park

Today when I went down to the park, it was cloudy, a bit wet and windy. When I was out on the breakwater, I saw some seagulls. When I was coming back from the end of the breakwater, I saw some sort of seashell wedged into one of the cracks of the board walk. When I was in the park, I saw a Varied thrush on the side of the trail that flew away when I walked by. When I was out by the battle site bench, I saw some sea gulls. When I was on the trail by the river, I saw some ducks on the other side of the river, and two ducks and one Gadwall flew from the side of the river I was on to the other side of the river. When I was on my way back home on the seawalk, I saw a bunch of shorebirds, some sea gulls, and one Black oystercatcher on sage rock. I watched the the shorebirds and black oystercater for a bit, then I walked back home.
How did that seashell get wedged into the crack of the boardwalk?
Why was that black oystercatcher on sage rock?
Why were there no other black oystercaters with the other black oystercatcher on sage rock?