Location: Chapter 13, discussion after Problem 8
It is (second paragraph)
In each of the problems 1, 2, 3, 4, we derived a sage bird from three proper combinators: one was irregular, the mockingbird, and the other two were regular.
It should become
In each of the problems 2, 3, 4, 6 we derived a sage bird from three proper combinators, one irregular (either \(M\) or \(Q\)) and the other two regular.
In Problem 1 a Sage bird is constructed using the Mockingbird, the Robin and the Bluebird: of these, two are irregular (\(M\) and \(R\)) and only one is regular, \(B\). Hence, the problem list given in the text is to be revised.
One can include Problem 6, where a Sage bird is derived using the Queer bird (irregular), the Lark and the Warbler (both regular): but then, the explicit mention of the Mockingbird has to be expanded, since the irregular bird used here is another one.