It’s jaeger time! Jaegers are some of the most exciting birds around. The identification challenges they present are notorious. When I asked one expert about distant jaeger identification, they said something like, “the more experience you get with jaegers, the more most birds become easier to identify, even at a distance, while some birds remain challenging, even up close.” The more jaegers I see, the more I tend to agree with this mentality. Some will argue that if a seasoned expert can mess up a jaeger ID with an extended, close flyby, then it is impossible to be certain about that distant bird on the horizon, no matter who is making the call. Maybe the truth lies somewhere in between these two attitudes… but I’ll leave that question to the reader.
One thing that no one can argue with is that the proliferation of digital photography, particularly from pelagics, has significantly broadened the understanding of jaeger plumage. Although new knowledge has emerged, public awareness has been slow to catch up. The last major publication on jaeger identification is from the late 90s. This is about to change, as more publications are said to be coming out within the next couple years. In the meantime, one of the best resources available to anyone, free of charge, is eBird’s Macaulay Library.
Here I will present a collection of photos from the Macaulay Library, not to act as a comprehensive identification guide, but to focus on a specific topic: color patterns in juvenile jaegers. The impetus for this project comes from the fact that I think there are some very useful things to know about color in juvenile jaegers, but that they are often over-simplified to the point of erroneous thinking. I will also discuss how the color patterns in these birds might create impressions at a distance that can aid in species identification in certain cases. At the end, I will show some side-by-side study skins, with some quizzes, and then a little speculation on how the different melanin types might be playing a part in the appearances of each species.
This blog is geared towards those of us watching jaegers on Lake Michigan and the Great Lakes, but it should be helpful to anyone wanting to learn more about jaegers in a modern context. Aside from coloration, I’ll also be touching on a few other useful field marks that I think could use some clarification.
Sections
Part I - What is a juvenile jaeger, and why should you start here?
Part II - The “cold-toned” Long-tailed.
Part III - The “cinnamon” Parasitic.
Part IV - The steady Pomarine.
Part V - Specimens and thoughts on melanin types.
Part I - What is a juvenile jaeger, and why should you start here?
Three key examples:
There is too much variability in juvenile jaegers to show a “typical” individual for any species (Pomarine may be an exception), but the examples above were selected because they display colors that are either typical or good to to know for each species, particularly in the pale highlights. Before digging in to each species, below is a small explanation of what I understand about ageing jaegers that are seen during the late summer and fall. The vast majority of jaegers seen on Lake Michigan are juveniles, but there are exceptions, and in those cases the rules change. Still, FEEL FREE TO SKIP AHEAD TO THE NEXT SECTION, while keeping in mind that eventually older birds have to be added in–especially if you are taking a trip to the ocean any time soon.
Juveniles of all three jaeger species typically show pale fringing on the upperwing coverts and relatively short, wide, central tail feathers.
Pale fringing on the upperwing wing coverts (circled in red) and relatively short, wide central tail feathers age this individual as a juvenile, born a few months before this photo was taken.
Long-tailed jaeger - Wojciech Janecki - 8 Oct 2023 - Pomorskie, Poland
On Lake Michigan, Parasitic is the most frequent species, followed by Long-tailed, with Pomarine relatively rare. Birds with adult and near-adult (“sub-adult”) plumage are seen every year, but these are far less common than juveniles. In between these age classes are one-year-old birds, which are said to spend their first breeding season on the ocean, not showing up at breeding grounds for another year or two. One-year-olds are not expected on the Great Lakes, but some examples have been documented. Both juveniles and adults/sub-adults are presumed to be coming from the breeding grounds and are heading to the oceans for the northern hemisphere’s winter.
Adult. Adult jaegers in breeding plumage can be aged by long central tail feathers and plain underwing and undertail coverts without pale barring. All pale adult jaegers show a distinctive dark cap and pale face. Adult Long-taileds are very rarely seen on Lake Michigan, while adult Parasitics probably turn up every year or so and can be aged by the same features.
Long-tailed Jaeger - Michael Topp - 22 Aug 2009 - Lake, Indiana, United States
Subadult-type (/nonbreeding adult?). This bird shows plain upperwing coverts and an adult-like dark cap and pale face, but has barred underwing and tail coverts, as well as some back feathers with pale fringing. The central tail streamers have probably broken off. How much of this plumage is "immature" vs. "nonbreeding" is a bit mysterious–how much of this plumage shows up in nonbreeding adults?–a point that will hopefully be clarified in upcoming publications.
Long-tailed Jaeger - Ryan Sanderson - 30 Oct 2023 - Lake, Indiana, United States
First-summer type. This bird was likely born a little over a year before this photo was taken. This is an age and plumage aspect that is not expected on the Great Lakes. Note the worn, pointed central tail feathers (which could describe both Parasitic or Long-tailed at this age), and the plain upperwing coverts without pale fringing, both of which age this bird as older than a juvenile. Signs of immaturity include the barring on the underwing and undertail coverts, the short length of the tail feathers, and the head pattern without any hint of dark cap or pale face. A post-juvenile jaeger with this extent of immature features is likely a one-year-old, or "first summer."
Long-tailed Jaeger - Ashley Fisher - 30 Jul 2023 - Cornwall, England, United Kingdom
When confronted with the above bird, I can hear the prevailing voice of jaeger wisdom in my head saying, "warm brown tones, more than two pale primary shafts, and pointed central tail feathers make this a Parasitic Jaeger,” but in reality, this should be an expected look for a one-year-old Long-tailed. More than two pale primary shafts are regularly seen in Long-taileds, and pointed tail feathers are the norm in Long-taileds after juvenile plumage. Relevant here, the orange-brown hue in places is seen both in one-year-old and juvenile Long-taileds, and so should be expected on the Great Lakes.
A word about central tail feathers.
Central tail feather shapes of juvenile jaegers - from the Pyle guide. Note how subtle the differences in shape are.
Central tail feathers of Parasitic Jaegers at different ages, with juvenile at the left, and each subsequent plumage to the right (example E "rare") - from the Pyle guide. Long-taileds skip to F and then H directly after juvenile plumage; in other words, after juvenile plumage, both Parasitic and Long-tailed Jaegers have pointed central tail feathers.
In general, people would do well to dial back the weight they give to central tail feather shape (i.e. “pointed” vs. “blunt”), especially if they are uncertain of the age of the bird, or if the bird is distant. In my experience, central tail feather shape is most useful for determining if a jaeger is a juvenile or not; the thick tail feathers of juvenile Parasitics and Long-taileds can be picked up and photographed at a moderate distance. As far as species ID, central tail feathers can be useful for closer juvenile Long-taileds, primarily due to the length of the feather projection, not the shape–the longer projection adds to the thin, long impression of the rear-end in juvenile Long-taileds. Post-juvenile Pomarines (not shown here) often have distinctly wide central tail feathers that can be picked up at a distance (if you see a post-juvenile Pomarine on Lake Michigan, I hope you have a camera ready!).
Part I recap.
Most jaegers on Lake Michigan are juveniles. Older ages are rare but regular. Pomarines, even juveniles, are rare.
Juvenile jaegers during fall migration typically have pale fringes on their upperwing coverts and relatively short, wide central tail feathers.
With certain exceptions, don’t worry so much about the shape of central tail feathers for species ID, while they are useful for ageing birds as juvenile up to a moderate distance.
Part II - The “cold-toned” Long-tailed.
I hear people talk about “gray” and “cold-toned” with Long-tailed, and “warm” and “brown” with Parasitic, as if these terms are interchangeable. The reality is more nuanced. In the following photos, pay attention to the color (or lack of color) in the areas here referred to as the pale highlights, specifically:
the pale barring in the upper and undertail coverts
the pale fringing in the upperwing coverts
the pale barring in the axillaries (the “armpits”)
the pale areas in the head and body
Treat these areas and the coloration there as a separate entity from the dark base color seen here:
the dark bases of the upperpart feathers (back, wing coverts, uppertail barring)
the dark barring in the underwing and undertail coverts
the dark areas in the head and body
You’ll notice that the pale highlights in Long-tailed juveniles are usually white or have subtle off-white coloration.
The dark bases to the upperparts and body feathers in Long-tailed are more variable, with hues ranging from light copper-brown (more orange), to darker chestnut-brown (more red), to less saturated brown and dark gray. The lighter copper-brown hues seen in some individuals may be unique to immature Long-taileds. Anytime you see the word “unique” in reference to jaegers, you should take note (and also be skeptical). I have a strong hunch that this light-brown coloration is the result of a more general tendency for reduced dark pigment, specifically eumelanin, that can show in Long-taileds at any age. I will expound on this hunch in the last section.
Long-tailed juveniles vary greatly in the amount of dark they show in their head and body feathers, which when combined with relatively colorless pale highlights, results in some individuals showing distinctly pale and desaturated heads and bodies.
The dark bases in the noted feather groups show a copper-brown hue, while the pale highlights are mostly white, with cream color in the lesser coverts and the collar at the back of the neck. This photograph is intensely colorful, but the hue of this orange-brown in the upperparts may only be seen in Long-taileds.
The pale highlights of the undertail and underwing coverts are virtually colorless and white. The dark areas in the head and body are light brown, with somewhat yellowish-cream pale highlights, especially noticeable in the collar and belly. Note the distinctly thin dark chest band, a feature shown by many juvenile Long-taileds.
While not as common, this plumage exemplifies what can happen when the head and body nearly lack dark pigment. This tendency occurs in Long-tailed juveniles more so than in other jaeger species as juveniles. Dark bases to the upperparts are practically black, while the pale highlights are mostly white (maybe some off-white in the lesser wing coverts).
Long-tailed jaeger - Bruce Arnold - 25 Sep 2023 - Lucas, Ohio, United States
The dark bases to the upperparts here are darker, less colorful, but still subtly brown. The pale highlights range from off-white in the tail coverts to creamy yellow in the lesser coverts and collar. The dark base color is reduced in the head, leaving a mostly pale head.
Long-tailed Jaeger - Qin Huang - 5 Sep 2018 - Guangdong, China
A somewhat light brown dark base color, with limited dark in the head and body. The pale highlights show a definite cream color in the head, body, and wing coverts. The pale barring in the undertail is stark white by comparison.
Long-tailed jaeger - Brent Bomkamp - 31 Aug 2024 - Suffolk, New York, United States
This close up of the uppertail reveals that subtle color may show in the pale barring here, in this case contrasting with the white bases of the tail feathers.
Similar to the last bird, but with the saturation dial turned down a notch. How many pale primary shafts does this bird have?
Long-tailed Jaeger - Paul Dufour - 25 Aug 2021 - Finistère, Bretagne, France
An individual with extensive chestnut-brown base color to the body, head, and upperparts. This more reddish-brown hue is shown by juveniles of all three jaeger species.
Long-tailed Jaeger - Christoph Moning - 15 Sep 2013 - Bodenseekreis, Baden-Württemberg, Germany
Like the last bird, extensive dark bases to the head and body feathers, but less saturated overall.
Long-tailed Jaeger - Zbigniew Kajzer - 3 Sep 2023 - Lubuskie, Poland
In the darkest individuals, the trace of color is all but gone from the dark areas, leaving dark gray to black. The pale highlights are pale gray to white in the undertail. While many jaegers can appear "dark," this is often due to distance or low light. Long-tailed juveniles this dark in plumage are probably less frequent.
Long-tailed Jaeger - Eric Ellingson - 28 Aug 2020 - Whatcom, Washington, United States
Note the lighter, subtly orange-brown color in the upperparts here. This particular hue may only appear in Long-taileds. Note the two pale primary shafts–oh wait, actually don't(!)–note the relatively long central tail feathers, and the strongly contrasting dark secondaries. Relatively dark secondaries are seen in Long-taileds at every age, though they are more obvious in older birds as these have more pale gray backs. The pale fringing in this October individual has somewhat worn off.
Color in Long-taileds at a distance.
Before guessing how these birds might appear at a distance, it must be said that even at a moderate distance, lighting has a much larger effect on the appearance of most birds than aspects of plumage. As distance increases, we perceive colors as less saturated and eventually reduced to values on the grayscale. With indirect or obscured sunlight, the transition to grayscale happens fairly close, while with full sun, the transition can happen much farther away and color can still be seen for some distance. In either case, differences in plumage can lead to perceivable differences in overall impressions.
As most pale highlights in Long-tailed juveniles are either white or only subtly colorful, at a distance these highlights will add a pale gray cast to whatever dark base colors are present. A paler gray impression will be especially apparent in the heads and bodies of those individuals with reduced dark pigment there, but the “graying effect” will also occur in the upperparts due to pale fringing.
Something I noticed in museum specimens is that the dark bases themselves were subtly paler in Long-taileds than in Parasitics, although my sample size was small.
The factors above would explain why even more colorful brown Long-taileds can look both more pale and more “gray” (and thus “cold-toned”) compared to most Parasitics at a distance; watching those same individuals as they move closer or get hit by the sunlight will reveal that they are actually quite brown. Birds with darker, less saturated base colors, or birds seen in low light, may still look subtly more pale compared to most Parasitics in the same context. This will be explained more in the next section.
Part II recap
The pale highlights in juvenile Long-taileds usually range from white to subtle off-white, cream coloration.
The dark base colors in juvenile Long-taileds show a range of grays and browns, with some individuals showing a distinctive light copper-brown. The average dark is probably paler than in other jaeger species.
Some Long-tailed juveniles have distinctive pale and desaturated heads and bodies, more often than other jaeger species as juveniles.
At a distance, many juvenile Long-taileds appear subtly paler than other jaeger species, from dark gray to muted brown.
Part III - The “cinnamon” Parasitic.
The most talked about field mark for juvenile Parasitic, as it should be, is the “cinnamon” color present in many individuals. The saturated yellow to yellow-orange color in the bird above may never be shown by Long-taileds, and is only very rarely approached by Pomarines–so it is good to know! As far as silver bullet field marks go in jaegers, this is about as good as it gets.
Relevant here, it is the pale highlights that show this color. Things are not that simple, however, as the pale highlights in juvenile Parasitics are also highly variable and regularly approach the coloration in the other two species. It is useful to remember that the pale areas in Parasitics are usually more intensely colorful than in Long-taileds, particularly in the axillaries and undertail coverts, regardless of the specific hue they show.
The dark bases in Parasitics, e.g. the upperparts, dark areas of the body and head, show practically complete overlap with the other species–chestnut brown to dark gray-brown and blackish in the darkest individuals.
This photo is probably benefiting from some saturation, but it highlights both the distinctive yellow-orange color in Parasitics and the regions that show this color most intensely. Note that the areas that are white or off-white in Long-tailed are here showing the same yellow-orange color.
Parasitic Jaeger - James MacKenzie - 21 Sep 2021 - Comox-Strathcona, British Columbia, Canada
The uppertail, lesser coverts at the leading edge of the wing, and the collar around the neck show the most intensely colorful pale highlights in Parasitics, if they show color at all. These regions often show color even when the pale highlights in other areas are not obviously colorful.
This bird has darker bases in the upperparts, but is still showing "the color" in the most obvious regions (hindcollar, leading edge coverts, and uppertail). Note the "pale blaze," a small region of pale feathers at the base of the upper bill–this is a nearly diagnostic marker of Parasitic which normally shows in older birds, but can often show up in juveniles.
Parasitic Jaeger - Nick Bonomo - 10 Sep 2019 - Middlesex, Connecticut, United States
The intensity of color shown in the pale highlights here should not be shown by Long-tailed, while the color in the dark bases could easily be shown by that species. With increased distance, the uppertail is often the last region to appear noticeably colorful. Note how the color in the uppertail is subtly more peach than the more yellowish hue to the wing coverts and collar–a typical pattern in juvenile Parasitics, and maybe all jaeger species. This individual also shows the "pale blaze" visible at the base of the upper bill, and the pale area at the underside of the primaries shows a connected crescent shape without any dark notch on the inner edge of p10.
Parasitic Jaeger - Zach Westfall - 11 Sep 2022 - Metro Vancouver, British Columbia, Canada
The pale highlights here are a more peach hue, more towards red and away from yellow. The brown color in the dark areas is extensive in the head and chest. This individual closely resembles many Pomarine juveniles in overall color (see next section).
Parasitic Jaeger - Keegan Corcoran - 19 Sep 2021 - Forsyth, Georgia, United States
The pale highlights here are much less saturated, and much closer to the subtle off-white hue shown in a number of Long-taileds. This is not the "cinnamon" hue seen in many juvenile Parasitics. Another angle reveals some key differences from Long-tailed:
The pale barring on the undertail and underwing coverts show obvious color, something that should not be seen in Long-taileds. The solid shape and large size of the pale bases to the primaries is also more consistent for Parasitic, as are the jagged dark marks in the axillaries (forming more even bars in Long-tailed). The fine details of the central tail feather shape are visible in this close-up.
Parasitic Jaeger - Caleb Putnam - 30 Oct 2016 - Allegan, Michigan, United States
I want to pause here to consider how this last bird might appear at a distance. The color features of this bird, with only subtly colorful pale highlights, brown bases to the upperparts, and the overall pale head and body, could easily approach the appearance of a Long-tailed at a distance. Make it an unusually small individual on a windy day, and the situation is ripe for mislabeling this as a Long-tailed at a distance.
Perhaps aided by lighting or photo effects, this color impression shown by this individual could easily overlap with that of a Long-tailed. The pale areas in the uppertail show only subtle coloration. Points for Parasitic include the long, irregular dark marks in the uppertail coverts, the shape and size of the underwing flash (seen better in other photos), and the overall shape.
Parasitic Jaeger - Jeremy Coleman - 15 Oct 2017 - Barnstable, Massachusetts, United States
This individual is fairly desaturated overall, with only subtle peach color in the axillaries and undertail coverts. Note the pale blaze, long neck, size and shape of the underwing flash.
Parasitic Jaeger - Adam Jackson - 16 Sep 2018 - San Bernardino, California, United States
Individual with more cream-colored pale highlights. The undertail coverts, which appear white here, show a little more color in other photos of this individual. Note the pale blaze at the base of the upper bill.
Parasitic Jaeger - Ron Pozzi - 13 Sep 2014 - Placer, California, United States
An example from last section. Compare to the previous Parasitic. Note the similar overall color palette, and also that this individual shows noticeable pale tips to the primaries, a trait more often found in juvenile Parasitics. The shorter neck, subtly lighter copper-brown hue to the dark bases in the upperparts, and clear white undertail barring are in line with Long-tailed.
Long-tailed Jaeger - Brent Bomkamp - 31 Aug 2024 - Suffolk, New York, United States
In some darker individuals, the pale areas are decreased or absent, as in the uppertail coverts here. The chestnut-brown in the dark areas of this individual can be shown by all three jaeger species as juveniles.
Parasitic Jaeger - Santiago Caballero Carrera - 2 Oct 2022 - Cádiz, Andalucía, Spain
This darker individuals show reduced pale areas in the axillaries, while the pale areas that are present show more intense color than is expected in Long-tailed. Note the greatly reduced underwing flash that can occur in some dark juvenile Parasitics.
Just to show how pale highlights and dark bases can work independently, and why Parasitics rule, is this individual with distinctive yellow-orange highlights and nearly black bases to the upperparts. By the way, the "upperwing flash" shown by this bird could refer to either the pale primary shafts, or the pale bases of the primaries themselves, which are not visible on the upperwing of every individual. This distinction is almost always overlooked–both are used as supporting field marks for Parasitic.
Color and distance in Parasitic.
With many individuals showing colorful pale highlights and dark pigment in the dark areas, Parasitics will tend towards a dark blackish appearance at a distance compared to the dark gray or muted brown in Long-taileds. Next time you see a Cliff Swallow, watch how the “pale rump,” which is actually a colorful orange, turns dark at a distance before the white underparts. It is also useful to compare the small jaeger species to distant juvenile Herring Gulls. At a distance, most juvenile Parasitics look closer to the black shade of an adult cormorant than the paler gray-brown of a well-lit Herring Gull, while Long-taileds at the same distance and lighting can appear quite similar to the gull in hue and value. Those tricky Parasitics with desaturated highlights can certainly mimic the color impression of a Long-tailed at a distance or in low light, so it is far from a clear cut field mark.
Importantly, the distinctive yellow-orange hue in many Parasitics can still be seen in the uppertail at a modest distance or with good lighting.
Part III recap.
The pale highlights in some Parasitics show a distinctive yellow-orange hue, while other individuals show highlights that are similar to the other two jaeger species.
Most Parasitic juveniles show more colorful pale highlights than Long-taileds.
At a distance where plumage details are lost, many juvenile Parasitics will look darker black than many Long-taileds. The colorful uppertail can often be picked up at a distance with good lighting.
Part IV - The steady Pomarine.
For whatever reason, juvenile Pomarines appear to buck the trend in juvenile jaegers for intense variation in appearances. There are certainly oddballs, mostly in the form of intensely dark or pale individuals, but these are harder to come by. As with the other species, it is helpful to consider the pale highlights as separate from the dark base color in each feather group. In juvenile Pomarines, the pale highlights almost always show a narrow range of peach hues, while the dark bases are dark brown to dark gray.
P0marine juveniles consistently show a dark, somewhat reddish-brown hue in the dark bases throughout.
The pale highlights are typically colorful in most of the same areas that are colorful in Parasitics, with one notable exception: the pale barring in the axillaries (the armpit) is mostly white in Pomarine juveniles. Other general field marks for Pomarine include a large underwing flash, pale bases to the first row of underwing primary coverts (the “double flash”), pale orbital, plain head without vertical streaking, large head with relatively small eye and large dark lore. Note again how colorful these birds can be, similar to Parasitic, but with a different hue than the distinctive yellow-orange in some Parasitics.
The pale barring on the underwing is practically white, while the barring on the undertail is peach-colored.
Pomarine Jaeger - David Leatherman - 27 Oct 2023 - Weld, Colorado, United States
This individual has more desaturated dark areas, less red but still brown. This may be a fairly common variation. The undertail coverts show subtle peach pale areas, while the axillaries have nearly white pale areas, although these are small and hard to see due to the large dark marks there (typical for Pomarine). Aside from color, the plain face without streaking, pale orbital arcs, large round head, and large, parallel-edged bill are all features of many Pomarines.
Pomarine Jaeger - Brandon Holden - 5 Nov 2005 - Simcoe, Ontario, Canada
The color in the pale barring here is subtle and mostly white in appearance, but it is still slightly more colorful in the undertail than the axillaries. The body and head show the typical reddish-brown hue as in other examples. The large head and body, large dark marks on the axillaries, and "double flash" on the underwing are good markers for Pomarine.
Pomarine Jaeger - Ryan Sanderson - 28 Nov 2011 - Lake, Indiana, United States
Dark reddish-brown body and head, peach color to the pale undertail barring, white pale areas in the axillaries, double flash in the primaries.
Pomarine Jaeger - David Wheeler - 21 Nov 2020 - Salt Lake, Utah, United States
But remember this Parasitic.
This individual from the last section shows how some Parasitics almost perfectly match the color scheme shown by juvenile Pomarines. Here are some key differences that point to Parasitic in this individual:
a relatively small, triangular head
“pale blaze” - an area of pale feathers at the base of the upper bill
colorful pale barring in the axillaries (mostly white in Pomarine)
lack of “double flash,” although this is not shown by all Pomarines
the shape of the dark marks in the axillaries, a feature that will be described in detail in an upcoming publication
These features are easy to see up close like this, but the similarity in overall color to Pomarines means that like so many other field marks in jaegers, the color scheme shown by most juvenile Pomarines is only a supporting field mark at best.
Pale Pomarines like this are rare, but they do exist and there is at least one documented record from Lake Michigan. The colors in this bird more closely resemble the yellow and yellow-orange of juvenile Parasitics in places, like in the head and collar. With close photos like this, one can make out many of the Pomarine field marks discussed above:
white pale areas in the axillaries
large, rounded head
relatively small eye, pale orbital, large dark lore
plain head without streaking or contrasting dark cap
the shape of the dark marks in the axillaries (details that are soon to be published)
Having never seen a bird like this, I wonder how it would appear at a distance, and how much it might resemble some chunky Parasitics. Also note how this individual has neglected to show a “double flash.” I also haven’t seen many juvenile Pomarines at a distance, so can’t speak to what impressions they have color-wise. I would have to assume that they are not much different from Parasitics in this regard, and are probably also darker than many Long-taileds at a distance for the reasons explained previously.
Part IV recap
Most juvenile Pomarines show a consistent and recognizable color palette, with reddish-brown in the dark areas, and peach in the pale highlights EXCEPT for the axillaries, which are mostly white.
Some Parasitics closely match this color scheme, but they are much more likely to show color in the axillaries.
Paler Pomarine juveniles are rare, but they can show more yellow-orange pale highlights in places, leading to Parasitic-like impression.
Part V - specimens and thoughts on melanin types.
From left to right: Pomarine, Parasitic, and Long-tailed juveniles. Specimens from the Field Museum.
A close-up of the uppertail region in the previous specimens. Note the color in the pale highlights: peach in the Pomarine, intense yellow-orange to pale yellow in the Parasitic, and dull cream in the Long-tailed. Also note the lighter brown in the dark areas of this Long-tailed.
Three examples that portray the same message as the “key three examples” at the beginning:
Pomarines juveniles usually show this hue of peach color in their pale highlights.
The hue and intensity of yellow-orange in the middle bird is almost exclusively shown by juvenile Parasitics.
Long-taileds show white to off-white pale highlights, less intensely colorful than the other species.
The color of the pale highlights here are similar to those in the previous photo, yet these are all Parasitic Jaegers. Note the shape of the dark marks in uppertail coverts, which create no even barring like the other two species, a detail that is probably not useful in most field settings.
The above photo shows that while the pale areas in some Parasitics can show the distinctive yellow-orange hue (middle bird), they are also highly variable and can approach the coloration of the other two species. The other two jaeger species are not nearly as variable in this aspect of plumage.
Some quizzes. Answers in the descriptions below each photo.
The bird on the left shows intense, yellow-orange coloration in many pale highlights that is a strong indicator of juvenile Parasitic; the color in the uppertail region in particular should be visible at a distance, with good lighting. The bird on the right shows a largely pale head with more subtle coloration, with cream-colored pale highlights throughout, and a paler brown dark base color to the upperparts. At a distance and with good lighting, the upperparts of this bird will appear gray-brown or muted brown, which when combined with the noticeably pale head (and likely a pale body), are supporting field marks for Long-tailed. Flying side by side at a distance or in poor lighting, the bird on the left should appear comparatively dark and blackish next to the bird on the right, which might appear a paler, dark gray. Parasitic left, Long-tailed right.
The dark bases in the right bird are slightly more pale, and the hue of the dark brown there is nudged slightly more towards the orange end of the spectrum. The pale highlights in both birds could fit for Long-tailed, while those on the right bird might be a bit dull for Parasitic. These individuals are similar enough that there is likely no difference in coloration visible here that would be helpful in field identification. Parasitic left, Long-tailed right.
Specimens from the Chicago Academy of Sciences.
One of the main impetuses for this blog is that “cold-toned” and “warm-toned” as general field marks fail to pin the right bird as a Long-tailed, while the copper-brown hue it shows is likely common–maybe even unique–in Long-taileds, enough so that the hue should be recognized as a field mark in its own right. Aside from the color of the dark bases, the pale highlights in this individual are relatively colorless, with subtle cream-color to the head and uppertail barring, all supporting features for Long-tailed. The individual at the left shows less colorful pale highlights in some areas, while the highlights in the wing coverts and uppertail coverts show a saturated yellow to yellow-orange that is distinctive of juvenile Parasitic. At a distance, the difference in overall darkness of these two individuals should be apparent. The light copper-brown in the right bird, muted at a distance to a light gray-brown by the colorless pale highlights, should put one in mind of Long-tailed. Parasitic left, Long-tailed right.
What’s up with the light brown in some immature Long-taileds?
In learning about the topic of coloration in jaegers, I stumbled down the rabbit hole of the two types of melanin, eumelanin and pheomelanin. At varying concentrations, eumelanin appears dark black to pale gray, i.e. “grayscale” tones, but it can also create brown and reddish brown at medium concentrations (the reasons for this are complicated and not covered here). Varying concentrations of pheomelanin, on the other hand, appear light cream, to yellow, orange, and reddish-orange at higher concentrations.
Examples of birds with color known to be primarily due to pheomelanin.
Common Quail - Christoph Moning - 3 May 2012 - Erding, Bayern, Germany
Oilbird - Sam Woods - 28 Nov 2017 - Pichincha, Ecuador
Rock Pigeon (Feral Pigeon) - Barbara Wise - 20 May 2022 - San Diego, California, United States
It is important to note that eumelanin and pheomelanin are often present in combination, and they show some overlap in the colors they can produce, so in many cases it is speculative to say which pigment is responsible for which color. At the extremes of black and gray, however, it is almost certainly eumelanin that is present.
Right to left: Pomarine, Parasitic, Long-tailed adults. Specimens from the Chicago Academy of Sciences.
In pale-morph adults and sub-adult jaegers, it is well-known that Long-taileds have more pale and gray upperparts (back, upperwing and tail coverts), while Parasitics are midway in value, and Pomarines have the darkest upperparts. It is likely that the differences in these regions are attributable to different concentrations of eumelanin due to their more grayscale, desaturated tones.
It is also true of many bird species that immatures produce more pheomelanin than adults (Guay et al., 2017). This may also be the case in immature jaegers.
Left to right: Pomarine, Parasitic, Long-tailed juveniles. Specimens from the Chicago Academy of Sciences.
The color in pale highlights of the above juveniles, as well as the more orange-brown hue in the dark areas of the Long-tailed, hint at a possible contribution of pheomelanin. If the following conditions are met:
Juvenile and immature jaegers produce more pheomelanin than adults;
Long-tailed jaegers have a general tendency towards decreased eumelanin concentrations in certain areas (e.g. the upperparts) compared with the other jaeger species;
The tendency for decreased eumelanin concentrations in Long-taileds can manifest in certain immature individuals;
then the light brown hues in juvenile Long-tailed may be the result of decreased eumelanin concentrations, with a relatively larger proportion of pheomelanin contributing to the coloration. Although perhaps less obvious, applying the same logic to Pomarines leads to another possibility: the more reddish-peach highlights in most juvenile Pomarines may be caused by a comparatively higher proportion of eumelanins, as the same highlights in the other juvenile jaegers tend towards more cream, yellow, or yellow-orange coloration. This would match with the tendency for higher eumelanin concentrations in certain areas of adult Pomarines, i.e. the upperparts and undertail.
Again, the explanations here are speculative, but they provide some testable hypotheses. Other bird species show similar tendencies across ages in the pigmentation of certain feather regions. For example, Lesser Black-backed Gulls show comparatively dark upperparts even as juveniles when compared to the juveniles of more paler-backed gull species.
Part V recap
The “gray-brown” description of Long-taileds is well-known, but some individuals show a distinctive orange-brown in the dark areas that does not fit well with the idea of “cold-toned.”
A tendency for decreased eumelanin concentrations in certain areas of Long-taileds could explain both cases, with higher levels of pheomelanin in some immatures creating more orange-brown hues, and lower levels of pheomelanin creating more pale-gray tones.
The same pigment, i.e. pheomelanin, may be largely responsible for the “warm-tones” in the pale areas of juvenile Parasitics, while increased eumelanin concentrations in the dark areas of this species would lead to darker, more reddish-brown coloration.
Even higher concentrations of eumelanin in Pomarines might be responsible for the more peach-colored pale highlights in most juveniles of this species.
Thanks to Ben Marks at the Field Museum, Dawn Roberts at the Chicago Academy of Sciences, and Yasmeen Erritouni for the info on melanin.