain, may obscure the dotted 

 appearance, but it is usually obvious at a glance. 



This progressive ossification is scarcely perceptible in any New 

 York species before the end of October, and seems to be com- 

 pleted in the frontal bones about two months later. The mi- 

 grants that press further south seldom show more than the begin- 

 ning of the process for they ha\^e nearh' all departed b\- the 

 middle of October. Resident species, such as the Chickadee 

 {^Panis atricapillus), and early nesting species, may complete the 

 ossification before the middle of December ; early broods of the 

 Song Sparrow {^Mclospisa fasciatd) at about the same time, late 

 ones a month later ; and late nesting species, such as the Cedar 

 Bird {Anipelis ccdroniui) and Goldfinch {Spiiiiis fristis), often 

 as late as February. Man}' of our winter visitors arrive with 

 skulls incompletely ossified ; the Horned Lark {Otocoris alpcs- 



