Have they been abandoned?

[pigeon.911.uk]

Hi,

We were fortunate enough to have some pigeons move into our (fake) tree on our balcony and lay two eggs...



The eggs hatched on the 18th of April, and these two little yellow balls of fluff...



(bonus video of the parents getting rid of the empty shell: 18042009073.mp4 (45 MB))

have rapidly turned into two fat brown balls of fluff...



All seemed to have been going well, and the parents have been pretty much constantly sat on/with the birds since they were born about 16 days ago, but as of this morning, I haven't seen the parents once. It is now gone midnight and the two baby birds are still left alone in the nest. Is this normal? I am worried that they might have been abandoned, but I don't want to go and interfere with the babies if the parents are still around and looking after them. They do have a good set of feathers on them and they are huddled against each other to keep warm, but I don;t know if they are being fed any longer.

Please could somebody advise as to whether they are okay?

Many thanks.
Stu