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I have wondered that phenomenom decades. There are many reasons.
1) It is early spring and first flowers. There are so many visitors in one frower, that it is cleaned at once when it opened.
2) Too many visitors in willow flowers. In low temperatus bees cannot fly very far. If it is much wind, bees do not fly over big fields. Pollen is gathered as soon as the new flowers open. Not enough pollen to all visitors, if bees forage only inside radius 1000 metres.
3) In Australian reports it was said that bee forage pollen with full speed in +16C temperature.
Their tiny legs and fingers work with full speed.... but assisted with sun radiation.
When bees work, their wing muscle temperature is 39C.
4) I folloved hives on spring rape field. Temp was 18C, sun and some wind.
Bees came out and went in with with speed. They had empty nectar bellies and not much pollen.
Near ground sun shine radiation is stronger and at same time bees can gather bigger pollen balls from dandelion.
5) There are many plant species from which bees gather only small pollen droplets. Raspberry is such.
6) In the morning bees use to forage pollen and in the afternoon it is rare. Perhaps most pollen has been cleaned from flowers.
7) Mullein offers pollen only before midday. No bees are in flowers after midday. Perennial Verbascum nigra is my favorit.
8) I moved hives into willow bushes. The willow bushes on open large field was full of open flowers. Temp was +8C. When sun came behind the cloud, bees attacked onto flowers, and when sun went behind clouds, bees were in a hurry to come home.
Wind was quite strong
At same time in the middle of trees it was calm and bees foraged several kilos nectar and pollen in few days. Temp beween trees is higher and no wind from any direction.
9) what is day temp and working hours
If the night temp is, let's say +5C, it takes many hours that the day temp reachers its maximum +18C. It may be 12 a'clock. Then 15 a'clock temp is allredy going down.
Last bees come with pollen load to the hive. They are tired and drop into shadows. Air temp is 12C and they cannot rise to their wings and die into shadow. Then there are lots of brave dead workers in the evening around the entrance.
Effective working hours may be 3-4, even if the whole day seemed very good.