Mr. Dunne, who, naturally,

Mr. Dunne, who, naturally, is working on his own bird guide, said birders did not see the guides as competing because they bought everything, and more than one copy. He cited an informal survey he did recently of him and two other birders while they were out in the field. How many Sibley's did each of them own, he wondered. The answers were five, two and five.
The New York Times: Vital Gear for Birders: A Good Book. "The Sibley Guide to Birds [is] a 544-page work that now has 700,000 copies in print."
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