A number of factors can come into play here. The lack of birds and the
presence of bees might not necessarily indicate that bees are excluding
birds.
The blossom might have little nectar meaning that it is not attractive to
birds but bees might be collecting pollen.
Similarly, if nectar is being produced then birds may have reduced the
nectar levels below a critical thresh hold. Birds are active a lot
earlier than bees. If nectar levels are reduced early in the day then
birds may not find it economical to continue foraging in that tree.
There is some evidence to suggest that bees might exclude birds but this
is a difficult thing to prove. The tendency is to enact the precautionary
principle and assume that bees are having an adverse impact.
David Geering
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