514 BREWSTER’S WARBLER. 
uals composing the brood, or it may be connected with sex, since the female 
of Brewster’s Warbler, I think, always retains a tinge of yellow on the breast, 
even in the adult. I fancy the rapid loss of the veiling yellow feather-tips is 
occasioned by their disintegration and dropping off rather than by mere mechani- 
cal abrasion and bleaching. 
When the female pinus was first observed, although she was a typical pinus 
in plumage, both Dr. Tyler and I surmised that she was a so-called impure pinus 
in blood, produced in accordance with the Law of Mendel from one of the many 
crossings of chrysoptera with leucobronchialis that, to our personal knowledge, 
have occurred in this locality during the last four years; in other words, that she 
belonged to the same family stock as the chrysopterae and leucobronchiales in the 
swamp. We even calculated, with lively anticipation, the chances of seeing a 
Lawrence’s Warbler among her offspring,— a more than even chance provided 
both she and her Golden-winged mate belonged to the mixed stock and raised 
a brood of four or five young. This expectation (like many a hope staked on 
young promise for its fulfilment) was not realized; all the young birds grew up 
to be Brewster’s Warblers; yet we had the full satisfaction of demonstrating 
the true nature of Brewster’s Warbler and removing the question forever from 
the realm of conjecture. That all of the offspring of this pair of birds were 
Brewster’s Warblers would indicate that both of the parents were of pure blood. 
By the Mendelian Law of transmission a pure chrysoptera mated with a pure 
pinus should produce nothing but leuwcobronchiales; a pure chrysoptera and an 
impure pinus will produce, on the average, chrysopterae and leucobronchiales 
in equal numbers; an impure chrysoptera and a pure pinus, in like manner, 
pinus and leucobronchiales in the same proportion; while an impure chrysoptera 
united with an impure pinus would give rise to chrysopterae, pinus, leucobron- 
chiales, and lawrencet in equal proportions. 
It is interesting to recall to mind in this connection, that Dr. Tyler saw a 
male Blue-winged Warbler near his house on the 6th of May of this year (Auk, 
July, 1913, 30, p. 435); this bird sang the normal song of the Blue-winged 
Warbler,— pretty sure evidence that he was a stranger from the South and not 
a member of the native Lexington colony of mixed breed, all of whom without 
exception sing the Golden-wing’s song. It is highly probable that on the night 
of the 5th-6th of May, when there was a heavy migration of Warblers in this 
region, a small flight of Blue-winged Warblers invaded eastern Massachusetts 
and that the female Blue-wing that mated with the Golden-wing in the Lexington 
swamp came in with that flight. 
