In 1928–1929 Rollo Beck discovered in New Guinea a spectacular new species of bowerbird, Sericulus bakeri, which according to his specimen labels he collected near the previously well-studied lowland town and former colonial capital of Madang. That seemed so implausible that suspicions arose that Beck had intentionally falsified the locality—especially when it eventually turned out that the new bowerbird is instead confined to the nearby Adelbert Mountains. Beck made this discovery in the course of amassing large collections in northeast New Guinea that, in fact, have never been published as a whole, although Ernst Mayr (1941) in his List of New Guinea Birds included some of Beck's records. Much doubt has remained about Beck’s collecting localities. Hence we have now reconstructed Beck’s itinerary on the basis of his field diary and specimen register; the letter by his wife who accompanied him; a spreadsheet of his cataloged specimens in the American Museum of Natural History (AMNH); and correspondence, records, and photographs archived at the AMNH. We show that Beck collected at 10 sites grouped into three areas: the foothills of the Adelbert Mountains and adjacent lowlands, the westernmost foothills of the Huon Peninsula, and the Cromwell Mountains in the east of the Huon Peninsula. We assemble a table listing all species that Beck collected at each of the 10 sites. For each site, we discuss the upland species, open-country species, and other groups of species collected there. Those results illuminate the upland avifaunas of the Adelbert Mountains and the Huon Peninsula, range borders in Northeast New Guinea, and a possible Massenerhebung effect in the Cromwell Mountains. It is clear that Beck's labeling of his Sericulus bakeri specimens as collected at Madang was not done with intent to mislead, but is instead readily understandable from Beck's previous collecting experiences and his preparation for his New Guinea trip.