Thursday, December 5, 2013
By Herb Wilson
Shorebird migration is peaking now in Maine as southbound birds head for warmer climes for the non-breeding season. We get two cracks a year with these passage migrants as they move through Maine each spring and late summer and fall.
The greater yellowlegs, shown here, and the lesser yellowlegs are somewhat similar in appearance, but a good identification can be made based on the distinct difference of their calls.
Herb Wilson Photo
Two species of yellowlegs, lesser yellowlegs and greater yellowlegs, are common migrants through Maine. They are more likely to be seen on coastal mud flats but regularly appear in inland wetland areas as well. The bright yellow legs make it easy to identify a large sandpiper as a yellowlegs, but which one is it? Separating the two is an identification challenge.
As befits the name, the greater yellowlegs is larger than the lesser, weighing twice as much as its smaller relative. Greater yellowlegs are chunky, bulky birds compared to the slim, graceful shape of lesser yellowlegs. A lesser is 10-11 inches long while a greater is 14-15 inches long. Nevertheless it is difficult to accurately gauge the size of birds when there are not other known species around to provide a yardstick. Fortunately, other characteristics differentiate the two species.
A somewhat subjective but useful feature is the facial appearance. The face of a lesser has a kind, gentle demeanor compared to a greater's harsher mien.
A greater, with its bulkier body, has a relatively small head and thin neck. The neck frequently has a kink in it, like a heron. The breastbone is strongly angled and bulges from the chest. The neck of a lesser is relatively thick and shorter, rarely showing the kinked appearance. The head does not seem small relative to the body.
The bill of a greater is absolutely and relatively longer (relative to the head). You can use the length of the head viewed from the side as a measuring stick. The bill length of a greater exceeds the length of the head. Measurement of museum specimens indicates the bill averages about 1.3 times the length of the head. A lesser's bill is pretty close to the length of the head.
The bill of a greater yellowlegs is slightly upturned compared to the straight bill of a lesser. Outside of the breeding season, the bill of a greater yellowlegs is broader at the base and usually two-toned (gray at the base and black at the tip). The tip of the bill is somewhat blunted. A lesser has a straight, thin, needle-like bill with a sharp tip. The bill is uniformly black.
Your ears can provide the most convincing cues for a correct identification. A greater yellowlegs gives a loud, piercing whistle that is often rendered as "tew tew tew." A lesser yellowlegs gives a softer, less strident call rendered as "tu." It may be given as single notes or a couple of notes. A great website to hear recordings of these two species is AllAboutBirds.org. Just type in the species name of interest in the search box and press "Enter." On the species screen, you will see a tab called "Sound." Click on that tab and you will be taken to links to a number of recordings.
The difference in the vigor of the calls is reflected in the foraging behavior. A greater yellowlegs is a hyperkinetic forager, dashing back and forth along a mud flat. A lesser is more sedate, less frenetic.
Most of the other large members of the sandpiper family that pass through Maine lack yellow legs and hence are not likely to be confused with one of the two yellowlegs. However, stilt sandpipers can cause some confusion. These birds have greenish-yellow legs and are about the size of a lesser yellowlegs. In fact, they often forage with lesser yellowlegs. Proportionally, the head of a stilt sandpiper seems large. The bill is relatively heavy and shows a pronounced droop.
Herb Wilson teaches ornithology and other biology courses at Colby College and perches in South China. He welcomes reader comments and questions at: