Southern Hemisphere Breeders
Birds that breed in the southern hemisphere come north during our summer to feed. These vacationers are seen during most of the summer. Examples are Greater Shearwater, Sooty Shearwater, South Polar Skua, and Wilson's Storm-petrel. Pelagic birding is the only way to count these birds for your North American list.
 

Northern Hemisphere Breeders
Northern breeders visit the waters off our coast after the breeding season. These birds are more likely to be seen from August into winter. Examples are Cory's Shearwater, Leach's Storm-petrel, Red and Red-necked Phlaropes, Jaegers, Alcids, and Great Skua. Some juvenile northern breeders feed off our coast throughout the summer. This is perhaps why we see Manx Shearwaters all summer long. Seabirds are not easy to see on the breeding grounds so the best way to count these birds is a pelagic trip.

A special case of Norther Hemisphere Breeders are the petrels that breed in the Carribean.  These birds are highly endangered and some were thought to be extinct.  Recently a Black-capped Petrels was seen on the continental shelf edge off Cape Cod raising hopes for seeing these birds from New England.