Blackbirds

The second clutch of blackbirds has not been as successful as the first. Last time, three young ones left the nest, although a week later only maybe one of them was still alive.

There were four eggs in the second clutch, but I think only three hatched. Here's how the nest looked a week ago:

And here's the view today:

Definitely only one left.

The adults have behaved differently this time. One or the other of them has been sitting on the nest for a lot of the time since the eggs hatched. We never saw Mr Blackbird on the nest the first time, but he has been lately. Both parents spent all their time feeding the first batch, but the second batch seem to have been fed much less frequently. Is that why fewer have survived? Or are the parents feeding less because there are less mouths to feed? Are they sitting on the remaining nestling to keep it warm, because it doesn't have siblings providing warmth? Or are they protecting it from whatever happened to its siblings?

Many questions, but no answers, I'm afraid!