Mostly, terns put three eggs. But it is normal that not all females lay three eggs. It always depends on how fit the females are. In years with less juvenile fish fewer eggs are laid. The females need a lot of energy for the egg production. There seems to be enough fish this year, as we even found two clutches with four eggs! One had five eggs. But they are probably not from one female. Probably another female accidentally put her eggs in the wrong nest. In the photo you can see that the lower two eggs have a paler colour.