My first bees and hives delivered and placed next to a field of rape on Thursday. ... my concern is what are they feeding on?
Seems that the opinion is to have feed available, they are in full size hives with one brood box.
The supplier perhaps ought to have indicated
something about the stores situation within your purchase.
Bees naturally store enough to get themselves through lean times - particularly the long winter.
But right now, normally stores would be low-ish and the number of bees to feed increasing rapidly - so its an important time of the year to know what the situation actually is.
Beekeepers get experience of estimating the stores, without opening up, by feeling the weight of their hives ("hefting"). But you won't have that yet. Never too soon to start feeling the weight though.
Thing is, very soon after a colony gets onto flowering rape, they can bring in so much that they run out of space before you notice it. That would be particularly so with the relatively small brood box on the WBC. (So make sure you have a super or two absolutely ready to add at short notice.
Or are they already on the hives?)
The problem is that you don't seem to know whether you are closer to feast or famine.
If you can't get guidance from your supplier (shame), then the risk of not feeding (starvation) has a bigger downside than the risk of overfeeding (swarming from a hive with not enough freespace).
So feeding a little (a pound or two of sugar, as syrup) is probably the safe option.
That is until you can grab a weather opportunity for a quick look (something like 10c and good sun, at a minimum).
For a "quick look" you need to be looking for two things in the brood chamber
- at least one or two frames seriously heavy (pretty full) with honey --and --
- at the very least a couple of frames of comb that are half empty - without stores or brood, just empty cells (but hopefully not a box half full of completely empty combs, or of frames with foundation rather than comb)
And close up again as soon as those observations are made.
If you've got some stores and some space, then relax until the weather gives you a chance for a proper inspection...
If not, give them more space or more food... as appropriate.
For making the syrup, add the hot water to the sugar (off the heat), stir to dissolve and then leave to cool.
ADDED --- of course you shouldn't be feeding syrup with (honey crop) supers on the hive ... !