MORTALITY STUDIES IN R. I. REDS 17 



from none to 50 percent among the different groups,, indicating that many of 

 the birds must have died of functional disturbances that left no significant gross 

 evidence. 



The percentages of dead birds examined during the period were variable. It 

 is believed, however, that the records give a reasonable approximation of the 

 incidence of diseases and disorders that were present. One fact that stands out 

 is that when the avian leukosis complex is present there is likely to be a relatively 

 high incidence of other diseases and disorders and consequently a high total 

 mortality. 



Relation of the Avian Leukosis Complex to Total Mortality in Females 



All of the data on the incidence of the various diseases and disorders in the 

 flock from 1937 to 1942 have been based on that part of the flock that was sent 

 in for necropsy. These data have shown the relative importance of the different 

 diseases and disorders in the flock, but not the relation of any particular disease 

 to the total mortality rate. 



Chart 4 is therefore presented to give this information for the avian leukosis 

 complex. The chart shows total mortality from all causes for females for one 

 full laying year (6 to 18 months of age), the percentage of birds examined and 

 the percentage of the total population which exhibited the avian leukosis complex 

 over an eleven-year period. Birds developing the leukosis complex before being 

 placed in the laying house are not included. The average number of pullets 

 housed each year was 732. 



The flock as a whole did not show a high incidence of the avian leukosis complex. 

 The maximum incidence was 9 percent in 1933, with an average of 3.40 percent 

 for the eleven-year period from 1933 to 1943. For the period 1937 to 1942, 

 included in the previous sections, the mortality rate from the avian leukosis 

 complex was 2.40 percent. Total mortality over the period was highly variable. 



There is some evidence to indicate that when the leukosis complex was at a 

 high level in the birds necropsied, there was generally a high total mortality in 

 the flock. The data also suggest that periods of from three to five years may 

 intervene between generations showing a high incidence of the complex. There 

 is no evidence that the avian leukosis complex is increasing in importance in 

 this flock. 



