Robert Brenchley asks

> Does anyone have any idea on what basis bees choose to collect one pollen
> rather than another?

I am not sure that anyone knows the answer to this.  To demonstrate, when I
was trapping pollen several years ago, I had about 30 hives with pollen
traps on them.  I would collect the pollen once a week.  The hives would
average about 2 kilos a hive in the good times.  Top individaul weight was
4.5 kilos in a week.

What I did notice when collecting the pollen was that most traps had
similiar looking pollen in them when judged on colour.  However, there would
always be one or two hives that looked different due to the colour of the
pollen in the traps.  If we assume that the colour indicates a different
type of pollen, then most hives seemed to be going for the same type of
pollen in the same proportions but some were different.  Why?

The hives were on the same site but some hives were collecting pollen in
different proportions.  Why was one pollen more attractive to bees in one
hive compared to bees in the hive next door?  We are told that bees collect
pollen with the highest protein value but why do they collect pollen from
plants such as sunflower which recent studies in Australia showed the crude
protein value at 13% well below the 20% often quoted as the minimum
requirement for bees.

Is there a competition factor here?  Are some bees forced onto other pollen
sources because they are being outcompeted on the common source?

The plot thickens.

Trevor Weatherhead
AUSTRALIA