Some Brown-throated Martins were nesting near the No 4 fishing site. Their nests are little holes in the sand bank.
Before going home in the evening, I have to check that everyone is gone so that I can lock the gates. I use this opportunity to check for fishing line and other debris that can harm the animals or birds, left behind by the day visitors, before locking the area for the night.
The martins were flying overhead and I felt something was amiss. As I surveyed the nest holes, I noticed that they were blocked by fresh mud. I opened the holes, as I suspected the children blocked them to trap the birds.
The next day the birds were using their nesting tunnels again.
Here is a link to some photographs sent to Google under the label of Brown-throated Martins. Note the Sand Martins have a white throat.