I sent a summary of our recent discussion to Project FeederWatch.   
Here's what I got in reply:

"The winter finches you listed are irruptive and only come south when  
food supplies in the north are scarce. Redpolls are typically every  
other year, and this would be their year, but the cone crops in the  
north are plentiful this year, so they did not move south again this  
year. All the other species are far more irregular and Evening  
Grosbeaks have been missing for years and probably have declined  
significantly. You can see where birds are being reported this year,  
and compare to past years, in the Explore Data section of our web  
site. The historical maps in the Map Room can show distributions every  
year since the project began, in animation so you can easily compare  
and see patterns over time.

I have received a few other comments about less birds in general, but  
no more than in any other year. I suspect food supplies are probably  
affecting all species. There are often fluctuations between years.  
Once all the data is in, we'll do our analysis to see if there were  
any significant differences this year."