The single osprey nestling is now 28 days old--slightly more than halfway to its first flight which typically occurs around 52 days of age. The nestling has become much more active. It can be seen standing, moving about the nest and flapping. Mom is doing very little brooding as the youngster now is able to thermoregulate. The father is still doing the vast majority of hunting for the family while the mother stays in close contact with the nest---more for protection at this point than for warmth.
The downy feathers that covered the nestlings body earlier is now being replaces by a solid layer of body feathers. In the coming days the nestling will begin to appear darker and darker against the background of the nest. Also the primary, secondary and tail feathers are partially grown in at this point. They are in a stage referred to as "blood feathers." When the feathers are growing the shafts are full of blood." the feather needs this blood supply in order to grow. Once they are fully formed the feather follicle closes cutting off the blood supply and the feather shaft hardens.
The feathers are pretty fascinating---if the bird is under stress or nutritionally deprived during the period when the feather it is growing, you will actually see stress marks on the feather--bands where it is more poorly formed and susceptible to breakage. Nestlings grow all their flight feathers simultaneously meaning that a period of stress will potentially weakening the entire set. Later on in life, the feathers will be replaced sequentially meaning that a period of stress may affect only a few feathers rather than an entire set. If you watch closely, you can see the youngster dropping its wings down when it is standing. That is because the feathers are full of blood and really heavy. It also has not yet built up its wing and chest muscles.
In the coming days watch for a lot more movement in the nest including more flapping and walking about by the nestling, as well as more food deliveries from the father. To give you a sense of how much fish they are consuming Heney et al report that a pair of osprey raising two young will typically go through upwards of 375 pounds of fish in a nesting season.