MORTALITY STUDIES IN R. I. REDS If 



houses according to age. No culling was practiced and all were trapnested as 

 long as retained. 



Mortality rates for the adult period are reported by generations in table IC 

 and presented graphically in chart 3. 



Males consistently showed a higher death rate than females in all four lines. 

 There is no question but that the male death rate was greatly increased by 

 injury from fighting. This will be further considered in the next section. 



Adult mortality rates in the four lines were tested for significant changes by 

 the method of Van Uven (Pearl, 1940). The control line had a calculated mor- 

 tality rate of 23.98 percent in 1937 and 36.76 percent in 1942. The rate of change 

 as measured by the slope was 2.13 + 1.217. An increase in mortality rate was, 

 therefore, only apparent since the slope was not much greater than its standard 

 error. The high mortality line fluctuated very widely through the six-year 

 period. The slope of its mortality rate line was 1.96 + 2.038 percent. It seems 

 logical to assume, therefore, that no significant change in mortality rate occurred 

 during the period of observation. The mortality rate in the low line remained 

 rather stable through the six-year period. There was a slight downward trend 

 which proved to be insignificant. The slope was —.61+1.077. The miscel- 

 laneous group of birds exhibited a rather definite increase in mortality rate dur- 

 ing the experimental period. The initial calculated rate was 24.01 percent and 

 the final calculated rate was 39.25 percent. The slope of this line was 2.54 + .827 

 which is clearly a significant slope. 



In general, when the mortality rates are considered for the combined sexes, 

 no significant changes were observed in the control, high, or low lines; but the 

 miscellaneous group showed a significant increase in the six-year period. These 

 data fail to indicate that selective breeding as carried on was effective in reducing 

 deaths from all causes between the ages of 6 and 18 months. 



Comparison of Changes in Mortality Rates 



A study and comparison of the rates of change in mortality between the ages 

 of 6 and 18 months is important. When the high and low mortality lines were 

 compared in rate of change the difference was 1.96 — ( — .61) or 2.57 + 2.31. 

 This difference is just above the standard error of difference and is not significant. 

 A comparison of the high mortality line with the control line shewed a difference 

 of 2.13 — 1.96 or .17+2.37. There was no evidence of any difference here. The 

 low mortality line and the control line were next tested for difference, which 

 proved to be 2.13 — ( — .61) or 2.74 + 1.63. Here again there was no significant 

 difference between the two lines with respect to rate of change in mortality rate. 



These facts further point to no significant differences between the high, low 

 and control lines in mortality rate between the ages of 6 and 18 months. 



CAUSES OF DEATH 



Sick or dead birds were sent to the Department of Veterinary Science for 

 necropsy. It is believed that the birds examined represent a fair cross section 

 of the different lines. The majority of diagnoses were made on gross autopsy 

 records and an attempt has been made to classify the birds examined with re- 

 spect to the diseases or disorders observed. Obviously some individuals showed 

 more than one disorder so that when the percentages of birds showing the differ- 

 ent diseases or disorders are added for a particular line, the sum may exceed 100. 

 Birds younger than 8 weeks were not examined. 



