Half of the chicks now dead

Things are not going well at all at this moment. Three more chicks have died over the past ~15 hours. Now there are only five left. As mentioned in a previous post, one chick disappeared yesterday. Early this morning there were still seven chicks alive.

The video below is a 33 sec compilation of three fragments within the period 8:12-9:28am. The first part shows six chicks and one of them is clearly dying. Chick number seven is then already missing. After that two dead chicks are removed by the mother. (8:18am and 9:28am).

Chick dying, 2 dead chicks removed by mother. 6 May 2014 8:12-9:28am

It's sad to watch and I don't know what is causing all these sudden deaths while the chicks appeared to be developing well. Lack of food?

As I write this, one of the remaining chicks is quite far behind the others and looks weak, so I fear he won't see the end of the day.