Quite a dramatic difference in the appearance and size of the two oldest chicks now. Their sibling which at 34 days, obviously a falcon because It is even larger but is not as developed as the older siblings with still lots of fluffy down evident. In the image here the bird being fed appears to be a young tercel.  It will not be many more days before this one fledges the nest ledge. I was quite surprised to see the falcon feeding the chicks today. this is the first time for several days that I have seen the chicks being fed. Prey has been delivered to the ledge and the chicks then left to fend for themselves.  This seemed to be what I have observed before and I have a theory.  The tercel catches smaller prey at a size that the young eyass can cope with whereas the falcon being a much bigger bird, catches larger prey such as Magpie and other corvids.