Spot on. No nectar or water, no brood. Simple as that.
OK there will be reduced brooding (but not wholesale), and if the weather catches out this rash of early brooding, it would very sharply slow or almost halt the spring expansion. So a water supply to enable the bees to use honey (only 20% water) as opposed to nectar (which is more likely 50%) is a requirement for the bees, if heavy brooding is to occur, until an adequate supply of nectar comes along.
Mine, at inspection, had some nectar coming in, but there are many bees collecting water, so they are either moving honey nearer the brood nest (or the nest is expanding) for the purpose of larvae feeding.
After all, when you think about it, relatively little stores are used up in the depths of winter, with little or no brooding and only thermal energy required from the stored honey; so no point in adding any more water than necessary (reduces the effective heat content by the latent heat of evaporation of the water present).
This may be one reason why the bees will readily consume fondant (11% water) in preference to honey (20% water). An added advantage of fondant might be a drier hive (less water vapour generated), but the downside is a shortage of brooding space.
It is bees that will collect a surplus of nectar (as honey), so the early crops (like OSR) may be wasted, as a large harvestable surplus will not be achieved if the hive is expanding rapidly during the flow - needing most of the collected nectar (from the lesser foraging force) to support the useless (for foraging) nurse bees and brood.
Then, surprise, surprise, at the end of the OSR there is a huge surplus of house bees (under three weeks old) - and emerging brood is also adding to that problem; there is no prospect, at the time, for foraging duties for all those up-coming foragers, so they go into swarm mode. Simple really.
RAB