Monday, November 30, 2015

POLYCHROMATISM (colour morphs) in OWLS

This collage of 3 photos of Tropical Screech-Owls (Megascops choliba) from southeastern Brazil illustrates the major colour morphs: grey, brown, and red. Despite the different lighting in each photo, these photos give you a sense of the range of colour variation in this species; however, there is a great deal of subtle variation and many “intermediate” forms (some people consider the brown morph to be an intermediate between grey and red in fact) and there are also greyer and redder birds than shown here.

Polychromatism is relatively common in owls. In fact, approximately one third of owl species exhibit polychromatism and this is amongst the highest percentage of any avian order. Polychromatism exists in several owl genera including Megascops, Otus, Psiloscops, Strix, Lophostrix and Glaucidium (possibly also in one or two Tyto and Bubo species). It is important to recognize that these different colour morphs are not subspecies – they occur together in the same population in the same habitats and it is common to see two owls of the same species together showing different colour morphs or offspring of the same pair with different colour morphs. Colour morph differences are not the same as clinal colour differences or subspecific differences, e.g. in the Great Horned Owl in North America, one finds a climate-related cline of paler birds in the cold, dry north; more rufous birds in the warmer but humid east; and very dark, blackish birds along the very wet west coast (in this case there is geographic separation along an environmental gradient with only limited overlap). 

Some evidence suggests that colour morphs in owls are determined by Mendelian inheritance; however, the large number of intermediate forms occurring in many species makes this a little more complicated. In the Eastern Screech-Owl at least, the red morph appears to be genetically dominant (in much the same way that dark eyes are dominant and blue eyes are recessive in humans), although I am not certain if this is also true in the Tropical Screech-Owl.

There are some interesting consequences of polychromatism in owls. For example, in the Eastern Screech-Owl, the red morph is much more prevalent in warm and humid climates, whereas grey dominates in cold and dry climates. This is believed to be related to the properties of melanin (pigment). In owls, plumage colour appears to be related primarily to concentrations of phaeomelanin and eumelanin. Melanin imparts rigidity to feathers (this is why so many gull species have dark wing tips for example) and makes them more resistant to abrasion from dust, but it also gives better thermoregulation. In cold and dry climates therefore, grey birds have higher survival. In Manitoba, Canada, where I studied Eastern Screech-Owls at the northernmost and coldest part of their range, less than 1% of the population of Eastern Screech-Owls is red. In these very cold climates, red females are more likely to survive than red males because they are larger and hence have better thermoregulation due to their surface area to mass ratio. Elsewhere, red morph Eastern Screech-Owls have exhibited higher mortality in very cold winters. Nonetheless, red plumage has some advantages in warmer, humid climates. Red is a weak wavelength that is easily dispersed. In dim light when owls are active and/or in very humid conditions, red is actually very cryptic. If you have ever tried to watch a red morph owl fly in a dimly lit forest you may have noticed how difficult they are to see. Fred Gehlbach wrote about this in a book on Eastern Screech-Owls published in 1994.

As for the reasons polychromatism exists, this is largely unknown, although different hypotheses have been advanced.  The most prevalent idea is that fluctuating environmental conditions benefit one morph or another at different times and so two or more morphs can therefore survive in the species in the long-term. This hypothesis hinges on the concept of differential survival between morphs. A study in Finland of Tawny Owls showed that grey owls had more offspring than brown owls and that, at least for the grey males, more of their offspring survived to breed (produce “population recruits”). This provides strong evidence for differential survival and shows that there is selection pressure on colour morphs.

Interestingly, there is some evidence for assorted mating amongst the colour morphs in Eastern Screech-Owl; however, the Finnish study found no evidence of assortive mating in Tawny Owls. Assortive mating in this case means that individuals preferentially select mates that are of a different morph than themselves. This behaviour might be a result of differential survival and be a form of minimizing risk or “not placing all one’s eggs in the same basket”, e.g. if the climate gets wetter or colder at least some of an owl’s offspring will have a better chance of survival. Nonetheless, there may be more to it than that; for example, one fascinating finding is that lighter and darker Barn Owls differ in the composition of their prey species, suggesting that colour could also be associated with different behavioural patterns.  Colour can also signal important information to birds including clues about an individual’s auto-immune system. One study of Barn Owls showed that the offspring of “spottier” females (i.e. females with more dark spots on their feathers) had better parasite resistance. Colour may in fact be an influential factor in sexual selection for both males and females (i.e. “mutual mate choice” as opposed to just “female mate choice”). Grey morph Tawny Owls were found to have a higher immune response than red owls in Italy. In Swiss Tawny Owls, darker plumage was associated with a stronger auto-immune response; however, lighter coloured individuals were better at retaining body mass. This suggests that different strategies may co-exist. 

1 comment:

  1. Just learned again about the difference of this owls. Thank you so much for sharing! :)

    Bird Watching Philippines


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