NORTHERN AND MARYLAND YELLOWTHROATS 547 



marshes had a foundation of cattail shreds, dried leaves, and grass 

 stems and were lined with fine grasses only. 



The nest is cup-shaped but in some instances loosely attached mate- 

 rial extends above the main rim and may partially roof over the top 

 of the structure. The average measurements of several typical nests, 

 not including protruding materials, are outside diameter 314 inches, 

 outside depth 3% inches, inside diameter of the cup 1% inches, and its 

 depth 1% inches. 



All nests that I have seen were completed when found, and I have 

 no information as to the time required or the manner in which the 

 nest is built. The female apparently builds the nest, but on one 

 occasion I saw a male bird carrying nesting material and it is probable 

 that he sometimes assists his mate in its construction. 



Eggs. — The usual set of the northern yellowthroat is four eggs 

 but complete sets vary from three to five, and as many as six eggs have 

 been reported. The eggs have a ground color of white or creamy 

 white specked chiefly at the larger end with reddish brown, umber 

 and black and with shell markings of stone gray. As in any large 

 series of eggs of a species there is more or less variation from the 

 typical. In some eggs the markings are in a distinct wreath near 

 the larger end, in a few, some of the marks are in the form of small 

 streaks and in still others the marks are faint and much reduced. The 

 average size of two sets of four eggs each is .69 by .52 inch. 



[Author's Note: The measurements of 50 eggs of the northern 

 yellowthroat average 17.5 by 13.3 millimeters; the eggs showing the 

 four extremes measure 19.4 by 14.5, and 14.2 by 11.7 millimeters. The 

 measurements of 26 eggs of the Maryland yellowthroat average 16.7 

 by 13.2 millimeters ; the eggs showing the four extremes measure 18.3 

 by 13.8, 17.5 by 14.0, and 15.4 by 12.1 millimeters.] 



Incubation. — The incubation of the eggs requires 12 days and is 

 performed entirely by the female. The male sings throughout the 

 incubation period and is ever alert in defending his territory. He 

 sometimes delivers foods to the female while she is incubating the eggs. 



H. Mousley (1917) has found that although the eggs of different 

 individual northern yellowthroats are subject to great variation, the 

 successive sets of any one bird are strikingly alike in shape, size, and 

 markings. Mr. Mousley did not succeed in getting the yellowthroat 

 to lay a third set of eggs after the first two sets had been taken. It 

 is doubtful if the yellowthroat rears more than two broods a year, 

 although Aretas A. Saunders (1938) believes that some of them do, 

 since he has found the last birds leaving the nest in August. Usually 

 an ^gg is laid each day until the set is completed but L, H. Porter 

 (1908) reports finding a nest on June 4 in which the set of four eggs 



