Only if they are dead :-)

I would leave it for now and see what May and June bring. You will know for certain by then.

ps several newish varieties of yellow flowered Magnolia have appeared on the market lately and four to five years would be the outside for most with many flowering quite well after three.

Scrape away a little of the bark with a fingernail. If there is green underneath it is still alive. General advice, leave all suspected deaths til at least June before digging up.

I have just ordered a magnolia nigra .

A newbie to growing magnolias ,do they need protected from frost.

And when ready to plant into garden ,should I I use any special compost or feed?

The majority of Magnolia's are quite frost hardy including M. liliflora Nigra. What they usually need though to do well is a sheltered site and a free drained soil with plenty of organic content. Being trees or understorey trees by nature (the majority of species anyway) they like that semi-woodland, glade type position. Most will take full sun too though not on very dry shallow soils.

I bought (at huge cost) a yellow magnolia 'Yellow Bird' two years ago. It flowered the first year but now (March 2011) it looks very sick. Can't see buds coming along even. I don't think it is very hardy. Planted it with a wall behind for shelter too. Disappointing.