Can a magpie steal chicks from my tit box?

Pete.8

I have a tit box facing the house on my pergola and in Feb and March have watched the tits stealing fibres from the twine I use to tie in my roses to make their nest.
It now gives me great pleasure to watch them whizzing in and out with food for their chicks.

BUT a couple of days ago I noticed a magpie hanging on the front of the box trying to get its beak inside! I ran down the garden and scared it off - a couple of hours later it was back and I ran down the garden waving my arms about again.
I was relieved to see the tits still going in and out with food a little later.
Yesterday I noticed the magpie again, and it was trying to pull the box from the pergola causing the box box to be at quite an angle.
I gently adjusted the box level again and tied string around it to secure it making sure I wasn't leaving somewhere for the magpie to get a grip.
Minutes later the tits were still in and out feeding their chicks and they still are.

Are the chicks still at risk do you think?
Anything else I can do to help?

I know the magpie probably has chicks to feed too, but I don't want MY tits to become magpie food

Thanks

Greenbird

Not much you can do other than keep chasing it away.

If its a purposely made tit box with the correct sized hole, the magpie will struggle to get into it without dislodging the box, so make sure both the box and lid are secure.

We had to put steel protectors around the hole of ours before, as the Great Spotted Woodpeckers or Squirrels would drill/gnaw the hole larger until they got in.

wild edges

The survival rate for blue tit chicks in the first year is about 37% so 2/3 chicks will be eaten by something sadly.

Fairygirl

It might be worth trying a cage of chicken wire if you can attach it without disturbing the nest. The adults will be able to get through it no problem.
I appreciate that it's probably not ideal for trying just now, and may be worth thinking of for next year, or for the gap between this clutch of babies and the next one. 
I'd agree that if the hole is the right size and can't be chewed, a magpie probably won't be able to access it, but it's not great if it disturbs the adults from feeding the youngsters 

Crazybeelady

I have my fingers crossed for your babies!

Pete.8

Thank you all for your helpful replies.
It is a bought tit box, albeit a cheap one (£5) - 2 yrs old now and in good nick.

The hole hasn't got any larger so I'm hoping at least the magpie wont be able to get its head through it, but it must be pretty scary for the birds inside to see a huge deadly bill trying to break in. But the entrance hole is quite high up so it'd have to get its head right inside to cause harm.

The chicken wire makes sense, but it would also give the magpie something to get its claws into and give it better grip, so I think I'll screw the box directly to the pergola after I've cleaned it out this winter then it can't move at all and hope that my efforts to secure it for this year will work out ok.

They've been so fascinating and entertaining to watch over the last few months and of course I feel very protective of them

Pete.8

Thanks @Crazybeelady