Robert Dunn first arrived in Shetland on March 21, 1831, and stayed for more than seven months. He returned in 1835. He spent seven months in Orkney in 1832 and another nine months in 1833. From these journeys he produced The Ornithologist's Guide To The Islands of Orkney and Shetland, published in 1837.
A notice appearing in the Magazine of Natural History in 1836 said: "The author has been in the habit, for a series of years, of making excursions to the Orkney and Shetland Isles, for the purpose of procuring specimens of natural objects; and, having sustained considerable trouble and inconvenience, and great expense, from the want of some guide, by which to regulate his movements, he proposes to publish the Guide, in order to prevent the like results to others. The author does not affect to publish a scientific work on the subject, but trusts he shall be able to furnish much information, of a nature not only interesting to the ornithologist and the general reader, but to the traveller to these islands."
Dunn's guide on Google books.