Imagine going on holiday but leaving someone behind.
That's exactly what happened to a family of swans from Iceland!
Every year large groups of Whooper swans leave Iceland for the winter and fly to the UK where it's a little warmer.
Thousands of them arrive at Martin Mere wetland centre.
Wildlife experts count the swans when they arrive to make sure they've all arrived safely.
They check identification rings on the swans' feet to see if they all survived the trip.
This year they noticed a young swan was missing.
The missing swan was nicknamed Wally and sparked an international search.
Now the mystery has been solved - he was spotted by a birdwatcher in Ireland.