I've encountered this first hand.
Four years back I was returning from doing some fishing. Passed a loon on the surface that wasn't doin' too hot. Seem very lethargic. The next day I went back out and at the boat ramp there was the loon. Washed up. Barely movin'. I tried petting it, but it really didn't want anything to do with me. I went into the bait bucket and started grabbing some minnows. I was there for about 45m feeding it the fish. It started gaining energy and looked like it was doing better. A guy from the loon center arrived in a car. Picked the loon up and placed it in a cardboard box. I rode with him to Winterhaven Vet with the loon in my lap. Arrived there and Ms. Wing took an x-ray. Clear as day there was a splitshot in the gizzard. They had to put it down right there.
Between Dad and I, we must have 20 tackleboxes. We went through every one and ditched every weight in there; regardless if they were lead or not. We bought all new weights because we didn't want to chance it.