25 May. Fall migrants arrive 20-25 September. Peak abundance occurs 1-20 October and departure by 10 November. During peak fall migration, Franklin's gulls are commonly found in association with ring-billed gulls on freshly plowed agricultural fields in western St. Croix County. Franklin's gull is virtually absent east of R. 17 W. in St. Croix and Polk counties. Largely restricted to semipermanently and permanently flooded wetlands.
Extensive use is made of agricultural fields during fall migration.
Uncommon migrant throughout the Valley, fairly common on the prairie wetlands of St. Croix and Washington counties. 25 May. Fall migrants arrive about 10 September. Peak abundance occurs 20 September to 1 October and birds depart by 25 October. During the fall, Bonaparte's gulls are most commonly found along the St. Croix River, becoming rare to absent elsewhere. Regular migrant and casual summer resident.