> We already know that they shift their foraging behavior in response to need, availability and yes, nutrient levels.


I'm assuming you mean that bees respond to nutrient levels in pollen. If that's what you meant can you please provide a reference. 




The authors of this study indicate that bees do not forage for pollen based on protein content but rather they will forage on a related group of plants. I'm assuming that's what I'm observing each year in fields where goldenrod and mugwort and ragweed are co-located. 


They make two interesting observations based on their data. 


> Despite the apparent importance of pollen protein concentration to pollen consumers, there is no indication in our data that pollen protein concentration has responded evolutionarily to the dietary demands of pollinators. 

>Furthremore, despite the importance of dietary protein content to bees, bee species collect pollens that differ widely in protein content and do not show an apparent association with protein-rich species. 


Cane, James H., and Stephen L. Buchmann. "What governs protein content of pollen: pollinator preferences, pollen–pistil interactions, or phylogeny?." Ecological monographs 70.4 (2000): 617-643. 

 
https://www.researchgate.net/profile/Jim_Cane/publication/272581405_What_Governs_Protein_Content_of_Pollen_Pollinator_Preferences_Pollen-Pistil_Interactions_or_Phylogeny/links/54f4de6b0cf2eed5d735a60b.pdf

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