Dead great tit chicks in nest box nearly 3 weeks old

We can't say why they are dying - we can speculate.  Sadly late nest failure is not unusual, lack of food, inappropriate food or poisoned food may contribute and the recent temperatures may have contributed - to have 25 degrees early in May is not normal.  By poisoned food I thinking of use of insecticides in the local area as the parents will forage anywhere suitable 

Thanks for your reply. Another died yesterday and another dead this morning. It’s just heartbreaking to watch. There are only 2 left but neither are jumping for the food like they were. I wish I could help them. I read that there are less caterpillar due to the lack of rain etc. I guess we’ve been lucky the past few years with having them all fledge successfully. Is there any particular food I could put out for them? I have suet, sunflower seeds and mealworms. Would berries help them?

If you can access some live meal worms that might help - or wax worms - Are you pre-soaking your mealworms? A little mild grated cheese.  Have you noticed the feed rate of the adult birds?

Re lack of caterpillars, normally it would be put down to 'cold snap' or 'heavy downpours washing caterpillars off trees'. I am not aware of dry weather being an issue.....at least, not for the types of caterpillars blue and great tits tend to target.

As well as not south facing, is the box in full sun anyway? e.g. on a fence out in the open?

• Thank you. I got some live today and soaked the dry ones. Mum seems to far prefer soaked ones. She has def been feeding those ones. I had suet up , which she was visiting. I’ve noticed the bigger chick of the two left is constantly pruning himself. I just hope the dead ones still in box aren’t going to make them ill. The weather is a lot cooler today. It also looks like no Dad helping anymore today. He was there all the time previously. The parents seem to feed quite a lot when dad was around. I’ve not even seen him in garden today, which is odd.

• It’s on side of garage wall , which is sheltered by side of our house so it’s never directly in the sun I don’t think. We’ve had same box for at least 5 years and always had the chicks fledge successfully. They’ve had hot weather before for certain but the lack of any rain is different this year and they’re a few weeks earlier nesting than normal.

If they are 3 weeks the remaining ones should hopefully fledge any day now - they go between 18 and 22 days - keeping everything crossed 

Thanks. I have to say that I'm really not a fan of supplementary feeding at this time of year, esp non-live food. It's widely recommended not to use fat. I include suet as that. Can get on feathers, and if chicks are constantly preening, it may be that. Chicks need protein and moisture. Fingers crossed re getting fledged birds.