Tuesday, August 21, 2018

Avian Quadrate; a difficult bone to describe and understand functionally.

Sifting through a box of bird bones can be a long and arduous task. Even so, this labor of love has allowed me to become comfortable around most specimens, creating a sense of personal satisfaction.  

Despite my deep admirations that I have for a bird’s internal workings, there are some parts of the skeleton which I still find rather intimidating.

Along with the vertebrate, the quadrate is always one of the last bones in a collection to be acknowledged. Why is that? I don’t have any real clear-cut answers, but I suspect it has a lot to do with the bones complexities, as-well-as simply not knowing how it truly functions.

Surprisingly, there is not a whole lot of information out there regarding the quadrates interaction with other bones, both in print and on the internet. I was wondering, how can that be?

By looking at a bird skull, we can see that the quadrates connect the lower mandible to the cranium. 

Spot-billed Duck (Anas poecilorhyncha) quadrate
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
Yes, this bone is obviously very important. In fact, the quadrate bones articulate with the zygomatic arch and the pterygoid-palatine complex, allowing the jaw muscles to pull this flexible complex of bones upward to swallow large food items whole (source; Proctor and Lynch. 1993. Manual of Ornithology). 

Avian quadrate
Image Credit: Proctor and Lynch. 1993. Manual of Ornithology
https://avianmusing.blogspot.com/
One of the best accounts that I have read on the workings of the quadrate can be found in the book; The Inner Bird, by Gary W. Kaiser.

The system for moving the lower jaw in birds looks rather uncomplicated. A pair of joined bones is moved up and down by a set of muscles, just as in a mammal. Unlike in mammals, however, the capacity for sideways movement is greatly limited. In comparison, the arrangement for moving the birds upper jaw looks amazingly complex, partially because we are more familiar with the mammal’s upper jaw, which does not move at all. Even though the birds upper jaw includes a surprising number of bones, its operation is straightforward. The force needed to move it is generated by muscles that stretch either from the sides of the braincase or from behind the eyes to the pair of quadrate bones on either side of the skull, near its base. When these muscles contract, they pull the quadrate forward to raise the upper jaw; when the posterior muscles contract, they pull the quadrates back to lower it. When the quadrates move forward, they press on two sets of pushrods, the pterygoids and the jugal bars, that extend forward to the base of the beak. The pressure from the quadrates is transferred through the pterygoids to the lower edge of the palate, near its midline, while the jugal bars push on the outer edges of the base of the beak, just in front of the eyes. The only way for the beak to ease that pressure on its lower edges is to rotate at the hinge, or bending plate, on its upper edge, just ahead of the eyes. The quadrates can rotate only a short distance because the muscles are short. Even with the help of the pushrods, the movement of upper jaw increases the overall gape of the mouth by only an extra 5%-10%. This small advantage must be significant to birds, however, because they all retain this feature, regardless of the kind of food they eat. 

The Inner Bird by Gary W. Kaiser
Avian Quadrate Workings
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/

To find out more about the quadrate bone please visit;  http://jeb.biologists.org/content/214/12/2036#F1

Mallard (Ana platyrhynchos) quadrate in red 
Image Property: jeb.biologists.org
https://avianmusing.blogspot.com/

See more avian quadrate images below.


American Crow (Corvus brachyrhynchos) quadrate
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
Blue Jay (Cyanocitta cristata) quadrate 
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/

Wednesday, August 15, 2018

Avian Mobbing; influence and variability.

Its an all too familiar site that even the most casual of bird observer will recognize. A Red-tailed Hawk (Buteo jamaicensis) perched majestically on a neighborhood light pole. Beside it, a highly agitated Northern Mockingbird (Mimus polyglottos) doing its best to make it go away.

Northern Mockingbird 'mobbing' Red-tailed Hawk
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
Northern Mockingbird 'mobbing' Red-tailed Hawk
Photo Credit: Paul Cianfaglione
Northern Mockingbird 'mobbing' Red-tailed Hawk
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
Our interpretation for this type of behavior is pretty straightforward. The predatory hawk, most would speculate, is either a threat to the mockingbirds young, or to the mockingbird itself.

Unfortunately, the end results of these interactions often go unseen. So, I’m often left asking, does being overly aggressive toward a predator really work at lessoning the threat?

In Ornithology, the term used to properly describe this aggressive behavior is called ‘mobbing’, and is done singly or in groups, depending on the species.

Mobbing occurs mostly during the height of the breeding season as a way to distract and encourage a predator to relocate to another area. Also referred to as the ‘move on’ hypothesis (Curio 1978), previous studies have confirmed that predators are profoundly distressed by mobbing and avoid the areas where they have been harassed. The ‘move on’ hypothesis appears to be most clearly linked to territoriality (Pavey and Smyth1998).

Before I get into some sure examples of avian mobbing, I’d like to touch upon a study that recently came out (July 2018) implying that ‘American Crows (Corvus brachyrhynchos) are always the bullies when it comes to fighting with Common Ravens (Corvus corax)’.

The study used more than 2,000 publicly collected and submitted observations from across North America via eBird to analyze the interspecific aggression between crows and ravens. From these records, it was determined that crows were the predominant aggressor. Crows primarily attacked in small groups rather than one-on-one confrontations with ravens. The breeding season was when most of the attack observations were made, suggesting that nest predation by ravens influences this behavior. Aggression during the winter is potentially explained by crows preemptively deterring nest predation and defending resources needed for nesting later in the year (source; Sciencedaily).

The results of this study offered little in the way of surprises. Nest predation by ravens is undoubtedly the cause of the crow’s aggression. But does this aggressive behavior toward ravens really translate to lower incidents of nest predation? Some of you will be surprised to learn that the answer is no.

Despite the crow’s best parental efforts, ravens are still successful at providing a steady flow of crow nestlings to their own growing young.

In retrospect, one should probably ask; do ravens knowingly coincide their egg hatching with that of crows? Crow nestlings appear to be an important and well-timed resource. In fact, an overlap in chick raising is curiously supported by the species annual cycle of breeding (source: Birds of North America Online). 

American Crow annual cycle of breeding
Image Credit: Birds of North America Online
https://avianmusing.blogspot.com/
Common Raven annual cycle of breeding
Image Credit: Birds of North America Online
https://avianmusing.blogspot.com/
So, are ravens in any way influenced or intimidated by crows? The answer may lie where the two species interact outside of the nesting season.

In my backyard, a small flock of crows will freely surrender their food source to a single large raven, returning only when it has long departed. In this setting, ravens are clearly the dominant species and are not intimidated by the presence of crows, even when heavily outnumbered. 

Common Raven at a backyard feeding station
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
This may explain why ravens are so effective at securing crow nestlings. Do crow’s offer any resistance when confronted by a prowling raven? Why does ‘mobbing’ fail in this situation?

Crows, on the other hand, may have better luck intimidating, or ‘moving on’ less active Red-tailed Hawks and Great Horned Owls (Bubo virginianus).  

Not only are the hawks and owl’s threats to the nest, they pose danger even to the adults at times.

Crow mobbing aimed at these predators can be particularly harsh, sending hawks into the paths of moving cars or buildings.

One of the most disturbing attacks that I have ever witnessed happened at a local cemetery, where a Great Horned Owl was observed being harassed at its daytime roost. Surrounded closely by a crowd of angry crow’s, the terrified owl slipped off a branch, lodging its neck tragically into the fork of a tree. 

Great Horned Owl death as a result of avian 'mobbing'
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
Is the death of a dangerous predator the intended outcome of every avian ‘mobbing’? Probably not. But it is for all intents and purposes, a result many birds would gladly welcome.

Birdwatchers will often tell stories of a more familiar form of ‘mobbing’ in some of our smaller bird species.

Warbling Vireo (Vireo gilvus) eggs for instance are often the target of opportunistic Blue Jays (Cyanocitta cristata), yet I have seen vireos on many occasions, successfully mob and distract jays well away from their leaf-hidden nests.

One of the simplest ways to find roosting Northern Saw-whet Owls (Aegolius acadicus) in winter is by listening and following the scolding calls of Black-capped Chickadees (Poecile atricapillus) and Tufted Titmice (Baeolophus bicolor). 

Black-capped Chickadee (Poecile atricapillus)
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
David Allen Sibley writes in his book, The Sibley Guide to Bird Life and Behavior; parids are social, bold, and inquisitive, traits that endear them to humans but make them among the most tenacious and aggressive songbirds when mobbing owls or other predators. Chickadees and titmice usually take the lead in announcing the presence of a small owl.

Does being overly aggressive toward a predator really work at lessoning the threat? In the vast majority of cases, ‘mobbing’ is an effective way of protecting eggs, nestlings and themselves.

The unusual crow-raven relationship is little bit harder to understand, and accept. The crow’s overall population however seems little affected by this ‘taking’ of birds.

One has to wonder if the raven’s sudden expansion into urban areas has to do with the presence of nesting crows more than anything else.

Tuesday, August 7, 2018

Owl Cervical Vertebrae; adaptive features for head rotation.

The owl has been a part of pop culture for as long as I can remember, from movies to television to advertising. Today, owls continue to dominate our society, so much that we don’t even realize that they are around us. By that I mean on top of shopping mall buildings, car dealerships and even in our gardens, replica owls are used as decoys to repel unwanted pests, animals and rodents. 


Owl Decoy
https://avianmusing.blogspot.com/
What makes the owl decoy so effective as opposed to other predatory birds? Is it their large eyes? Angry expression? Fearsome reputation? All are classic owl traits. But it’s the rotating head that really sets them apart; a distinctive life-like feature and a realistic profile. I jokingly ask, what good is an owl decoy if it can’t fully rotate its head?

So how does an ‘real’ owl spin its head around? What major biological adaptations are known today to allow for such a unique behavior? 

For this, I turned to a new book that I just recently completed titled; The Ascent of Birds, How Modern Science is Revealing their Story, by John Reilly (2018). 

The Ascent of Birds by John Reilly (2018)
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
It is here in chapter 10 that the author nicely describes the ‘The Owl’s Story’; revealing a number of the owls previously unknown and unique adaptations, including its ability to rotate its head.

Given that owls are closely related to raptors and mousebirds, it is likely that the earliest species were diurnal and relied primarily on sight to obtain their prey. As a result, all owls have large eyes that are very efficient at capturing and processing light. They are not spherical organs, as in most birds, but elongated structures, held in place by bony protuberances known as sclerotic rings. Such large, forward-facing eyes provide owls with an improved depth perception, but they are essentially fixed and cannot be moved within their sockets. To compensate, all owls can rotate their heads through 270 degrees in either direction without having to move their bodies. For us, this would mean turning our head to the left so that we end up looking over our right shoulder. Not a maneuver I suggest you try, as your delicate blood vessels would be severely damaged, resulting in a major stroke or even death. So how do owls manage it?

To find out, a team from John Hopkins University School of Medicine, led by medical illustrator Fabian de Kok-Mercado, injected radio-opaque dye into the bloodstream of dead owls and took a series of CT scans while the head was twisted. The results revealed a number of previously unknown and unique adaptations. Firstly, the owl’s cervical vertebrae contain transverse foramina (‘holes’ for the vertebral arteries and veins) that are much larger than those of other species, including humans. In our neck, the foramina are roughly the same size as the blood vessels, whereas in the owl they are almost 10 times the diameter of the arteries, with the extra space acting as air sacs that cushion the vessel during twisting motion of the head. Furthermore, the last few cervical vertebrae lack transverse foramina, an anatomical adaptation that provides the cord-like arteries with some slack when the head rotates.

Owls also possess connections or anastomoses between their carotid and vertebral arteries, a feature that ensues a constant blood flow to both sides of the brain even if the vessels on one side of the neck are occluded. Finally, the main arteries – carotid, mandibular and maxillary – contain contractile reservoirs that are hypothesized to ensure an uninterrupted blood supply to the brain and eyes, even during the most extreme of head movements. It seems that the ‘wise old’ owl has not just evolved one answer to the neck-twisting problem; it has come up with a whole raft of solutions (end of book citation).

These were extraordinary findings, so much so that I felt compelled to see it for myself.

First, I compared three avian cervical vertebrae side-by-side; a Barred Owl (Strix varia), Mallard (Anas platyrhynchos) and an American Crow (Corvus brachyrhynchos). The claim made in the book regarding the owl’s cervical vertebrae containing transverse foramina that are much larger than those of other species, is still a mystery to me. The holes, in relation to the size of the cervical vertebrae, appeared nothing out of the ordinary. 

From top to bottom; Barred Owl, American Crow and Mallard Cervical Vertebrae
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
Avian Foramen transversarium
Handbook Of Avian Anatomy (1993); Baumel and Witmer
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
I was however more intrigued by the fact that the last few cervical vertebrae were reported to lack transverse foramina altogether. This was a feature that should be easy to detect. 

Barred Owl (Strix Varia) Cervical Vertebrae
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/

Surprisingly, my inspection of the cervical vertebrae instead revealed holes on the transverse processes, albeit very narrow ones.

Barred Owl (Strix varia) 14th Cervical Vertebrate
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
What was going on here with the owl’s last few cervical vertebrae? Why the retention of holes? Could the existence of holes in the transverse processes imply a more recent evolutionary adaption for extreme head and neck rotation?

Given their other set of raptor-like characteristics, large eyes, binocular vision and powerful talons, one could envision head rotation along with the unequal positioning of their ear openings for acute hearing, as adaptations later evolved by owls for a more successful nocturnal existence.  


John Reilly mentions in his book that 40% of extant species are commonly active in daylight, especially around dawn and dusk, only two are truly diurnal, a fact largely dictated by prey preference.

Why retain holes in the transverse processes if they are bypassed and no longer of use? Were today’s owls forced into a nocturnal lifestyle, not long ago, by more aggressive daytime competitors like hawks and corvids?

I believe this is a reasonable idea, loosely supported by this final snippet;

‘Most of the early owl lineages, however, were subsequently displaced by other bird orders, and only two survived: Barn Owls (Tytonidae) and the typical or ‘true owls’ (Strigidae). At the time, the Strigidae would have resembled the Spotted Owl or the Tawny Owl, and their marked diversity of morphology and anatomy only evolved during the last 15 million years (The Ascent of Birds, How Modern Science is Revealing their Story. Reilly J. 2018).  

Thursday, August 2, 2018

Brazilian Merganser (Mergus octosetaceus). All Birds Considered.

Inspired by David Attenborough’s book, The Life of Birds, is a monthly spin-off segment of my blog called “All Birds Considered”, which I hope will bring attention to some of the more unusual, and lesser known birds of our world.

The Life Of Birds by David Attenborough
Photo Credit: Paul Cianfaglione
https://avianmusing.blogspot.com/
Species; Brazilian merganser (Mergus octosetaceus)

Introduction; The Brazilian merganser (Mergus octosetaceus) is a duck in the typical merganser genus. It is one of the six most threatened waterfowl in the world with possibly fewer than 250 birds in the wild and currently 4 kept in captivity at 2 different Brazilian locations. The origin of its name is from its long, sharp-edged beak that has a great number of teeth-looking edges.

Brazilian Merganser (Mergus octosetaceus)
Image Credit: Wkipedia
https://avianmusing.blogspot.com/


Population; The total world population for Brazilian mergansers is believed to be less than 250 birds. Originally, the duck's geographical distribution comprised central-south Brazil and adjacent regions in Paraguay and Argentina. Currently, all confirmed populations are located in Brazil and a more recent population in Argentina and information on most populations is very scarce. The Brazilian Merganser population in the Serra da Canastra region is the most significant and best known, with populations occurring hundreds of kilometers away from each other. There are 47 individuals—28 adults and 19 young—in the Serra de Canastra region as of 2006. Most mergansers are found in the Serra da Canastra National Park. 70 birds have been seen near the park's headquarters in Rio São Francisco. In Jalapão region are estimated 13 individuals in Novo river on 2009/2010 (four couples and some solitary adults).

In 2002, the species was also found on the Arroyo Uruzu in Misiones, Argentina, the first record in the country for ten years, despite extensive surveys done by local researchers conducted throughout previous years. The bird was last reported seen in 1984 in Paraguay, where very little habitat remains, however, some local reports show that a few individuals may still be living in the area.
Description; Narrow bill. Head, neck and long crest black with green sheen. Back blackish; breast, flanks, and belly finely barred dark and light grey. Wings blackish with white secondaries and a white line through wing-coverts.

Voice; The Brazilian mergansers are generally silent birds, but may make barking calls in certain situations. Four calls have been recorded. A harsh krack-krack acts as an alarm call emitted in flight. Males make a barking dog-like call, females make a harsh rrr-rrrr and the contact call ia a soft rak-rak-rak. Ducklings give a high pitched ik-ik-ik.

Behavior; The species spends the days interspersing foraging and resting periods. Generally, they rest on rocks or branches which are projected out of water, as well as on river beaches. They also can stay in the water motionless or with very slow movements.

Brazilian Merganser (Mergus octosetaceus)
Image Credit: Wikipedia
https://avianmusing.blogspot.com/
Habitat; Streams and rivers in sub-tropical forests.

Justification of Red List category; Critically Endangered (IUCN 3.1) The Brazilian mergansers are very sensitive to habitat degradation and loss primarily due to human actions. A major threat to the birds' survival is the issue of silting of rivers caused by the expansion of farming activities, mining, watershed degradation and soil erosion, as well as deforestation.

Source:

https://neotropical.birds.cornell.edu

Wikipedia

The Life of Birds by David Attenborough, 1998.

Birds of Southern South America and Antarctica. 1998. Martin R. DE LA Pena and Maurice Rumboll