This question totally depends on the species of swan. For instance, a Mute Swan eats approximately 8 lbs of vegetation daily while the larger Trumpeter Swan eats approximately 20 lbs of vegetation daily.

Defecation would depend on food resources at any time of the year as well as species and size of bird and types of food eaten. Also, sometimes, bird defecation is somewhat watery. So, this is a more difficult amount to accurately determine without staying with a swan for a day and measuring its output. The Regal Swan