326 U. S. NATIONAL MUSEUM BULLETIN 211 



Among the biological factors which accounted for 86.07 percent of 

 the mortality, predation was preeminent. Those predators that were 

 "persistent bird and egg feeders" were: Scrub jay (Aphelocoma coeru- 

 lescens), American magpie {Pica pica), crow (Corvus brachyrhynchos) , 

 bridled weasel (Mustela jrenata) , western ringtail (Bassariscus astutus), 

 desert bullsnake (Pituophis catenifer), striped racer (Coluber taeniatus), 

 and blue racer (Coluber constrictor). Those predators that could only 

 be "classed as occasional opportunists" were: Sharp-shinned hawk 

 (Accipiter striatus), horned owl (Bubo virginianus) , Steller's jay (Cyano- 

 citta stelleri) , Piute ground squirrel (Citellus mollis), Beechey ground 

 squirrel (Citellus grammurus), Sierra mantled ground squirrel (Callo- 

 spermophilus chrysodeirus) , Douglas chickaree (Sciurus douglasi), 

 striped skunk (Mephitis mephitis), and Great Basin rattlesnake 

 (Crotalus viridis). 



"Protective factors" included the floral elements; some types of 

 plants used afforded better nest protection than others. Also the 

 height of the nest emplacement was important; 57.94 percent of the 

 nests were placed 5 feet or less above the ground, 32.71 percent from 

 5 to 10 feet, and 9.34 percent above 10 feet. It was found that 

 "mortality progressively increased" from the highest to the lowest. 

 Still another protective factor was the tactics of parent birds in driving 

 off predators. However, in some cases, La Rivers believes, "noisy, 

 quarrelsome, conspicuous birds" attracted predators to their nests. 



Bond (1939) in an examination of the remains of prey items from 

 beneath five nesting sites of the prairie falcon (Falco mexicanus) in 

 the region of the Lava Beds National Monument, Calif., found two 

 Brewer's blackbirds at one eyrie, none at the other four. He found 

 remains of eight Brewer's blackbirds beneath the nesting site of a duck 

 hawk (Falco peregrinus). A large number of pellets of the horned 

 owl and barn owl (Tylo alba) were also collected by this author from 

 beneath roosts in the same region. Two collections were made, one 

 on August 12, 1937, at which time only those pellets "were taken that 

 seemed, on the basis of state of preservation, to have been cast later 

 than the preceding winter." At the time of the second collection 

 November 5, 1937, only those were taken which seemed certainly to 

 have been deposited since the August collection. In the first collection 

 the remains of 12 Brewer's blackbirds were found among a total of 

 3,391 items of bird, reptile, mammal and insect remains, which 

 amounted to 0.0035 percent of the total. In the second collection 

 two Brewer's blackbirds were found out of a total of 994 bird and mam- 

 mal remains, or 0.002 percent. Considering the effect of predation 

 on birds by both hawks and owls in this area, Bond states that the 

 Brewer's blackbird was among the five species of birds most often 

 killed by hawks. In the combined owl pellet collections the total of 



