For a few weeks now mine have been coming in loaded with pollen like that in the early morning, say 6:30am to 9:30am, then they seem to switch to nectar since I don't see any with pollen after around 9:30-10:00, though traffic is still heavy. And yeah, really heavy loads of pollen, big bulges. The hive is about 100 yds from my door so I usually walk out to see what they are doing from time to time. I guessed that the sun warms up flowing plants, nectar starts flowing, and the bees switch over. Dunno.