From page 289 of Waterfowl of North America, Europe & Asia:
Trumpeter Swans prefer freshwater wetlands for breeding, which usually occurs between mid-May and late June. They typically choose habitats that include lakes and shallow ponds that are large enough for take off, with plenty of submerged vegetation and water that is neither too acidic nor eutrophic.
Waterfowl of North America, Europe & Asia: An Identification Guide
This is the ultimate guide for anyone who wants to identify the ducks, geese, and swans of North America, Europe, and Asia. With 72 stunning color plates (that include more than 920 drawings), over 650 superb photos, and in-depth descriptions, this book brings together the most current information on 84 species of Eurasian and North American waterfowl, and on more than 100 hybrids. The guide delves into taxonomy, identification features, determination of age and sex, geographic variations, measurements, voice, molt, and hybridization. In addition, the status of each species is treated with up-to-date details on distribution, population size, habitats, and life cycle. Color plates and photos are accompanied by informative captions and 85 distribution maps are also provided. Taken together, this is an unrivaled, must-have reference for any birder with an interest in the world’s waterfowl.
• A guide to the 84 species of ducks, geese, and swans of Europe, Asia, and North America
• 72 color plates with more than 920 illustrations of most plumages and subspecies, both in flight and standing
• More than 650 color photos
• Details on taxonomy, identification features, determination of age and sex, geographic variations, measurements, voice, molt, hybridization, habitat and life cycle, range and populations, and status in captivity
• 85 distribution maps
• Descriptions and illustrations of more than 100 hybrids regularly encountered in the wild