EASTERN BROWN-HEADED COWBIRD 439 



The average measurement of 127 eggs in the U. S. National Museum 

 collection is 21.45 by 16.42 millimetres, or 0.84 by 0.65 inch; the 

 largest egg measures 25.40 by 16.76 millimetres, or 1 by 0.66 inch; 

 the smallest, 18.03 by 15.49 millimetres, or 0.71 by 0.61 inch. 



Incubation. — Friedmann (1929) says that the incubation period 

 of the cowbird "is ten days, about the shortest of any of our passerine 

 birds." But Mrs. Nice (1939) says: "On Interpont with the Song 

 Sparrow as host the Cowbird egg has never hatched in ten days. 

 Sometimes it hatches in eleven days, sometimes in twelve, and 

 occasionally in thirteen or even fourteen days. It requires about 

 one day less of incubation than the Song Sparrow egg, hence it normally 

 hatches first and the bird gets an advantage from the start. Some 

 eggs have been laid after incubation has started; these have hatched 

 from one to five days later than the Song Sparrows, and most of the 

 little birds perished." 



Hervey Brackbill gives me the following personal observation on the 

 subject: "The incubation period that I observed for one egg at Balti- 

 more was about 11^ days. This egg was laid on May 18 before 8:47 

 a. m. (studies by other investigators indicate that laying is usually 

 done at about 5 a.m.), and after steady incubation by a Wood Thrush 

 hatched on May 29 at 1 : 25 p.m. ; that is, at that hour I found the bird 

 enclosed in only half of the shell and, when I touched it, it wriggled 

 free of that. The incubation period of the thrush's own eggs was 

 12 to 13 days." 



Young. — It will be seen from the above that the young cowbird 

 usually hatches at least 1 day ahead of the young of its foster parents. 

 It does not, apparently, make any effort to oust its nest mates, as the 

 European cuckoo does, but it is so much larger than the young of the 

 smaller foster parents, and grows so much faster, that some of the 

 smaller young are often crowded out of the nest; also, it gets more than 

 its share of the food brought to the nest, a condition that sometimes 

 proves fatal to its nest mates. Mrs. Nice (1939) says on this subject: 



Many writers assert that each Cowbird is raised at the expense of a brood of 

 young. This is not true with Song Sparrows. Sixty-six successful nests without 

 Cowbirds on Interpont raised an average of 3.4 Song Sparrows, while twenty- 

 eight successful nests with Cowbirds averaged 2.4 Song Sparrows. So, taken 

 by and large, each Cowbird was reared at the expense of one Song Sparrow. * * * 



Song Sparrows often raise all of their own young that hatch along with a pen- 

 sioner, anywhere from one to five Song Sparrows having been fledged in such 

 nests. With two Cowbirds of like age in the nest, the Song Sparrows have been 

 able to bring up only one or two of their own children. Smaller birds undoubtedly 

 suffer more than do Song Sparrows, but there is little information on this subject. 

 Once I found a nest containing three young Maryland Yellowthroats and a Cow- 

 bird just ready to leave. 



880928—57 29 



