Breeding is often a dangerous activity for many animals, requiring a great deal of energy and sometimes an increased exposure to predation. In contrast, animals that have reached breeding age have more experience, and they may be more able than their less experienced relatives to escape predation as a result. Greisser et al. (2017) studied populations of the Siberian jay, a bird which lives in small family groups in northern Eurasia. They followed 586 individually tagged birds over several weeks and recorded deaths due to predation. The following figure shows their estimates of the survival probabilities of both breeders and nonbreeders in this population. Time is measured as number of weeks since each bird was tagged.

A comparison of the survival curves of breeders relative to nonbreeders yielded the following results:

a. Based on this plot, which group has a higher mortality rate, breeders or nonbreeders?

b. Based on this plot, which group has a higher survival rate, breeders or nonbreeders?

c. What statistical test can yield the output shown above?

d. Interpret the results based on the figure and the output. What is your conclusion?

e. What assumptions are made by this statistical test?

f. In order to evaluate one of the assumptions of this test, we generated the following plot. What assumption is assessed by looking at such a graph? Is there cause for concern about the suitability of our test in this case?