Osmia lignaria

Hi,  I put out a box of 7mm tubes next to my normal 8mm  tubes for the Orchard Mason Bees or Osmia lignaria. The  mason bees are doing very well filling their tubes. Attached is a picture of the 7mm tubes, they have 5-7 bees working in them. Most are horned Face (Osmia cornifrons) but a couple look like the regular Osmia lignaria but are much smaller as can be seen in the picture. My question is are they just small Osmia lignaria that prefer the smaller tubes or is there another species that I don’t know about? From what I can tell they look like the Osmia lignaria but are about the size of a Horned Faced. Thanks                  Norm

 Hello Norm, Thank you for this fine photograph.  I see two bees quite clearly, and the third is a bit too blurry to see what it is.    I do not have any experience with the horned Face mason bee, but it has brownish coloration as the lower bee (see link below).  The black one is  Osmia lignaria.   Both are early spring pollinators, and so both would be about  in early spring.  The size of female mason bees or Osmia lignaria varies quite a bit.  I do not know if this is genetic variation or the end result of varying levels of nutrition.  It is unlikely that there is a third species of a smaller size in early spring.  However, many more smaller black mason bees are around that come out late spring through to  late fall.  at what time of year was this picture taken?    Dr Margriet Dogterom