by Bryce W. Robinson
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.
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’.