Lev Frid and Stephanie Pereira reported a possible Cassin's Vireo from the Marshlands Trail in Simcoe County this morning at 7:45 am May 23. They report "It is a very drab "solitary" vireo with no yellow vent strap, a small amount of yellow only in the armpit, low contrast between the back and head/body, a very dark lore, and only a white bar on the outer rectrix rather than it being entirely white. However the song is what drew our attention to it. We originally thought it was a Yellow-throated Vireo when we heard it. It is very burry and unlike Blue-headed, and bang on for Cassin's. It was and still is singing at the silver maple swamp when the trail opens up. From the trail entrance (https://maps.app.goo.gl/eNopNyYv4q3Ucjrc9), walk 200 m or so down to just where the boardwalk starts in a flooded silver maple area. The trail is mostly a decrepit boardwalk so be very careful if you go. Parking is roadside."