Asynchrony

Several people have asked why the birds are still copulating after they have already started incubating. Ospreys begin incubating as soon as they lay the first egg. They continue to copulate to fertilize subsequent eggs. Eggs take about 72 hours to be formed, fertilized and pushed out, so they lay the eggs about one to  three days apart. (They typically lay 2-3 eggs. Occasionally there are four.) The chicks will also hatch one to three days apart, so one chick may be as much as six days older than the youngest.  This gives that first chick a head start and a distinct advantage when competing for food.  Alan Poole, who wrote the well known book "Ospreys, a Natural and Unnatural History" says the youngest chick can weight 25-30% less than its older nest mates. This is called asynchronous hatching. Ducks, on the other hand, do not begin incubating until all eggs have been laid, and then all chicks hatch at the same time. This is called synchronous hatching. 
We do see runts on Osprey nests with clear differences in size in the early days, and some of them do not survive, but in some cases, by fledging time they have caught up, and it's hard to tell which chick is the youngest! 

I watched a female lay an egg yesterday. She was upright on the nest edge when I arrived. I was hoping to read bands on both birds so stayed and watched for several hours. During that time she did not leave the nest but moved into the center of the nest and assumed  a hunched over position. She began rocking back and forth slightly, as if she was shifting her weight from one foot to the other and she repeatedly looked  down between her legs. After about 20- 30 minutes of this she began poking around in the nest cup and finally sat down in incubating posture! Then the male came with a fish and she stood up and they both looked into the nest cup, and then he assumed incubating posture! Chicks are on the way!