Blackbird chicks

I have a blackbird nest in my garden and until a day or so again, an adult blackbird (can't tell if it's male or female) has been sitting on the eggs, presumably incubating them. I can now see that there are a few little heads popping out of the top of the nest, so I assume the eggs have hatched. My partner also claims that she heard the adult feeding them.

Unfortunately I haven't seen an adult blackbird on the nest for a couple of days. Does this mean that they've abandoned the chicks? If so, is there anything I can do (legally) to help them? I know there are some people who'll say "never interfere with nature," but if the adults have abandoned the chicks then I am determined to do all I can for them. 

Any advice would be appreciated.