Her mate figuring out how incubation works when left in charge of the egg for the first time. It seems like this might be his first nesting season, but he did manage to shuffle on top of it eventually and keep it guarded while Nuppu was away.
At nest#5/LS Sara already laid her first egg, too, but her equally inexperienced mate (banded as a nestling in 2019. Ospreys reach sexual maturity at 3-4 years old, so this is likely his first nesting season as well) managed to kick it over the rails while arranging the nest material. Well, it happens. Learning to be a parent is hard enough with guide books for beginners, let alone without.
ETA: Sara with her second egg that she just laid this morning:















