THE EGG 
77 
this kind of parasitism. The former lays its egg in some convenient 
place and, taking it in its bill, inserts it into the foreign nest; but the 
Cowbird, as far as is known, lays its egg in the nest selected. 
Of the nearly one hundred species of North American birds in whose 
nests the egg of the Cowbird has been found, one fourth are Warblers, 
indicating how often the Cowbird chooses as a foster parent a bird 
smaller than itself. The Warbler may nest upon the ground, or eighty 
feet above it, and still be preyed upon by the Cowbird, which never 
having had a home of its own has formed no attachment for any particu- 
lar site. Many theories have been advanced to account for the lack of 
the nest-building instinct in the Cowbird, but none is conclusive (see, 
however, Herrick 7 11). In any consideration of the subject, however, it 
should be remembered that some of our Cowbird’s seven tropical Amer- 
ican relatives have advanced toward the prevalent custom of monogamy, 
Molothrus badius of Argentina always incubating its own eggs and 
sometimes building a nest. (See Bendire, “The Cowbirds,” Rep. U. S. 
N. M. for 1893, pp. 587-624.) 
The Egg 
Number Laid . — The number of eggs comprising a full ‘set’ or 
'clutch 7 ranges from one to as many as twenty. No law governing 
this number is known, though birds of temperate zones usually lay 
more eggs than their representatives in the tropics. Our northern 
Terns (Common, Roseate, Arctic), for example, usually lay three eggs, 
while southern Terns (Noddy, Sooty, Bridled) lay but one. Our 
Thrushes lay normally four eggs, but tropical Thrushes lay only two or 
three. 
Generally speaking, birds of the same family lay approximately 
the same number of eggs, but there is much variation between birds 
of closely related families (e. g. Loons and Grebes, Thrushes and Wrens), 
while birds of similar nesting-site may not lay the same number of 
eggs ( e . g. Bob-white and Meadowlark). 
The young of praBcocial birds require less care than do those which 
are wholly dependent on their parents, and among Grouse, Quail, and 
Ducks we find the largest families. But, on the other hand, the eminently 
prsecocial Snipe and Plover have but four young. These birds, however, 
are too small to cover more than four of their disproportionately large 
eggs, while the gallinaceous birds can readily cover a dozen or more. 
Therefore, size of the parent, as well as condition of the young at birth, 
is here a governing factor. 
The number of eggs laid is no indication of the fecundity of the 
bird. At the time of laying, the ovary contains a large number of partly 
formed eggs, of which, normally, only the required number will become 
fully developed. But if the nest be robbed, the stolen eggs will 
frequently be replaced. The oft-cited case of the Flicker which laid 
seventy-one eggs in seventy-three days is in point, while the long con- 
