- longer and thicker bill with curved culmen
- darker and much thicker streaks on the underside
- darker streaks on the upperside and coarser and darker markings on the face
- Belding's never (as far as I could see) raises a crest, while the northern birds often look slightly crested and show a distinct peak on the rear crown
- Belding's in early March were singing and territorial, chasing each other across the marsh, while the northern birds were in small, loose flocks moving together widely across dry grassy and weedy areas around the marsh
- Belding's flight was low and weak, with round body and fluttering wingbeats, reminding me of Sharp-tailed Sparrow, unlike the stronger, higher, more "bounding" and undulating flight of northern Savannahs
- in early March the Belding's were all in clean fresh plumage, while the northern birds were molting and scruffy with missing feathers all over the head and body
I was struck by how much stockier and larger-billed the Belding's looked in comparison to the northern subspecies, in many ways intermediate between northern and "Large-billed" Savannah Sparrows. Apparently some populations intermediate between Belding's and northern birds are resident farther north along the California coast, but alongside the migratory northern subspecies the Belding's Savannah Sparrows in southern California seem quite distinctive.