Most serious of these problems
was the fact that many of the nests in my sample had not been found until
after incubation had begun. That others did not mention such difficulties
was not reassuring but rather aroused the suspicion that some of their findings
might not be so exact as the cold finality of their figures seemed to imply.
Consequently, I proposed a new way of analyzing data of this kind (Mayfield,
1960. The Kirtland’s Warbler. Cranbrook Inst. of Sci., Bloomfield Hills,
pp. 182-209). There, however, the method was incidental to the results, and
it was complicated at every turn by the effect of Brown-headed Cowbirds
(Molothrus ater) in the nests with the warblers. So I am offering here a simplified
explanation for the benefit of field workers with little training in mathematics