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The Many Plumages of Late Summer Forster's Terns
It is often interesting what you discover when you start sorting through bird photos. This past weekend David Fix, Jude Power, Jennifer Brown and I spent the weekend birding around three of the larger playa lakes--Abert, Summer, and Goose--in south central Oregon. At Summer Lake, the northernmost of these three basins, I spent over an hour taking flight photos of Forster's and Black Terns. As is usually the case when photographing birds in flight, about one out of every ten images is worth saving.
The images below were all taken at the Summer Lake Wildlife Management Area, Lake County, Oregon on 15 August 2009. They depict some of the many looks that Forster's Terns can present at this season. Given that one species can show this much variation on a single calendar date, one has to wonder how the editors of standard field guides ever decide which illustrations they will use. We'll start with a juvenile (hatched during summer 2009) and work forward through several age classes to a definitive alternate-plumaged bird. Most of these birds are showing some signs of molt that we will point out along the way.
These two photos (above and below) show hatch-year Forster's Terns still sporting juvenile plumage. Note the warm orangish buff wash on the crown and nape, the isolated black eye patch that does not connect across the nape, extensive black on the underside of the primaries (outermost flight feathers). Other than the outermost tail feathers, the tail is fairly short. I was somewhat surprised that these birds showed so much black on the corners of the tail. This is not shown well in the standard field guides.
One of the interesting things that I learned when I started researching Forster's Tern molt sequences is that they do not attain their first alternate plumage (often referred to as "summer" or "breeding" plumage) until their third year of life (Mcnicholl et al. 2001). In other words, birds hatched this summer will not attain an alternate plumage until the spring/summer of 2011. Instead, they molt from Basic I directly to Basic II. Basic plumage is a non-breeding or winter plumage acquired via pre-basic molt.
This Forster's Tern appears to be transitioning into "basic II" plumage, which tells us it hatched during the summer of 2008. It has none of the orangish-buff in its plumage shown by the hatch-year birds seen above. Its bill is starting to show some orange towards the base, whereas the bills of hatch-year birds are entirely dark. Also, note the wear on the flight feathers, which will be replaced later this fall. Since these primaries and secondaries are nearly a year old, they are frayed and somewhat irregular in length. Compare them to mint condition of the weeks-old flight feathers of the juvenile pictured above. This bird does show a bit of gray mottling above and behind the black eye patch. Some first summer (after hatch-year) Forster's Terns molt in a few alternate-type feathers, which may explain the gray across the hindcrown and nape of this bird.
In their third year, Forster's Terns finally attain their first alternate plumage, which is referred to as "alternate I" by many sources (Mcnicholl et al. 2001). In most aspects, alternate I birds look exactly like older adults in definitive alternate plumage. They are entirely white below, unmarked gray on the back and upper wings, and they have bi-colored bills that are at least 50% orange towards the base and black towards the tip. However, birds in this plumage do not have the long streaming outer tail feathers and they show clean white foreheads and forecrowns. The black on the head is limited, starting in front of the eye and then extending through the eye and around the back of the head, connecting across the nape and hindcrown.
Here is an example of an apparent alternate I Forster's Tern, which would have hatched during the summer of 2007. The eye patch is solid black and connects across the nape and yet the forehead and forecrown are clean white. The bill is more than 50% orange with a black tip. It has abbreviated outer tail feathers, but otherwise the plumage resembles that of a definitive alternate adult.
Finally, Forster's Terns attain a definitive alternate plumage in their fourth year. This plumage, held during the breeding season, is characterized by long streamer-like outer tail feathers and a solid black cap. The black cap extends from the base of the bill across the forehead, crown and nape and surrounds the eye. The bills of full adults are generally brighter orange towards the base and the orange covers about about two-thirds of the total bill length. Once the breeding season begins to wane (late July-early August), adults commence their pre-basic molt. Typically, the first indication of this molt is the appearance of a few white feathers on the otherwise black forecrown. Within a few weeks, adult birds will show completely white foreheads, and by winter--when the pre-basic molt is complete--the head will be entirely white except for a black patch surrounding the eye. Unlike the basic plumages of other similar-sized Sterna terns, the black on the head of a Forster's does not wrap across the nape. Only Gull-billed Tern, a species which has a much more limited range in the U.S., shows a similar head pattern in basic plumage. We did not see any adult Forster's sporting a basic plumage head pattern on the day these photos were taken.
This Forster's Tern, which is in definitive alternate plumage, has commenced its pre-basic molt, as evidenced by the emerging white feathers on the dark forecrown. Note how the black on the forehead extends right to the base of the bill. It also has longer outer tail feathers (not shown well from this angle). The flight feathers of this individual are in surprisingly good shape, although the primary tips are looking a bit frayed. Most of the adults we saw on 15 August 2009 had extremely battered outer primaries.
While the images above show an array of ages and plumages, they do not capture the full range of variation that we saw on this date. At this time of year--late summer, early fall--nearly every bird you see will be replacing feathers (molting). The fall (pre-basic) molt results in a generally less colorful basic or "winter" plumage. The late winter/spring (pre-alternate) molt results in a typically brighter alternate or "breeding" plumage." Unfortunately, not all sources use the same terminology to describe the plumages that result from these molts, which creates a bit of confusion for those trying to better understand the feather replacement process. It is important to remember that the timing of molts and migrations is not sychronized among all individuals of a particular species, so it can be expected that certain individuals in a large homogenous flock may not look like all the rest. Just yesterday, I received an e-mail with a "mystery" shorebird photo. The image showed a dull, basic-plumaged adult Least Sandpiper. This bird, which was in a plumage one typically sees during the winter months, had been seen with a large flock of juvenile Leasts, all of which were brightly plumaged. The resulting confusion was understandable.
Molt also results in birds that bear no resemblance to the paintings in your field guides. At this season, hatch-year birds are usually the sharpest looking birds in the crowd. Their newly acquired feathers seem perfectly matched and show no signs of wear. Conversely, adults often have wings and tails that look like they went through the lawn mower. In some cases, after hatch-year birds have newly grown feathers right next to old faded ones. The new feathers may look like someone stuck them on the wrong bird. A basic understanding of these fairly logical molt sequences can be a major help when it comes to properly ageing and identifying those birds that don't look anything like the illustrations in your favorite field guide. We encourage you to take a closer look at the feather condition of the common birds in your midst.
Literature Cited:
Mcnicholl, Martin K., Peter E. Lowther and John A. Hall. 2001. Forster's Tern (Sterna forsteri), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/595
It’s worth noting that your molting first and 2nd cycle adults are showing a molt limit in the primaries in which the inner primaries have been recently replaced, but the outer ones are old and darker. This contrast between the inner and outer primaries is often touted as a field mark of the Common Tern. Standard field guides seldom show it in Forster’s Tern.
The difference is that Common Tern molts inner primaries twice a year while Forster’s does it only once. Thus the molt limit is seen year round in the Northern Hemisphere in Common Tern, but only briefly during the late Summer/Fall molt in Forster’s.
Also of interest is that the outer tail feathers in Forster’s are replaced twice a year, both in the Spring and in the Fall. The long feathers of Alternate plumage are replaced by shorter ones in the winter. Thus the ratio of tail to wing on perched birds differs substantially depending on the Season. Only in Spring/Summer do Forster’s show the extensively long tail extending well past the primary tips.
This is a well-constructed summary that relies upon the Noble Content Editor’s research and clear wording as well as his own photos. Keep it up!