Kentucky Ornithological Society

Identification of Forster's and Common Tern by Diagnostic Tail Patterns

Text and Photos by David L. Roemer

Identification of the mid-sized Sterna terns is often not as straightforward as the field guides depict. Individual variation of characters such as bill color, darkness of upper and underparts, pattern of dark in primaries, as well as the effects of lighting and angle can complicate identification of these birds.

The pattern/color of the uppersides of the primaries is often touted as a reliable field mark to distinguish Forster's from Common Tern, with those of Forster's typically being silvery-white and those of Common showing dark outer primaries. This feature is not diagnostic however, as Forster's can show varying degrees of dark outer primaries and those of Common sometimes being relatively uniform.

The pattern of pale inner/dark outer primaries is due to the molt sequence of these terns, which molt their inner primaries twice annually, late summer before their southward migration, and in spring before their northward migration. The outer primaries are molted annually after arrival on the wintering grounds. When fresh, these feathers are covered with a pale powdery bloom, and darken as the feathers wear. This results in the newer inner primaries being paler than the older outer ones. Forster's and Common Terns share this molt sequence, but the ground color of Forster's primaries is paler so there typically isn't as much contrast. Both species can show the same primary pattern however, and immature Forster's typically show relatively dark outer primaries.

In contrast to some birds sharing the same primary pattern, the placement of dark in the tail feathers is diagnostic at all ages. In Forster's, the outer webs of the outer tail feathers are white and the inner webs are dark gray. The outer webs of the outer tail feathers of Common are blackish, and the inner webs are white. This character can be seen in the field given decent views. In addition, the dark in the inner webs of Forster's is typically confined to the distal 1/2 of the feathers, where in Common, the dark in the outer webs typically extends to near the base of the feather. For more information on these birds, see Kaufman's excellent treatment of mid-sized Sterna tern identification in his Advanced Birding guide.

Images A-D are of Forster's Terns, and the dark inner webs of the outer tail feathers can be seen.

A

B

C

D

Images E-G are of Common Terns, and the dark outer webs of the outer tail feathers are visible. Note the transluscent inner primaries of the Common Tern in image G with the bird strongly backlit. Forster's typically show little or no transluscence to the primaries. When they do, the flight feathers usually appear to be relatively uniform in their transluscency, whereas an apparent window is usually visible in the inner primaries of Common.

E

F

G

These terns were photographed at various locations during May 2006 in Kentucky.