A marker bird is one you can recognize because of some unusual feature - an injury, an extra-long bill, or abnormal coloration.
Examples are Bill of the Bird's leucistic junco, Snowflake, the long-billed White-breasted Nuthatch in Mary's View, or Julie Zickefoose's Red-bellied Woodpecker, Ruby. An extreme example is the gynandromorphic Northern Cardinal recently featured at Minnesota Birdnerd's blog. This half-male/half-female cardinal is certainly a remarkable bird.
My bird is not so dramatic, but he is recognizable. "Barry" is my "White-winged House Finch." He showed up today, right after I read a post on the Ohio Birds listserv about a similar House Finch reported by Amy W. in Medina County, Ohio.
I snapped one quick picture through the bathroom window, but am hoping for a better photograph later, if he sticks around.
Despite the poor quality of this photo, I think you can see the strong white wing bars which caught my eye. Barry's appearance at my feeders today, along with Amy's report, has the unintentioned consequence of reducing my Life List by one species. In December of 2007, I recorded a "White-winged Crossbill" feeding with the finches at my office feeder, based on the wing bars. Now, I feel obliged to retract that ID, thanks to Barry.