In Guns, Germs, and Steel, Diamond (or his editors) made the decision not to use footnotes, which, given the wide variety of secondary material he is referencing, may be problematic in the minds of some readers. However, he lists his sources in the "further readings" section beginning on page 429 of the paperback edition. Essentially, this section is broken down into a series of bibliographical essays, arranged by chapter. Within the main text of the book itself, fe specifically names many of his primary sources, most notably in the extended accounts of the Spanish conquest of the Inca by several of Pizarro's companions. He also names many classical theories related to the subjects he pursues, such as the work of Charles Darwin, Gregor Mendel, or Thomas Malthus. By and large, however, the modern scientists and historians that he draws upon for his argument are relegated to the back of the book. While this goes against academic convention, it is fairly common in books that are intended for popular consumption, as many readers find footnotes, or even endnotes, unduly distracting. Diamond never passes off anyone else's argument as his own, and in fact one of the most useful aspects of the book is the fact that it exposes general readers to up-to-date scientific and historical thought.
Source: Diamond, Guns, Germs, and Steel, 429-457.