Montana Field Guide

Forster's Tern - Sterna forsteri

The Forster's Tern is a medium-sized, primarily white tern with a black cap and dark eyes. The back and wings are a pale silvery-gray, contrasting with the white of the neck and belly. The primaries and the deeply forked tail on the breeding adult bird are also a pale gray, with the primaries appearing as white as they become worn. During the breeding season, the large bill is orange and tipped in black, and the legs are bright orange or orange-red. In non-breeding plumage the bill is black and the legs are a duller red-brown (McNicholl, Lowther, and Hall 2001). During non-breeding season, the primaries are dark silvery-gray and the crown is white with an evident large black patch encompassing and extending behind the eye (Sibley 2000). The bird is approximately 13 inches (33 cm) in length with a 31 inch (79 cm) wingspan.

The common call of this bird is a simple descending "kerrrr", described as lower and more raspy and wooden-sounding than the Common Tern (Sibley 2000). Sibley (2000) notes the species also has a "kit" or "kuit" common call; a begging "kerr kerr kerr" during courtship; and a very low "zaaaar" during defensive attack.

White wings and underparts give the Forster's Tern a lighter and brighter overall appearance than the Common Tern. The Forster's is also distinguishable from the Common Tern by its longer and stouter bill, longer tail, and more orange, rather than red, colored bill (McNicholl, Lowther, and Hall 2001).

Montana Range

After departing the southern U.S. and Mexico, normal arrival and departure migration dates for the Forster's Tern in Montana are generally May 7 to 24 through the third week in September (Reichel 1996, Sibley 2000). The extreme migration dates for this species are April 28, 1993 at Freezout Lake and September 27, 1958 in Ennis (Reichel 1996). Normal migration periods in Bozeman are May 7 to 12 and September 15 to 19 (Skaar 1969). The normal arrival date in Missoula is May 14, and in Billings it is May 24 (Skaar et al. 1985).

In general, this species is described as a short- to medium-distance migrant. It is present throughout the year in all but the most northerly portions of its breeding range (McNicholl, Lowther, and Hall 2001). Dispersal north and south of breeding areas takes place post-breeding. The Forster's Terns migration habits alter from that of the Common Tern by generally being earlier in the spring and later in the fall (McNicholl, Lowther, and Hall 2001). The species, which commonly travels in small groups, though larger flocks have been reported, migrates primarily through interior North America.

Large marshes with extensive reed beds or muskrat houses that provide nesting structures are the preferred breeding habitat for the Forester's Tern. It is also occasionally found along marshy borders of lakes and reservoirs in Montana. The species generally nests colonially, with as many as five nests recorded on one muskrat house (Johnsgard 1986). Preferred nesting locations include both nesting and foraging sites within close proximity. Alkali bulrush (Scirpus maritimus) was used as nesting substrate at Benton Lake National Wildlife Refuge (Montana Bird Distribution 2003). A study in the Lewistown District of the Bureau of Land Management documented that five of the six sites selected by the Forster's Tern were larger than 100 acres, with emergent vegetation covering more than 25% of the shoreline (Feigley 1997). Four of the nesting sites were on permanent bodies of water, with the remaining two on temporarily flooded sites (Feigley 1997).

No food habit information is available for Forster's Tern in Montana. Other source material indicates insects (e.g., dragonflies, caddisflies) are caught in the air or snatched off the surface of the water (e.g., dead beetles) while the tern is in flight. The Forster's Tern also dives into water for fishes, mainly submerging the bill and a portion of the head, though sometimes the entire body is submerged below the surface of the water (Terres 1980, McNicholl, Lowther, and Hall 2001).

Forster's Terns are usually found in small, loose colonies in Montana. Little other ecological data is available from within the state. However, information provided from other states in the species' range states that during the non-breeding season, the Forster's Tern can be found singly or in small loose groups.

Limited information is available on disease and parasites common in Forster's Tern. In a Colorado study, 9 of 10 were infected with the fluke Diplostomum spathaceum (Trematoda) and several lice species (Actornithophilus funebre, Philopterus melanocephalus, and Saemundssonis parvigenetalis) (McNicholl, Lowther, and Hall 2001).

Forster's Terns nest in Montana from mid-May through mid-August, though little detailed information is available on the timing of reproduction for this species in the state (Reichel 1996). Forster's Terns were observed on the nest as early as June 1, 1995, and reports in North Dakota indicate newly hatched young in late June and early July (Reichel 1996).

Fewer than 500 breeding pairs are anticipated in Montana, with nesting occurring as single pairs or colonies up to 60 pairs (Reichel 1996, Montana Bird Distribution 2003). A study performed in the Lewistown District of the Bureau of Land Management found Forster's Terns nesting from one pair up to 20, depending upon location and available habitat (Feigley 1997). Most reproductive information comes from sources in other areas of the species' range. Nests are most frequently found on mats of floating vegetation or muskrat houses; in these situations, the nest is lined with grass and reeds (Johnsgard 1979, Godfrey 1986). Occasionally Forster's Terns will nest on islands or beaches like Common Terns, using a lined depression in the mud or sand (Johnsgard 1979, Ehrlich et al. 1988). At times, nests will be very close together on a favored site, such as a muskrat house, where up to five nests have been reported together (Johnsgard 1979).

This tern produces eggs from late May to mid-June. Both sexes incubate the clutch of usually 3 to 4 eggs for about 23 to 24 days. The eggs are subelliptical to oval, smooth, non-glossy and very pale, tinted olive or cream, spotted or speckled in brown, blackish-brown, or gray and average 32 x 23 mm (Baicich and Harrison 1997). The semi-precocial young are tended by both adults until capable of flight, fledge at 3 to 4 weeks, and remain with the parents well into the fall (Byrd and Johnston 1991). The Forester's Tern often renests if the first nest is lost; this often occurs at coastal nesting sites as a result of tidal flooding. They nest in loose colonies or singly, with some colonies ranging in size from less than 500 (Atlantic coast) to up to several thousand (Louisiana) (Spendelow and Patton 1988).

No management activities in Montana specific to Forster's Tern are documented. However, recommendations on surveying techniques stress that limited time should be spent at a colony particularly just before egg-laying and just after hatching; the prior, because of the likelihood of desertion of the colony, and the latter, to avoid causing mortality of young chicks (Reichel 1996).