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. 

Brazilian Nightlife: Owls of Southern Brazil

I had posted previously on Brazilian nightlife (the beautiful owls of southeastern Brazil) but a second visit to the area has given me the chance to update this blog post with some new photos of a few species. For now I will just focus on the owls of southern Brazil, and we will follow taxonomic order, but I may add some nightjars and potoos later. First, I will repeat my brief introduction to the area. 
 

The humid Atlantic Forest of southeastern Brazil is an area of rich endemism and biodiversity. Although not nearly as long or tall as the Andes in western South America, the Serra Do Mar Mountain Range reaches nearly 3,000 m ASL and is separated from the Andean montane forests by a large area of more open habitat types. These open habitat types are the lowland Chaco region (Gran Chaco, in essence a large depression of Bolivia, Paraguay, northernmost Argentina and a small part of western Brazil) and the Pantanal. The atlantic rainforests are also separated from the dense Amazonian rainforests by the intervening belt of relatively open and drier Caatinga and the Cerrado habitats (paler green in map below). On the Google Earth image below this belt of darker rainforest can be seen in the pink oval, although note that it also extends northward a fair distance in a very narrow belt along the coast and (now) patchily inland.

The Atlantic forest, in particular the lowland portions, is a highly endangered ecosystem, possibly now reduced to a mere 7% of its former extent (according to www.atlanticrainforest.org/index.php?page=facts). The endemic bird area of Atlantic forest lowlands is in critical shape (http://www.birdlife.org/datazone/ebafactsheet.php?id=71) though the mountain EBA is not considered quite as severely threatened (due to access limitations). You can view a polygon map of the Atlantic forest on the WWF site at http://wwf.panda.org/what_we_do/where_we_work/atlantic_forests/ although note that much of this area no longer contains intact rainforest. With the already major and expanding urban centers of Sao Paolo and Rio de Janeiro placed in the heart of the Atlantic Forest, it is not hard to understand the pressure that these forests now face.

To give you a little sense of the forest that once covered this rugged coastal region, here are two photos from Intervales State Park, part of an important connection of five protected areas. The first photo shows a view from the Carmo Road and the second shows flowering trees (Ype) around the lake.

To introduce the owls of this area in taxonomic sequence we must start with the Barn Owl family (Tytonidae). The Barn Owl is the only tytonid found in South America and it occurs in this region too. Some taxonomists treat this as the "American Barn Owl (Tyto furcata)" but the IOC still lists this species as Tyto alba.  We start with an image of a roosting adult Barn Owl, followed by a series showing two families of immature (recently fledged) youngsters. As you can see, they are fairly common around human habitation, including urban areas.

Next we move to the Strigidae or “true owl” family. The genus Megascops or “screech-owls” is found throughout the Americas and there are three species in southern Brazil The first is the widespread Tropical Screech-Owl, smallest of the three screech-owl species that occur here.   Tropical Screech-Owls are polychromatic (contain 2 or more colour morphs) so here are photos of the three main morphs of this species (there is quite a range of variation): first grey, then brown then red. I added a second photo of the red morph bird to illustrate it hunting, focused on a sound on or near the ground.  The call of the Tropical Screech-Owl is very distinct and is the best way to distinguish this species from the others!  

There are two screech-owl species that are endemic to the Atlantic rainforest region. The Black-capped Screech-Owl (shown below with its dark crown) has the more northerly distribution of these two similar species, mostly along the coastal forests from Sao Paolo state northward and also somewhat inland in forested areas. The photo below show a rufous morph Black-capped Screech-Owl at Intervales State Park. Note the dark cap of this owl and the large size (noticeably larger than Tropical Screech-Owl). The song is also very different. I couldn’t resist compiling the collage of three photos below showing slightly different postures.

Larger still than the Black-capped Screech-Owl, the range-restricted Long-tufted Screech-Owl, also known as the Santa Catarina Screech-Owl,  has a more southerly distribution within this region and is endemic of the southern portion of the Atlantic rainforest from southeastern Brazil to Uruguay and northeastern Argentina. This is a large screech-owl with a well marked pattern below and a beautiful rich face colour with a dark and thick facial disk. The species gets its English common name from the bushy ear-tufts but these do not show well in these photos (posture relaxed, tufts lowered). The adults in these photos are of the brown morph, which seems to be the most common (grey and red morphs also exist). This series shows a pair bringing a moth and another invertebrate prey item (appears to be a beetle) to a recently fledged chick. If anyone is able to identify either of the prey item please get in touch with me. The third photo shows one of the adults diving off the perch to hunt (it looks as though one leg is still touching the perch but in fact both feet are off the perch, one being mostly visible and the other being already tucked mostly into the feathers). Interestingly the chick shows the same rich facial colouration as the adults.

One of the larger owls of southeastern Brazil is another regional endemic in the Pulsatrix genus (the “spectacled owls”, name for the pale facial marking surrounding the eyes), the Tawny-browed Owl. The Tawny-browed Owl is found in the regions humid forests and also in the regions high elevation Araucaria forests (Araucaria is a genus of coniferous trees from the Southern Hemisphere with a bizarre distribution in only South America and Australasia, including some Pacific islands, and which includes the famous "monkey puzzle tree" of Chile). The rich colour and distinctive patterning of this large owl is reason enough to admire them but their indescribable call, like other Pulsatrix owls, will send shivers down your spine!   At 44 cm, the Tawny-browed Owl is a large owl; however this is the smallest of the three species in the genus Pulsatrix (some authors split it to give a fourth species though not widely accepted). This genus is Neotropical in distribution with the largest species, the Spectacled Owl, being widespread in the region, the Band-bellied being Andean and the Tawny-browed being endemic to the Atlantic forest. Many other owls in the Atlantic forest seem to fear the Tawny-browed Owl or at least they seem to stop singing when there are tawny-brows around. I shared three photos below:

The genus Bubo (large “horned” owls) does not have an endemic species in the Atlantic forest region; however there is a disjunct population of Great Horned Owls here. In this region (as often elsewhere), this species mostly occurs in more open lightly wooded habitats rather than in rainforest proper, including more savannah like habitat such as the “Caatinga”.   Despite this disjunction, they look and sound quite similar to Great Horned Owls elsewhere (voice was a little deeper than I am used to in Canada and eyes more orange). I found this species when I heard the distinctive hissing scream of a begging immature owl. While I was trying to sneak a look at the youngster, this adult flew in right above my head. You can tell that this owl is stressed by the posture, the erect ear tufts, and especially by the raised rictal bristles that reveal the nostrils (when relaxed, the rictal bristles lay flat to cover the nostrils to protect them from dust). I realised this of course as I was raising my lens, so I very quickly snapped three photos and then left the area.

The region also has a Pygmy-Owl who may be endemic – I say “may be” because the taxonomy of this tiny owl is still somewhat controversial. More “generous” (i.e. more likely to split species complex into multiple species) taxonomies consider the East Brazilian Pygmy-Owl, also known as Sick’s Pygmy-Owl  (Glaucidium sicki), whereas others consider this taxon to be part of the more widespread Least Pygmy-Owl (Glaucidium minutissimum) complex. At only 14cm in length, this is one of the smallest owl species in the world. We had to work very hard to get views of this little bird calling high in the canopy!

In addition to the endemics discussed above, there are various other non-endemic species of owls who occur in southern Brazil. I have already given an example above with the comparison of Black-capped Screech-Owl, Long-tufted Screech-Owl and Tropical Screech-Owl. In many cases in fact, the various genera shown here have both a regional endemic and a more widespread species that occur in sympatry in southeastern Brazil. As a general rule, the endemics are the more habitat specialized and the endemics have stronger associations with particular regional habitat types. In the case of the Pygmy-Owls, the widespread congenitor of the East Brazilian (Least) Pygmy-Owl is the Ferruginous Pygmy-Owl. This species is a little larger than the East Brazilian Pygmy-Owl and has differences in crown pattern, tail pattern and call. You can compare the two by looking at my photo below of Ferruginous Pygmy-Owl; however, the above photo of Least Pygmy-Owl is really too poor to show the differences well at all.

Needing no introduction, the widespread Burrowing Owl, the Athene species of the Americas, also occurs here but in more open habitats such as fields and urban areas and also in sandy coastal areas. The first photo shows a bird in open “cerrado” habitat. The second photo shows an unusual dark-eyed and dark-plumaged individual from an urban area. I had not seen this dark-eyed mutation previously anywhere in South America. 

The genus Strix (large round-headed forest owls) is also represented in southern Brazil by three species; two widespread South American species and one regional endemic, the beautiful Rusty-barred Owl.  Although very similar in appearance to other owls of their genus, I found the rufous and white barred pattern much more beautiful than depicted in any illustration. The last two photos show the wing droop display posture as two birds (possibly a third) interacted. 

The Mottled Owl is a widespread forest species that extents all the way from this region to southern Mexico, although sometimes the Central and North American form is considered distinct leaving Mottled owl as confined to South America. This species often stays fairly high in the canopy as seen here.

Related to the above two species is the more widespread Black-banded Owl. The extremely distinctive black and white owl species (Black-and-white Owl and Black-banded Owl) that were formerly considered in their own genus Ciccaba are now considered to belong to the genus Strix. The Black-banded Owl is mostly a lowland species and occurs in greater densities in the Amazon region, although there is a smaller disjunct population in southeastern Brazil. I was delighted to spot this bird perched high up by their eyes shine, while I was walking in a rather disturbed forest patch, as they were not calling at all in the post-breeding season (as with other species).

The genus Asio has two representatives here (there is also a third species, the Striped Owl that I did not manage to photograph that is sometimes considered to be an Asio). We start with the widespread and famous Stygian Owl. This species gets its name from the River Styx of Greek mythology (entrance to the underworld) because of its dark and “horned” (devilish) appearance. This exceptionally beautiful owl was roosting in the tall shade trees of a town square. This sequence shows how the roosting owl became alert when a flock of White-eyed Parakeets landed nearby (not quite clear if the parakeets were mobbing the owl but they certainly got its attention) and then settled back down and started preening. Note the change in posture and the position of the ear-tufts.

The well-known and cosmopolitan Short-eared Owl also occurs in open grassy habitats in this general area, although it is rather urban in distribution pattern in this part of Brazil. I was lucky to see this owl in the city of Americana where a conservation project exists to steward the owls and their habitat (see previous post).

Last but certainly not least comes the magnificent genus Aegolius and the incredible Buff-fronted Owl, the only Aegolius species found in South America and quite an avian enigma. After so many years of dreaming of seeing this bird and so many sleepless nights in the dry forest of Peru, Ecuador and elsewhere, I finally managed to see this species in the cerrado habitat north of  São Paolo with “a little help from my friends” Cal Martins, Gustavo Pinto, Norton Santos and the enthusiasm of young Matias Ternes who first told me about the Dourado site. Although my visit was at the wrong time of year and although we heard nothing on the first night, on the second evening, after heavy rain, we had an incredible moment of luck when we found this owl with a prey item that appears to be a small bird, possibly a seedeater (the diet of this species is very poorly known). This was well worth the “slide” there and back on treacherous muddy roads. I felt it worth sharing a few points of interest. When the owl turned its head, I noticed that this species has interesting markings on the back of the head that somewhat resemble a false face (vaguely similar to its congenitor Boreal Owl and not unlike the Northern Hawk Owl, but not nearly as obvious as in the pygmy-owls). Unfortunately I didn’t manage a good photo of this. Although I have read about this feature in Boreal Owl, I had not seen this referred to in the literature for Buff-fronted Owl. This bird from southeastern Brazil is the iheringi subspecies, which has sometimes been suggested to be specifically different from the Andean nominate race, although the differences seem relatively minor. The three photos below show the owl with prey, then a side view that show nicely how the long bill is angled down so as not to interfere with the owl's binocular vision and finally a back view that gives a little bit of a sense of the false-face-like pattern on the rear of the head.

Huge thanks to my Brazilian friends for their extraordinary hospitality! Comments most welcome!