The yellow warbler is not the only bird that has learned to do this occasionally, but it is the only one that does it regularly and persistently in spite of repeated contributions from the cowbird. Even if the warbler has one of its own eggs in the nest when the cowbird's egg is deposited it may bury both the eggs by building a story above them, but if there are two or three warbler's eggs in the nest before the alien egg appears, the warbler may feel obliged to incubate and hatch out the stranger, with the usual results of her own young being crowded out and lost. Two or more cowbird's eggs are almost sure to be deserted or buried. But the cowbird is very persistent and keeps on laying, as successive stories are added to the nest by the energetic and persevering warblers. Two-story nests are very common, and as many as three, four, five, and six stories have been recorded. Mr. Forbush (1929) was told by Dr. H. F. Perkins "of one case where a six-storied nest was built, with a cowbird's egg in every one." Mr. Du Bois tells me of a new nest he found in a low bush, with another nest, about half completed and only about a foot below it, containing a fresh, cold cowbird's egg. Out of 43 nests found by Dr. George M. Sutton (1928) in Pymatuning Swamp, Pa., "a Cowbird egg was found in only one nest. This is most unusual, but is due, as elsewhere stated, to the protection against these parasites afforded by the Red-winged Blackbirds which would not tolerate a Cowbird anywhere about the marshes." Snakes sometimes destroy the young, as related above; squirrels, blue jays, and other predatory mammals and birds rob the nests; and the adults must always be on the alert to escape the many enemies that prey on all small birds. Harold S. Peters (1936) records only one louse, _Philopterus subflavescens_ (Geof.), as an external parasite on the eastern yellow warbler. _Fall._--The striking feature of the fall migration of the eastern yellow warbler is its earliness. The birds begin to move away from their nesting haunts as soon as the young are able to take care of themselves, and the southward migration is well under way before midsummer. Smith (1943) says that, in Vermont, "during many seasons, the species is not seen later than July. Departure dates for local summer residents range from July 15 to the 30th. Later records occur between August 18 and September 9th." These later records are probably for birds from farther north. There seems to be a wide spread between the times that the earliest and latest birds leave.