An unusual situation

Our osprey pair successfully raised three young this season, a first for them.  We have watched them come along; being fed and growing in a fever pitch, covered in the rain and sun by the adults, trying out the wings and flying to our boathouse on those first flights.  They are trying out their water skills, plopping down in the drink and pulling themselves out and shaking off the excess.  Three young birds provide plenty to watch.
By this time in the progression, Mama osprey is bringing fish and feeding just a bit.  Papa has all but disappeared.  The young are big enough to eat a fish on their own.
The unusual situation?  Yesterday a fourth young osprey appeared on the nest.  Was it lost, abandoned, lonely?  In any case it was fairly well accepted by the younger crew.  Mama had a problem when she came around with a fish.  She screamed and screamed ... like something wasn't right.  But the fourth did not leave.
This morning the fourth was gone but by  five p.m. today it was back.  We watched as Mama delivered a fish like nothing was wrong.  Here she is coming in and the four young waiting.
The fish was given to one who soon lost it, too young to handle one so large and still kicking.  The crew was suddenly depressed and Mama flew off to fish again.
Then we got to see one of the young fly off the nest into the water and pull up a pumpkinseed ... what a   sight!  It got to eat that little fish in peace on its own away from the others.
But the fourth osprey, what a puzzle.