The webcam shows a Eurasian reed warbler nest. In that nest, last week a female cuckoo laid an egg. If all will go as expected, then this week the young cuckoo will hatch. Female cuckoos time laying their eggs in other birds’ nests so that the cuckoo egg will hatch just before the other eggs. As a cuckoo hatchling can push reed warbler eggs out of the nest, but has a much bigger problem doing that to reed warbler hatchlings.
You can watch the webcam, where the reed warbler now sits on one reed warbler egg and one cuckoo egg, here.