Dan, 

   Red-necked Grebes are somewhat an irruptive species much like crossbills, Red-breasted Nuthatches, Black-capped Chickadees, or Snowy Owls though not necessarily for the same reasons. The presence of Red-necked Grebes here is probably dependent on food availability farther north more than anything else. Numbers vary greatly from year to year. Some years there are hundreds (rare), other years there will be small numbers (usually the case), and some years there are none (rare). If you have the Maryland Birdlife CD you can read the spring season reports over the years since this information is not available on eBird. 

Bob Ringler 
Eldersburg MD