Wisconsin

The three medium-sized terns that arrive in Wisconsin during spring present identification challenges for even experienced birders. Terns are slender and buoyant and often seen flying at a distance or in tightly packed groups on a beach or sandbar.  Common, Forster’s and Arctic Terns are distinctively different and can be identified with careful observation. It is important when identifying terns not to rely on any one particular field mark but rather to consider a suite of traits when making your identification.

Text contributed by Steve Lubahn, WSO Records Committee.  Photos by Nick Anich, Dave Freriks, Brian Hansen, and Ron Howard.

In spring, the majority of Common, Forster’s, and Arctic Terns arrive in full alternate plumage with solid dark caps and red or orange-red bills. Most immature terns remain on the wintering grounds but few can be found in various stages of molt that can exhibit a range of both immature and adult plumage characteristics.

My focus here will be adult birds in alternate plumage. Common and Forster’s Terns are expected migrants in Wisconsin and both nest in Wisconsin. Arctic Tern is a rare occurrence. Arctic usually arrives later, mid-late May into early June is the typical window for them. If you think you are seeing an Arctic, make note of as many details as you can and try to get a photograph. The Arctic Tern requires documentation for the WSO Records Committee. http://wsobirds.org/?page_id=858

Perched Terns

Let’s start by differentiating standing birds. All three terns are roughly about the same size. Forster’s Terns stand taller and more upright while Common and Arctic appear more crouched. Arctic has the shortest legs and will appear shorter and lower to the ground. Beware of Commons that have puffed up belly feathers that conceal their legs. This is not a reliable field mark especially when birds are flocked together.

A more dependable field mark in spring is to compare the back with the color of the primaries. Forster’s Terns have a medium gray back which contrasts with paler, frosty gray primaries and a completely white throat and belly. Common Tern has a medium gray back which contrasts with darker primaries and a gray throat and belly. Arctic Tern has a uniform medium gray back and primaries with a deeper gray throat and belly. Caution should be given under poor light conditions and especially with immature birds. Special attention should be paid to the extension of the primaries in relation to the tail. Forster’s and Arctic have tails that extend beyond the wingtips while in Common the tail is shorter and does not extend beyond the wingtips.

Common and Arctic both can show a contrasting white cheek between the black cap and gray throat with Arctic usually being the most contrasting. All three terns have a dark cap but the amount of white above the gape of the bill varies. Forster’s shows the greatest amount of white, Arctic shows the least while Common is intermediate between the two.

Finally, let’s compare bills. Forster’s has the largest bill which is orange at the base with a black tip. Common has a slightly smaller and thinner bill which is a variable red-orange at the base with a black tip. Some Commons lack the black at the tip in spring and can lose this trait by mid-summer. Arctic has the smallest and thinnest bill which is a darker red color and sometimes dusky at the base but with no black tip.

Terns in Flight

There are a couple of key points to consider when identifying the medium-sized terns in flight. To begin with, the underwing pattern is consistent regardless of the bird’s age. This is important because the upperwing pattern can vary with age. Forster’s can show darker wings in their first winter and a wedge on the inner primaries in basic plumage which can suggest Common Tern. A less obvious field mark is the translucence of the underwing if illuminated from behind. Common Terns molt their inner primaries before migrating to the north. These freshly molted primaries are much paler compared to the darker outer primaries which have been retained from the previous fall.

Let’s compare the underwing pattern of all three terns. Common Tern shows broad dark tips to the outer primaries which contrast with a very pale inner primary triangle.  Forster’s also shows dark tips to the outer primaries but slightly paler gray with little or no translucence. Arctic Tern has very narrow dark tips to the outer primaries, much thinner and cleaner than Common or Forster’s. Also, Unlike Common or Forster’s, Arctic shows primaries appearing translucent when backlit.

The upperwing pattern as discussed earlier is also a good field mark in flight as well as perched. In spring, Common shows a medium gray back which contrasts with darker outer primaries. Most Commons show an obvious dark wedge on the primaries. Forster’s has very light pale gray outer primaries which contrast paler than the rest of the upperwing. Arctic has a uniform pale gray upperwing.

Judging the tail in flight can be very difficult. Forster’s and Arctic both have longer tails than Common. In Arctic the outer tail webs are dark gray and in Forster’s the inner webs are gray while the outer webs are white. Common is similar to Arctic with dark gray outer webs.

A final note about flight behavior. Forster’s is less graceful with stronger wingbeats compared to Common and Arctic. Arctic is the most buoyant of the three in flight. Common is graceful but strong.

TO RECAP:

Forster’s

Perched: Taller, more upright, white throat and belly, large orange bill with black tip, more extensive white above the gape, medium gray back and paler primaries, tail projects beyond the wingtips.

Flight: Underwing has pale gray tips to primaries, upperwing very light gray primaries, lighter than rest of the wing, long tail, inner webs dark outer white, larger bill, less graceful.

Common

Perched: Gray throat and belly, somewhat contrasting white cheek, orange-red bill with black tip, less white above the gape, medium gray back, darker gray primaries, shorter orange-red legs compared to Forster’s. Tail does not extend beyond the wingtips.

Flight: from below, broad dark tips on primaries and contrasting translucent inner primary triangle, upperwing shows dark primaries and wedge, smaller tail, dark outer webs, graceful yet strong.

Arctic

Perched: Darker gray throat and belly, even more contrasting white cheek, deeper red bill with no black tip which is thin and small, least amount of white above the gape, uniform gray back and primaries, very short red legs, tail extends beyond the wingtips.

Flight: uniform gray upperwing with thin dark tips to primaries, underwing translucent, long tail, dark outer webs, graceful and buoyant.