108 U. S. NATIONAL MUSEUM BULLETIN 211 



Major Bendire (1895) writes: 



The eggs of the Yellow-headed Blackbird vary in shape from ovate to elliptical 

 and elongate ovate; the shell is finely granulated, strong, and rather glossy. The 

 ground color varies from grayish white to pale greenish white, and this is pro- 

 fusely and pretty evenly blotched and speckled over the entire surface with 

 different shades of browns, cinnamon rufous, ecru drab, and pearl gray. The 

 markings are usually heaviest about the larger end of the egg, and sometimes a 

 specimen is met with which shows a few fine, hair-like tracings, like those found 

 on the eggs of the Orioles. 



The average measurement of 134 eggs in the United States National Museum 

 collection is 25.83 by 17.92 millimetres, or about 1.02 by 0.71 inches. The largest 

 egg in the series measures 28.96 by 19.81 millimetres, or i.14 by 0.78 inches; the 

 smallest, 23.11 by 17.53 millimetres, or 0.91 by 0.69 inch. 



Incubation. — Incubation is performed entirely by the female with 

 no help from the male, except that he sometimes feeds her on the nest. 

 The period of incubation has been reported by different observers 

 within rather wide limits. 



Roberts (1909) says that in "seventeen nests the period of incuba- 

 tion, inclusive of the day on which the last egg was laid, to the day on 

 which the first egg hatched, was nine days in one instance, ten days in 

 twelve, eleven days in three, and twelve days in one. Thus ten days 

 may be considered the usual period of incubation. The nine-day 

 period was in the case of the only set of five eggs that hatched." The 

 eggs hatched irregularly, though in three nests all hatched on the 

 same day, and in three others one hatched each day. 



Reed W. Fautin (1941b), who made some very extensive studies of 

 the nesting of the yellow-headed blackbird in Utah, writes: 



The females were not assisted by the males in any way in the incubation of the 

 eggs, 56.6 percent of them beginning incubation at the time the second egg was 

 laid, with a tendency for the beginning of incubation to be delayed longer the 

 larger the clutch. The length of the incubation period varied from 12 to 13 days, 

 74.6 percent of the eggs hatching in 12 days. 



The attentive periods during incubation ranged in length from 1 to 41 minutes, 

 with an average of 9.1 minutes. These periods were longest during mid-day 

 when the females were seemingly protecting the eggs from the sun. During 

 83 hours of observation the females spent an average of 63.9 percent of their time 

 on the nest, with a range from 53.1 to 69 percent. 



The inattentive periods ranged in length from 1 to 18 minutes, with an average 

 of 5.4 minutes. These periods tended to be longest during the morning and 

 evening hours when feeding was most intensive. 



The hatching success of the larger Provo River colony amounted to 75.7 

 percent, while that of the smaller Lakeview colony was only 60.6 percent, gi\ ing 

 an average of 70.9 percent for the two. Wind and predation were responsible 

 for the destruction of 90 (20.3 percent) of the eggs before the time of hatching, 

 and 39 (8.8 percent) failed to hatch because of being addled or infertile. 



Eighty- three females nested in the Provo River colony and 40 in the 

 Lakeview colony. There were about 35 males in the former colony 

 and only 12 in the latter, suggesting promiscuity or polygamy. There 



