Greater yellowlegs are larger than lesser yellowlegs, but size can be hard to judge unless both species are side by side. Greaters also have a longer, thicker bill, especially at the base, that is often two-tone. Lessers appear delicate in every way, including the all-dark needle-thin bill. Both have long, bright yellow legs.
Voice is the best way to tell these birds apart. Greaters tend to call in three or four loud, clear notes: teww-teww-teww-teww! Lessers call less, that is, their calls are shorter, with one or two mellow notes: ti-teww, ti-teww.
Greaters have longer, thicker bills and louder, longer calls, and you have a greater chance of seeing a greater yellowlegs in cold weather.
During migration, both species can be found on marshes (saltwater or freshwater) and mud flats and along streams and rivers. Lessers seem to prefer smaller bodies of water. Greaters remain farther north in winter.
Lessers, despite being smaller birds, are less easily spooked into flight than greaters. Many birders have noticed this as they’ve approached a flock of yellowlegs.