If you carefully keep your bird records by county, be sure to use the specific hotspots at this reservoir so that the birds will be assigned to the proper county. The northern part of the lake is in Berkshire County and the southern part is in Hampden County.

Otis Reservoir was originally formed across three ponds in 1865, by a dam across the Fall River, a tributary of the Farmington River created by the Farmington River Power Company. It was used for water storage, ensuring a steady supply of water power for mills along the Farmington River. In the early part of the twentieth century, the rise of the automobile led to the development of many seasonal cottages along the lakeshore. The building took place along most of the lakeshore and on many inland lots. These cottages were fairly inexpensive, being purchased by middle-class people: bankers, barbers, tradesmen, construction contractors, and others. A trend that accelerated in the 1980s was for wealthy people to purchase cottages and upgrade or replace them with expensive second homes. This trend continues. Many people from Connecticut and New York use these as weekend or vacation homes. In addition, many have converted their cottages into year-round residences.

This huge, 1,200-acre reservoir is a study in contrasts. While it is large, it is also relatively shallow, with a maximum depth of 48 feet and an average depth of about 15 feet. The southern third of the Tolland State Forest controls a third of the shoreline (primarily in the southern basin), yet the remainder is heavily developed with seasonal and permanent homes. One is likely to see canoes and cabin cruisers here on the same day. Aquatic vegetation is scarce, limited mostly to pond lilies growing in scattered clusters in the coves.