The creative study of birds through illustration, photography, and writing

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Dark Morph Red-tailed Hawk Illustration and A Note on Plumage Polymorphism in Raptors

by Bryce W. Robinson


Dark morph western Red-tailed Hawk (Buteo jamaicensis calurus). 20×30″ Gouache on watercolor board. Prints available in the shop!

Ornithologists have long been after explanations for why we see plumage polymorphism (multiple different color types) within a single bird species. There are countless examples of polymorphic species, the most belonging to the group referred to as diurnal raptors (members of Accipitriformes and Falconiformes), where polymorphism has convergently evolved multiple times.

There are two main hypotheses that aim to explain the mechanisms behind polymorphism in raptors, both of which assert that color morphs are a result of predator-prey dynamics. I’ve listed these below, along with an explanation of each:

Apostatic Selection Hypothesis: Color morphs result from predator-prey dynamics, where keen prey readily recognize common color morphs. Less common color morphs then have the advantage, where prey do not as readily recognize the threat giving the predator the edge it needs to be successful. Under this hypothesis we would expect polymorphism to be more prevalent in species that hunt in static environments. Tested many times, this hypothesis has not been shown to best explain the occurrence and maintenance of polymorphism in raptors. An additional complication is that we would expect the frequency of morphs to drift over time, much like the classic predator-prey dynamic patterns we see (consider Hudson Bay Trading Company data for Lynx and Hare populations in the 1800’s). This is because as one color type gains the advantage, they become more successful and vice versa until that color type is then the most common and the pattern begins to swing the other way. To my knowledge, I am not aware of this occurring in raptors.

Niche Variation (Disruptive Selection) Hypothesis: Color morphs result from predator-prey dynamics, where variable environmental conditions provide success to particular color morphs. This variable success may differ between populations, or species creating differing frequencies of color morphs, i.e. the ratio between dark and light birds. For instance, consider the ratio between color morphs in Red-tailed hawk (Buteo jamaicensis), where dark birds in the west (B. j. calurus) are the less common morph, but dark birds in the boreal north (B. j. harlani) are the most common morph. Ambient light conditions may favor one morph over another, depending on the time of day, weather, or if a bird hunts in a variety of habitats such as the bright open lands and dark understory. Thus, morphs are resultant from niche partitioning into variable environmental conditions. Supporting this notion are observations within a species where particular morphs differ in their foraging strategies. This Niche Variation hypothesis has been supported to best explain plumage polymorphism in raptors many times, and for various reasons. However, I’m still left with some questions for how this hypothesis fits in a few model species.

For a full understanding of the difference between these hypotheses and why disruptive selection best explains polymorphic plumages in raptors, see Tate and Amar (2017), Galeotti and Rubolini (2004), Roulin and Wink (2004), and Fowlie and Kruger (2003).

The Red-tailed Hawk is a widespread and common, highly polytypic species that exhibits varying degrees of plumage polymorphism throughout its range. For instance, B. j. borealis in eastern North America has relatively no polymorphism, whereas  B. j. calurus in western North America exhibits wide plumage variation unrelated to sex. Additionally, B. j. harlani in the boreal north is also polymorphic but differs from calurus where the proportion of color morphs is opposite, dark being the most frequent phenotype (as mentioned above).

In this species, the two hypotheses explaining the development and facilitation of polymorphism seems to fit. However, there remain a few questions unanswered.

Do different morphs of Red-tailed Hawk show different foraging strategies? I thought of this idea when I was considering the maintenance of polymorphism in calurus. I think this is an interesting question, and as far as I’m aware has not been investigated. It is a relatively simple undertaking to discover the answer, so I look forward to someone taking the opportunity to investigate.

In the interior west, I question the regularity of dark morphs and polymorphism as a result of varying environmental conditions because of the lack of large forest and this hawk’s hunting strategy even if these forests were prevalent, the less variable environmental conditions related to weather, and the fact that many dark morphs are resident rather than migratory, so likely hunt in the same geographic area year round. Still, their prevalence in this population could be an artifact of gene flow from coastal populations of the north west and those of the forests of British Columbia. I’m only speculating here, and perhaps I’m missing something so I welcome discussion on the matter.

Why is there relatively no polymorphism in borealis? This taxon frequents forest edge, in areas that seem to either not differ or have higher levels of variation in environmental conditions when compared to it’s polymorphic relatives. This is a burning question and I would love to have a discussion with anyone who has ideas.

Population size was suggested to be the main correlative factor involved in the presence of polymorphism, because larger populations experience more variable conditions, have higher mutation rates, and thus a higher chance for the development and maintenance of these traits. This makes sense, but still leaves me questioning why there are no dark borealis especially considering their presence west of the edge of the eastern hardwood.

Of course, patterns that we see throughout the range of the highly polytypic and polymorphic Red-tailed Hawk beg the curious to dive into research aimed at understanding why we see geographic patterns in plumage types (consider harlani, kriderii, fuertesi, etc.). Some are intuitive and likely resultant of the obvious explanation of environmental factors, i.e. taxa that inhabit more open habitats are lighter. However there remains puzzlers like borealis.

Apart from Red-tailed Hawk there are others that leave me questioning. Although plumage polymorphism in the Gyrfalcon may fit the Niche Variation Hypothesis to some degree, I’m not convinced. Why do we see polymorphism, and the patterns of such, in this species?

Always fun to consider is polymorphism in other taxa apart from raptors. One such group are also predatory, the Jaegers (Stercorarius spp.). Why are Parasitic and Pomarine  polymorphic, while Long-tailed is not? I haven’t entered the literature to explore this one quite yet, but it is a nice question.

I’ll probably add to and amend this list of questions over time. Ideally, I would like to amend with explanations or answers. If anyone reads this post that has ideas, answers, or additional queries then please feel free to engage with me and discuss. There’s always something new to consider.

Referenced Literature:

Fowlie, M. K., and O. Kruger. 2003. The evolution of plumage polymorphism in birds of prey and owls: the apostatic selection hypothesis revisited. Journal of Evolutionary Biology 16:577-583.

Galeotti, P., and D. Rubolini. 2004. The niche variation hypothesis and the evolution of colour polymorphism in birds: a comparative study of owls, nightjars and raptors. Biological Journal of the Linnean Society 82:237–248.


*An important note on terminology: the term ‘phase’ is widely misused to refer to color morphs of polytypic species. I implore the community to eliminate the use of this term  in speech and in publication, because it is fundamentally incorrect. Phase refers to a temporary or ephemeral state, one that changes over time. A species that is polymorphic such as the Red-tailed Hawk does not have a ‘dark-phase’, because dark birds remain dark throughout their lives. Their plumage classification does not change. Please do not use ‘phase’ when referring to polymorphic species. Use the term color ‘morph’ or ‘type’.


Idaho’s Endemic: The Cassia Crossbill (Loxia sinesciuris)

by Bryce W. Robinson


Cassia Crossbill (Loxia sinesciuris) and Rocky Mountain Lodgepole Pine (Pinus contorta latifolia). 11×14″ gouache on watercolor board. From top: Adult male, adult female, and juvenile. Original illustration donated to Golden Eagle Audubon Society in Boise, Idaho.

This article is an overview and summary of the Cassia Crossbill (Loxia sinesciuris), including it’s evolution, life history and distribution, taxonomic status, and conservation status and threats. Additionally, I’ve listed many resources available to get to know this excellent example of the processes of evolution in effect. This is a fluid post, meaning I will revise and add as I gather more material and information (such as photos, recordings, etc.) or as information becomes available through further studies of the species. My aim is to provide a resource from which curious birders and naturalists can delve into learning about this incredible species as well as provide a resource for seeking it out in southern Idaho. Any thoughts, suggestions, revisions, or additions are welcome.


The Cassia Crossbill represents our continued refinement of understanding the natural world. How peculiar it seems that in the 21st century, while beginning to recognize and understand incipient speciation in some taxa, we are also finding well established independent evolutionary lineages that have until now gone unnoticed. Even more peculiar is that the Cassia Crossbill is certainly not restricted to a place where ornithologists and bird enthusiasts rarely visit. They breed in areas with extensive road networks and occupy ranges with nearly year round access. My point is that we certainly haven’t missed them, we have only overlooked them. I don’t consider this an embarrassment, I find it extremely exciting. How many other patterns such as this have we yet to notice?

This bird’s name, Cassia Crossbill (Loxia sinesciuris) is loaded with information and in my opinion aptly applied nomenclature. I commend those involved with choosing these names. Below is an explanation of this loaded nomenclature, as well as an overview of the evolution of the species, it’s distribution, what separates it from the Red Crossbill (Loxia curvirostra), why it was finally elevated to species level by the AOS Check-list committee, and threats to its future in our ever-changing world. I’ve also included a list of resources from which I’ve extracted the information I present here.


The first aspect of the Cassia Crossbill’s loaded name is its species epithet, sinesciuris. Although I already knew about the unique situation that gave rise to the divergence of this species, I hadn’t put much thought to this name. I now realize the etymology of the word – “sin” meaning “without”, and “sciuris” referring to squirrels. So, the scientific name means “Loxia without-squirrel” – an excellent transition into the proximate reason this species developed in this small area in southern Idaho.

The South Hills and Albion Mountains in southern Idaho are unique in the respect that they lack a primary mammalian seed predator, the red squirrel (Tamiasciurus hudsonicus). Rocky Mountain lodgepole pine (Pinus contorta latifolia) in this region are thus relieved of this predatory pressure, however one important seed predator occurs in the region, the Cassia Crossbill. The crossbill fills the void of the squirrel, spurring a unique relationship that has resulted in the divergence of this crossbill type from the Red Crossbill complex (Benkman 2009).

From this absence of squirrels, lodgepole pine in this region has been relatively free of pressures on one aspect of the trees biology, serotiny. Serotinous cones are cones that remain closed until they are heated by fire. Because red squirrels are a selective agent against serotiny, the frequency of serotinous cones in South Hills and Albion Mountains has increased due to the squirrels’ absence. This has resulted in a large seed bank that is utilized by the Cassia Crossbill, an important aspect in it’s evolutionary trajectory.

A relatively stable and abundant food resource has given rise to a unique life history strategy of this crossbill when compared to the Red Crossbill. The Cassia Crossbill is sedentary. Furthermore, the crossbill as a primary predator and the pine as a primary food source are coupled in their life history, and thus locked in an ‘evolutionary arms-race’ where one species develops ‘armaments’ and defenses to lessen predation pressures whilst the other develops ‘weapons’ and tools to better access this resource, the seeds. This coupled relationship acts in selection and causes divergence of traits, such as we see between these two taxa where the crossbill has developed a larger bill relative to Red Crossbill types. From this relationship, a new species of crossbill has arisen.

LIFE HISTORY AND DISTRIBUTION – What separates L. sinesciuris from L. curvirostra?

The other part of the Cassia Crossbill’s loaded name refers to its relatively minuscule range, when compared to that of its sister species the Red Crossbill. This species occurs in only two counties, the core being in Cassia County, a truly uncommon trait for taxa in North America (See Fig. 1, and also be sure to check out the eBird data for this species).


Morphologically, Cassia Crossbill’s have a larger bill than Red Crossbill, and average larger in body mass (Benkman et al.  2009). Otherwise, it is difficult to distinguish the two sister species from appearance alone and in fact judging bill size in the field is entirely unreliable.

A better distinguishing characteristic, and one that is actually quite discernible is the call type. Cassia Crossbills have a much dryer and sharper call note than other Red Crossbill types, a difference that can be quite distinctive when heard in the field. Additionally, observers can record crossbills and look at the spectral characteristics of the call notes to identify between Cassia Crossbill, and all other Red Crossbill types (spectrograms to come soon from my own recordings). eBird has published an excellent article detailing each call type, their identification, the distributions of each, and some brief information on their biology (Young and Spahr 2017).

Additionally, the songs of the Cassia Crossbill differs from Red Crossbill in consisting of more buzzy notes (rather than whistled), and have more repetitive syllables (Benkman et al. 2009). I hope to record some songs later this year (2018) and include a spectrogram here.

Cassia Crossbill range also hosts two Red Crossbill types on occassion (Types 2 and 5). Although these taxa occur in sympatry, Cassia Crossbill mate assortatively at an extremely high rate, resulting in reproductive isolation which is a key mechanism for divergence.

The Cassia Crossbill also differs from Red Crossbill through shifted and set phenology of life history events. It has a more seasonal breeding strategy relative to the sporadic breeding nature of the nomadic Red Crossbill, where their breeding initiates at relatively the same time each year (Benkman et al. 2009). Additionally and related to their sedentary lifestyle and regular breeding cycle, the Cassia Crossbill molts at the same time in the late summer until early fall each year (ibid).



In the 58th supplement to the American Ornithological Society’s Check-list of North American Birds (Chesser et al. 2017), the committee approved the nomination to elevate Cassia Crossbill to species level based on high levels of reproductive isolation, and genomic differences.


Because of the small and restricted range of this species, it’s particular life history, and the predicted impacts of climate change in the region, the Cassia Crossbill has an uncertain future. Compounding these impacts are threats to lodgepole pine such as shifted fire regimes and the mountain pine beetle (Dendroctonus ponderosae), which have the potential to extirpate lodgepole pine from south central Idaho (see Benkman 2016 for a discussion of these threats).


For a brief guide to finding and identifying Cassia Crossbill, visit:



Singing Coastal Cactus Wren Perched on Coastal Cholla

by Bryce W. Robinson



The Cactus Wren (Campylorhynchus brunneicapillus) is a polytypic wren (Family: Troglodytidae) that occurs in the arid southwest of North America. The species comprises five subspecies (Following Rea and Weaver 1990). Birds on the coast of southern California differ in appearance slightly from interior groups, primarily in being paler on the flanks where they have less rich and warm tones. The taxonomy of this coastal group has been in flux, but it is currently recognized by Clements, Howard & Moore, and others as C. b. sandiegensis.

A few years ago, I had the pleasure of getting to know this species well while working with nesting birds along the I-10 corridor in California. I can still hear their iconic rattle song as they sing atop cholla in the intense heat of the Sonoran desert. Their nests also stick in my memory. Often in dense and formidable cholla, the species construct a tunnel nest out of grass. These are some of my favorite nests I’ve encountered in all of my time in the field.

I had the pleasure of illustrating this bird for silent auction at the Sea and Sage Audubon Society’s annual benefit dinner. Hopefully it generates some funds for them and finds a good home.

If you like this image and want a print, you can get one HERE.

Referenced Literature:

American Dipper (Cinclus mexicanus) Illustration with a Note on the Evolution of Cinclidae

by Bryce W. Robinson


American Dipper (Cinclus mexicanus). 11×14″ gouache on watercolor board. 

I enjoy supplementing each illustration I do with a bit of deeper discussion pertaining to the subject at hand. Because I’m beginning more in-depth study of evolutionary history and relationships in birds, I’ll give a brief synopsis of our current understanding (thanks to Gary Voelker) of the evolution of the five species belonging to the dipper family (Cinclidae) and the origins of the American Dipper (Cinclus mexicanus) to complement this illustration.

Although the phylogenetics (genetic history) of dippers was published in 2002, and thus utilized mitochondrial data to inform the inferences I’ll lay out below, I suspect applying new techniques wouldn’t change the outcome much. That’s just a hunch, and revisiting the phylogenetics with next generation sequencing methods is certainly warranted and needed.

Mitochondrial data shows two important evolutionary points:

  • Dippers (Cinclidae) are most closely related to Thrushes (Turdidae).
  • Dippers originated in the old world, where they diverged and colonized the new world later (~4 million years ago).

Fun and interesting information for understanding dipper diversity.

If you are a dipper lover and you’d like a print, you can purchase one here:×14-limited-giclee-print-american-dipper

Referenced literature:

Voelker, G. 2002. Molecular phylogenetics and historical biogeography of dippers (Cinclus). Ibis 144(4):577 – 584.