According to Albert R. Brand (1938) there is considerable variation in the pitch of the song of the eastern yellow warbler, from 8,775 vibrations per second in the highest note to 3,475 in the lowest note, and with an approximate mean of 5,900 vibrations per second. This is far below the approximate mean of 8,900 for the black-poll warbler, but well above the average of 4,000 for all passerine birds. _Field marks._--One hardly needs field marks to recognize a yellow warbler; it is the yellowest of all our warblers at all seasons, even the wing and tail feathers are edged with yellow, and there is no white in either wings or tail. The youngest birds likewise show some yellow on the under parts and in the flight feathers. See the descriptions of plumages for details. _Enemies._--The arch-enemy of the yellow warbler is undoubtedly the cowbird. This warbler is one of the very commonest victims of this parasite, and comparatively few of its nests are not visited at least once by a cowbird in regions where the latter is very common. Dr. Friedmann (1929) has about 500 records of such imposition on the eastern yellow warbler. Everyone who has examined nests of this warbler in any number has found one or more eggs of the cowbird in some of the nests. This parasitic habit has cost this species of warbler many extra hours of unexpected labor and the loss of many eggs and young. But the most interesting fact about it is that the warbler has found a way to combat the evil and, in many cases, to defeat the plans of the cowbird, by either deserting the nest in which the strange egg is deposited or by building a second floor over it and leaving the alien egg to cool off in the "cellar."