Peregrine falcon

Kingdom: Animalia

Fig1. Perergine falcon in flight 

Phylum: Chordata

Class: Aves

Order: Falconiformes

Family: Falconidae

Genus: Falco

Species: F. peregrinus

This species is probably one of the more widely distributed of the raptors, famous for the speeds they reach during the pursuit of prey (White, Pruett-Jones & Emison 1980). But what is more interesting is that they also show different approaches when it comes to hunting their aerial prey.

Peregrine falcons as mentioned hunt avian prey, which occurs mostly in the open skies, taking prey out at high speeds of up to 125ms^-1, which in most cases is the result after initiating a dive (Tucker 1998). Though in order for them to reach such speeds during their dives and flight aerodynamic efficiency is required (Tucker, et. al 2000). This is achieved by maintaining a streamline form and folding their wings nearly flush to their body depending on how much the incline is and speed required (Tucker 1998).

Although, straight flight and its’ accuracy are affected by distance. As the distance of approach increases the falcon is more inclined to turn it’s head to use its’ monocular vision for precision, reducing aerodynamic efficiency (Tucker, et. al 2000).  

Peregrine falcons have been seen to show other flight patterns to hunt avian prey, with a primary focus on approaches from long distances in order to maintain aerodynamic efficiency. This flight pattern is referred to as the curved flight pattern, this allows the peregrine falcon to reach its’ destination and maintain an aerodynamic form over long distances (Tucker, et. al 2000). But aside from efficient flight, as mentioned by Tucker, et al. (2000 p.3762), other explanations for this behaviour include, hunting out of the sun and misleading prey.  In either case, the curved flight allows falcons to utilise the morning sun, making it difficult for prey to detect the approach and not being a straight approach the prey is less likely to startle (Tucker, et. al 2000).

Reference:

Tucker, V.A., Tucker, A.E., Akers, K. & Enderson, J.H. 2000, "Curved flight paths and sideways vision in peregrine falcons (Falco peregrinus)", Journal of Experimental Biology, vol. 203, no. 24, pp. 3755-3763.

Tucker, V. 1998, "Gliding flight: Speed and acceleration of ideal falcons during diving and pull out", Journal of Experimental Biology, vol. 201, no. 3, pp. 403-414.

White, C., Pruett-Jones, S. & Emison, W. 1980, "The status and distribution of the Peregrine Falcon in Victoria, Australia", Emu, vol. 80, no. 5, pp. 276-277.

Raptorial digestion?

You would think that raptors all digest their prey similarly and that their digestive systems wouldn’t show any difference.

Although, there are in fact differences between the digestive morphology and processes found in some raptors (Ford 2010, Smith & Richmond 1972, Duke, et. al 1975). The differences that are discussed in this blog focuses on digestive morphology, the formation of pellets and gastric pH of the stomach.

Unlike Psittaciformes (parrots) and most other birds, raptorial diet consists highly of protein in the form of small lizards, rodents, birds or fish where the piscivorous raptors diet consists of fish (Fowler, Freedman & Scannella 2009). However raptors when eating, tear fairly large morsels of flesh from its prey, consuming a large amount in a short time, where it is stored in the crop (lacking in Strigiformes) (Ford 2010).

The digestive process in raptors is also more chemical than mechanical, where they have a gland that secrete mucus and another secreting HCl and pepsin (Ford 2010). Allowing the breakdown of protein structures, even so raptors such as hawks and owls form pellets (Fig 1) (Ford 2010, Duke, et. al 1975). 
 Pellets are undigested compressed materials that are egested (regurgitated) from owls and hawks, though it is observed that the amount of undigested materials (mainly bones) in Falconiformes is lower than that of owls (Moon 1940, Duke, et. al 1975). Reasons for this include Falconiformes initially not ingesting as many bones in comparison to owls (swallowing prey whole) and that the gastric pH in the of owls are higher than that of the Falconiformes (Moon 1940, Duke, et. al 1975, Smith & Richmond 1972).

Fig1. Simple illistration depicting the process of pellet formation (sourced by: https://www.tes.com/lessons/PBj-79gS-EQIjQ/science-owl-pellets) 

Reference:

Duke, G.E., Jegers, A.A., Loff, G. & Evanson, O.A. 1975, "Gastric digestion in some raptors", Comparative Biochemistry and Physiology -- Part A: Physiology, vol. 50, no. 4, pp. 649-656.

Ford, S. 2010, "Raptor Gastroenterology", Journal of Exotic Pet Medicine, vol. 19, no. 2, pp. 140-143.

Fowler, D.W., Freedman, E.A. & Scannella, J.B. 2009, "Predatory functional morphology in raptors: Interdigital variation in talon size is related to prey restraint and immobilisation technique", Plos One, vol. 4, no. 11, pp. e7999.

Moon, E.L. 1940, "Notes on Hawk and Owl Pellet Formation and Identification", Transactions of the Kansas Academy of Science (1903-), vol. 43, pp. 458-465.

Smith, C.R. & Richmond, M.E. 1972, "Factors Influencing Pellet Egestion and Gastric pH in the Barn Owl", The Wilson Bulletin,vol. 84, no. 2, pp. 179-186.

Hearing how does it benefit raptors?

Raptors are as mentioned in previous blogs have keen eyesight in which they rely on in hunting. 

Although, some raptors such as owls you find morphological adaptations, such as having facial disks, as well as the positioning of the ears that allow them to locate prey (Singheiser, et al, 2010). But aside from the owls’ adaptations, sensitive acoustic hearing that allows them to locate prey isn’t exclusive to owls; this form of detection is also seen in diurnal raptors (Rice, 1982).

Fig 1. diagram indicating properties related to hearing in Barn owls and their position (source from

All About Owls)

Firstly, owls (Strigiformes) being nocturnal hunters their vision allows them to detect movement though it is limited; therefore to precisely locate and capture prey they are guided by acoustic location (Hausmann, et al 2009). For example, barn owls and their dish-shaped face allows the sound of fairly low frequencies to be heard, as well as the positioning of the right and left ears being asymmetric helps the owl orientated and locate their prey in the dark (Singheiser, et al 2010, Payne 1971, Hausmann, et al 2008).

Fig 2. Shown in this figure is a mouse with paper tied and trailed behind it, determining whether the owl locates prey by sound or sight in the dark (sourced from Konishi, M. 2012 )

Secondly, acoustic detection and location are also seen utilised by diurnal raptors such as the marsh hawk which detect small mammals concealed in long the grass, even though it would play a larger role in nocturnal raptors (Rice 1982). Though the marsh hawks have facial ruffs and display quartering behaviour (not typical of Accipitridae but seen in owls), they do not show the accuracy demonstrated by the owl (Payne 1971, Hausmann, et al 2008). As mentioned by Rice (1982), some tested marsh hawks would strike several times in the attempt to flush out or would miss their target.

This blog is not to say all owls and other raptors have similar acute hearing or accuracy with acoustic location.

Reference:

Hausmann, L., Plachta, D.T.T., Singheiser, M., Brill, S. & Wagner, H. 2008, "In-flight corrections in free-flying barn owls (Tyto alba) during sound localization tasks", Journal of Experimental Biology,vol. 211, no. 18, pp. 2976-2988.

Hausmann, L., von Campenhausen, M., Endler, F., Singheiser, M. & Wagner, H. 2009, "Improvements of sound localization abilities by the facial ruff of the barn owl (Tyto alba) as demonstrated by virtual ruff removal", Plos One, vol. 4, no. 11, pp. e7721. 

Payne, R.S. 1971, "Acoustic location of prey by barn owls (Tyto alba)", Journal of Experimental Biology, vol. 54, no. 3, pp. 535-569.

Rice, W.R. 1982, "Acoustical Location of Prey by the Marsh Hawk: Adaptation to Concealed Prey", The Auk, vol. 99, no. 3, pp. 403-411.

Singheiser, M., Plachta, D.T.T., Brill, S., Bremen, P., van der Willigen, Robert F & Wagner, H. 2010, "Target-approaching behavior of barn owls (Tyto alba): influence of sound frequency",Journal of Comparative Physiology A, vol. 196, no. 3, pp. 227-238.

What are the use of raptorial feet?

This blog will focus heavily on families that hunt their prey such as Accipitridae, Falconidae, Strigiformes and Pandionidae: osprey. 
Beside their eyes, these raptors also have their feet that make them lethal hunters (Fowler, Freedman & Scannella 2009). Although while you might find that these raptors all have the characteristic feet, the talons and digit positioning between species have subtle but significant differences (Fowler, Freedman & Scannella 2009). But despite these differences all raptorial feet share a common function; that is to capture, restrain and immobilise their prey (Fowler, Freedman & Scannella 2009).

Accipitridae is distinguishable by their enlarged talons on the I and II digits, Falconidae by the elongated III digit, Strigiformes by their large but not very rounded talons and Pandionidae from the large equal sized, largely rounded talons found on all digits (Fig1.) (Fowler, Freedman & Scannella 2009). These differences are due to prey preference and the way in which these raptors capture and immobilise their prey (Fowler, Freedman & Scannella 2009). The enlarged talons on Accipitridae are thought to restraining larger prey items, where death occurs through being eaten alive causing blood loss and organ failure (Fowler, Freedman & Scannella 2009). Falcons on the other hand like the peregrine falcon predate birds, flying at high velocities in order to dislocate the spine but when this isn’t effective they’ll overpower the bird and sever the spine with their beak (Fowler, Freedman & Scannella 2009, Brown  & Amadon 1968). In contrast, owls will tend to prey primarily on smaller rodents, thought to be why owls have less curve to their talons and have the specific positioning of their digits, that they might be able to entirely grasp prey rather pierce their prey (Fowler, Freedman & Scannella 2009).  Finally, osprey unlike birds and rodent hunting raptors hunt fish, where their feet have sharp spicules for enhanced grip in addition to their largely curved talons restraining and immobilising their prey items (Brown  & Amadon 1968, Fowler, Freedman & Scannella 2009, Poole, A.F., 1994).

Fig1. Feet of Accipitridae (A, B), Falconidae (C), Strigiformes (D) and Pandionidae (E) with labelled digits I, II, III, IV (by Fowler, Freedman & Scannella 2009). 

References: 

Brown, L. & Amadon, D. 1968, Eagles, hawks and falcons of the world, Country Life Books, Feltham, pp. 17-29.

Fowler, D.W., Freedman, E.A. & Scannella, J.B. 2009, "Predatory functional morphology in raptors: Interdigital variation in talon size is related to prey restraint and immobilisation technique", Plos One, vol. 4, no. 11, pp. e7999.

Poole, A.F., 1994, “Family Pandionidae (Osprey)”, Handbook of the birds of the world, vol. 2, pp.42-50.

Is there olfactory use in birds of prey?

Is olfactory sensory insignificant and not used in birds?

This was thought true until studies showed Procellariiformes used olfactory senses to locate prey, this use of olfaction is also seen in kiwis and species of raptors (Malakoff 1999, Balthazart & Taziaux 2009, Cornfield 2015).

Focussing more towards raptors, diurnal and nocturnal species generally rely on eyesight to locate prey (O'Rourke et. al 2010, Jones, Pierce & Ward 2007). Although this might be the case for the vast majority of raptors, some buzzards, and few vultures show the use and reliance more on olfactory cues, while not discarding the use of visual cues (Lisney et. al, 2013, Yang, Walther & Weng 2015). It is also suggested that aside from location resources of food, olfaction allows communication and other purposes such as; predator avoidance and navigation (Cornfield et. al 2015, Malakoff 1999).

As for why these raptors focussing on the Oriental honey buzzard (Pernis orientalis) and Turkey vulture (Cathartes aura) are capable of locating food resources via olfaction is related to the development and size of the olfactory lobes, located in the frontal region of the brain (Fig1.) (Cornfield et. al 2015). The association being the larger the lobes the more adapted the species are to use olfaction (Cornfield et. al 2015). As well as olfactory receptor genes and its' sequence length are suggested to contribute towards olfactory capabilities (Yang, Walther & Weng 2015). 

Fig1. Brain diagrams indicating differing olfactory lobes highlighted in blue, on the left-hand side (A), whereas on the right-hand side (B) are the cross-sections 

For P. orientalis, it was decerned from the experiments by Yang, Walther & Weng (2015), that P. orientalis use of olfaction distinguishes pollen containing foods from non-containing food. It was also determined that P. orientalis have a larger olfactory receptor gene sequence compared to that of the sequences of falcons and eagles in the experiment, where 81.5% of the genes were functional, but olfaction was not as acute as seen in vultures (Yang, Walther & Weng 2015).  

As for C. aura, it is said to have large, highly developed olfactory lobes and is able to locate and detect carrion without visual cues from great distances (Lisney et. al 2013, Smith & Paselk 1986). This is also observed in Yellow-headed vultures (Graves 1992).

Reference:

Balthazart, J. & Taziaux, M. 2009, "The underestimated role of olfaction in avian reproduction?”, Behavioural Brain Research, vol. 200, no. 2, pp. 248-251.

Corfield, J., Price, K., Iwaniuk, A., Gutierrez-Ibanez, C., Birkhead, T. & Wylie, D. 2015, "Diversity in olfactory bulb size in birds reflects allometry, ecology, and phylogeny", Frontiers in Neuroanatomy, vol. 9, no. JULY, pp. 102-16.

Graves, G.R. 1992, “Greater yellow-headed vulture (Cathartes melambrotus) locates food by olfaction”, Journal of Raptor Research, vol. 26, no. 1, pp.38-39.

Jones, M.P., Pierce, K.E. & Ward, D. 2007, "Avian Vision: A Review of Form and Function with Special Consideration to Birds of Prey", Journal of Exotic Pet Medicine,vol. 16, no. 2, pp. 69-70.

Lisney, T.J., Stecyk, K., Kolominsky, J., Graves, G.R., Wylie, D.R. and Iwaniuk, A.N. 2013, “Comparison of eye morphology and retinal topography in two species of new world vultures (Aves: Cathartidae)”, The Anatomical Record, vol. 296, no. 12, pp.1954-1956.

Malakoff, D. 1999, "Following the Scent of Avian Olfaction", Science, vol. 286, no. 5440, pp. 704.

O'Rourke, C.T., Hall, M.I., Pitlik, T. & Fernández-Juricic, E. 2010, "Hawk eyes I: Diurnal raptors differ in visual fields and degree of eye movement", Plos One, vol. 5, no. 9, pp. 1-8.

Smith, S.A. & Paselk, R.A. 1986. “Olfactory sensitivity of the Turkey Vulture (Cathartes aura) to three carrion-associated odorants”, The Auk, pp.586-592.

Yang, S., Walther, B. & Weng, G. 2015, "Stop and Smell the Pollen: The Role of Olfaction and Vision of the Oriental Honey Buzzard in Identifying Food", Plos One, vol. 10, no. 7, pp. 8-15.

Sexual dimorphism in some raptors have been switched? how can this be explained?

In birds, sexual dimorphism is the morphological difference observed in appearance and size between males and females as well as in many species of vertebrae (Owens & Hartley 1998). This dimorphism is explained as being associated towards the social interactions in mating, the males tending to have distinct morphological differences in appearance (eg. colourful plumage and bigger in size) in most species being the competitive sex (Owens& Hartley 1998).

Although in the case of raptors like Falconidae as well as some other families, females show the morphological difference in bigger sizes, referred to as a reversed sexual dimorphism (RSD) (Olsen & Olsen 1987, Olsen 2013).  Even though it is not certain as to which sex changed, or when they changed in size, there are explanations in the attempts to distinguish why females and males show reversed dimorphism in their size (Montgomerie & Lundberg 1989).

 It is suggested that males being smaller allow efficient energetic output, longer hunting ranges as well as agility and speed when hunting prey items such as mammals and birds (Sonerud et. al 2013, Olsen 2013, Slagsvold & Sonerud 2007). This is because the male will dominantly provide and collect food for the female and chicks (Olsen 2013). Also, reversed sexual dimorphism is said to aid in flight, compensating for the increase in weight of the female prior to egg laying (Wheeler & Greenwood 1983). Meaning RSD is prominent in high flight performance dependent raptors (Wheeler & Greenwood 1983).

Another explanation is that intrasexual competition is the reasoning behind RSD as females will compete and command nest sites with a mate and that being bigger aids in bluffing and avoiding potential conflicts (Olsen & Olsen 1987)

Lastly, another explanation is that RSD allow mates to utilise different ranges of prey resources avoiding inter-sexual competition (Sonerud et. al 2013, Olsen & Olsen 1987). Although as mentioned by Wheeler & Greenwood (1983 p.148), this highlights the size relation to the diet where larger females and smaller males may predate large and small prey respectively, it doesn't address why the RSD occurs. 

These are some of the explanations attempting to illustrate why some species of raptors show this reversed sexual dimorphism. 

Reference:

Montgomerie, R. & Lundberg, A. 1989, "Reversed Sexual Dimorphism in Raptors: Which Sex Changed Size?", Oikos, vol. 56, no. 2, pp. 283-286.

Olsen, J. 2013, "Reversed Sexual Dimorphism and Prey Size Taken by Male and Female Raptors: A Comment on Pande and Dahanukar (2012)", Journal of Raptor Research, vol. 47, no. 1, pp. 79-81.

Olsen, P. & Olsen, J. 1987, "Sexual Size Dimorphism in Raptors: Intrasexual Competition in the Larger Sex for a Scarce Breeding Resource, the Smaller Sex", Emu, vol. 87, no. 1, pp. 59-60.

Owens, I. P. F.  & Hartley, I.R. 1998, "Sexual dimorphism in birds: why are there so many different forms of dimorphism?", Proceedings of the Royal Society of London. Series B: Biological Sciences, vol. 265, no. 1394, pp. 397-407.

Slagsvold, T. and A Sonerud, G., 2007. “Prey size and ingestion rate in raptors: importance for sex roles and reversed sexual size dimorphism.” Journal of Avian Biology, vol. 38, no. 6, pp.650-655.

Sonerud, G.A., Steen, R., Løw, L.M., Røed, L.T., Skar, K., Selås, V. & Slagsvold, T. 2013, "Size-biased allocation of prey from male to offspring via female: family conflicts, prey selection, and evolution of sexual size dimorphism in raptors",Oecologia, vol. 172, no. 1, pp. 93-103.

Wheeler, P. & Greenwood, P.J. 1983, "The Evolution of Reversed Sexual Dimorphism in Birds of Prey", Oikos, vol. 40, no. 1, pp. 145-148.

Owls: silent flight is it all the same?

Many owls have adapted features for silent flight mainly due to the serrated leading edges on their wings; other reasons are thought to be the soft downs on their wings as well as trial feathers (Weger & Wagner 2016, Bachmann et. al 2007, Kun et. al 2012).

These serrated edges are located on a few primaries and alula feathers (Fig1.) and reduce noise frequency below 2kHz (Weger & Wagner 2016, Kun et. al 2012, Bachmann et. al 2007). Although differences in these serrated comb-like edges are present between species, due to different activity times meaning diurnal and nocturnal owls are bound to predate different organisms (Weger & Wagner 2016).

Fig1. Labeled diagram indication location of serrations (from Weger & Wagner 2016 p.3)

This difference in prey would indicate different hunting methods. Such as nocturnal owls locate prey by bi-aural acoustics and hunt silently as well as slowly due to increased drag by the serrations allowing undetected approach (Weger & Wagner 2016). Whereas diurnal hunting owls that do not hunt in the cover of dark, the reduced flight speed from serrations would impede their hunting fitness thus you see lesser developed serrations (Weger & Wagner 2016).

Therefore, the silent flight is not an adaption that is observed in all Strigiformes species as serration formations vary in occurrence to the time of activity in the owls, but is seemingly advantageous in nocturnal species.



Reference:

Kun, C., Qingping, L., Genghua, L., Ying, Y., Luquan, R., Hongxiu, Y., Xin, C. 2012, "The Sound Suppression Characteristics of Wing Feather of Owl （Bubo bubo）", 吉林大学仿生工程学报：英文版, vol. 9, no. 2, pp. 192-195.

Jaworski, J.W. & Peake, N. 2013, "Aerodynamic noise from a poroelastic edge with implications for the silent flight of owls",Journal of Fluid Mechanics, vol. 723, pp. 456-457.

Weger, M. & Wagner, H. 2016, "Morphological Variations of Leading-Edge Serrations in Owls (Strigiformes): e0149236", PLoS One, vol. 11, no. 3, pp. 1-18.

Bachmann, T., Klän, S., Baumgartner, W., Klaas, M., Schröder, W. & Wagner, H. 2007, "Morphometric characterisation of wing feathers of the barn owl Tyto alba pratincola and the pigeon Columba livia", Frontiers in zoology, vol. 4, no. 1, pp. 1-2.

An insight to raptorial vision

In comparison to humans, birds generally have larger eyes considering their body and head size as the eyes can take up to 50% of the skulls volume (Jones, Pierce & Ward 2007). This means they depend on a more visually active lifestyle and that they use their eyesight to not only find food but also to avoid being predated, as well as observing their environmental surroundings (Jones, Pierce & Ward 2007, Garamszegi, Møller & Erritzøe 2002).

Fig 1. Orientation and visual field in avians from (Martin 2009)

With a focus towards raptorial adaptations, the orientation of the eyes in diurnal raptors is relatively laterally orientated, having a better monocular vision but narrow binocular vision, whereas forward orientated owls have a greater binocular visual field but lack posterior vision (Fig.1) (Iwaniuk, Hall & Wylie 2008).Also depending on what raptor is being observed there will be differences in the eyes size and shape (Jones, Pierce & Ward 2007). 

The eye shapes seen in birds of prey mainly consist of round and tubular shaped eyes (Fig 2.) (Sturkie 1987). These eye shapes are characteristic of diurnal and nocturnal raptors respectively, as diurnal raptors require the ability to see long distances in higher definition, in contrast to owls and with keener eyesight from their concave ciliary region (Jones, Pierce & Ward 2007, Sturkie 1987).

Fig.2 attained from Sturkie (1987 p.39)

But aside from orientation and exterior morphology, internal structures such as the retina and fovea will be discussed.
The retina (Fig3.) contains rods and cones, which allows diurnal and nocturnal raptors to see in their environment (Jones, Pierce & Ward 2007, Sturkie 1987). These raptors are cone dominated and rod dominated respectively, as cones have photoreceptors (Fig 5.) allowing them to filter out different wavelengths of light depending on the oil droplet they contain, whereas rod photoreceptors in low light environments discern shapes and movements (Jones, Pierce & Ward 2007, Sturkie 1987). 

The fovea (Fig 4.) is characteristically deep in diurnal raptors and is, in fact, something that most taxa lack, but raptors have two foveae, being the area that aids in the acuity of vision, this allows diurnal raptors see on more than a single plain stationary and whilst hunting (Jones, Pierce & Ward 2007, Sturkie 1987).  

Fig 3. Obtained from Glasser & Howland (1996 p.479)

Fig 4. deep (A) and shallow fovea (B) from Jones, Pierce & Ward (2007 p.77)

Fig 5. Labeled diagram of cone photoreceptor from Jones, Pierce & Ward (2007 p.77)

Reference:


Glasser, A. & Howland, H.C. 1996, "A History of Studies of Visual Accommodation in Birds", The Quarterly Review of Biology, vol. 71, no. 4, pp. 479.

Garamszegi, L.Z., Møller, A.P. & Erritzøe, J. 2002, "Coevolving avian eye size and brain size in relation to prey capture and nocturnality", Proceedings of the Royal Society of London. Series B: Biological Sciences, vol. 269, no. 1494, pp. 961.

Iwaniuk, A.N., Heesy, C.P., Hall, M.I. & Wylie, D.R.W. 2008, "Relative Wulst volume is correlated with orbit orientation and binocular visual field in birds", Journal of Comparative Physiology A, vol. 194, no. 3, pp. 267-268.

Jones, M.P., Pierce, K.E. & Ward, D. 2007, "Avian Vision: A Review of Form and Function with Special Consideration to Birds of Prey", Journal of Exotic Pet Medicine, vol. 16, no. 2, pp. 69-79.

Martin, G.R. 2009, "What is binocular vision for? A birds' eye view", Journal of Vision, vol. 9, no. 11, pp. 14-14.

Sturkie, P. D., 1987, Avian Physiology: Fourth Edition, Springer-Verlag, New York, pp. 39-45.

Distinguishing genetic relations and associations with Diurnal birds of prey and owls opposed to morphological evidence, is it important?

Birds of prey (Raptors) are characteristically carnivorous birds that hunt their prey and stated by an adaption of Baird are in the Order: Raptores, consisting of Vultures, Diurnal raptors (osprey, kites, falcons, eagles) and nocturnal raptors being owls (Sibley, C.G. & Ahlquist, J.E 1990).

The raptors classifications are generally distinguished by physical (morphological) characteristics for instance, some eagles and their feathered "booted" legs, also behavioural features that can be used in the field (Morris, F.T 1976, Weick, F. & L.H, Brown 1980). Because of this early scientists may have associated the two orders closely, as mentioned by Sibley& Ahlquist (1990 p.178) in their adaption of Bairds classification. But with the development of DNA analysis, scientists were able to classify organisms based on not just characteristics but genetics as well. This classification is determined specifically by the closeness and relationship between sequences of genes found along the DNA of organisms (Sibley, G. C. & Ahlquist, J. E. 1990, King, R.C. & Stansfield, W.D. 2006).

Thus leading to the classification of diurnal raptors and owls, looking at Order classification. Diurnal raptors are classified under the Order: Falconiformes, with five families within that order, sharing a common ancestor, whereas owls are classified by the Order: Strigiformes, with two families (Wink, M. and Sauer-Gürth, H., 2004, Brown, L. & Amadon, D. 1968, Christidis, L. & Boles W.E. 2008). Although with DNA analysis, the issue of properly distinguishing the relationships of owls and diurnal raptors was apparent. As mentioned by Christidis & Boles (2008 p.54), scientists formed associations with nightjars and owls,  others suggested that Strigiformes might have a weak relation with Falconiformes based on genetics. Further more with similarities in bone structures of the owl and diurnal raptors, Mayr (2005 p.639) suggested Strigiformes are a sister group to Falconiformes, although without sufficient data as proof for this, it was not conclusive.

Therefore determining an organisms’ relationship with DNA analysis, such as the owls and Diurnal raptors, is definitely something that should be of importance as it could indicate the evolutionary history of that particular organism. Overall determining how they might have evolved specific features and the relationships that they have with other organisms.

Reference:

Books:

• Brown, L. & Amadon, D. 1968, 'cLASSIFICATION AND dISTRIBUTION', in Balding and Mansell (eds), Eagles, Hawks and Falcons of the World, Hamlyn Publishing, Great Britian. pp.17-21
• Christidis, L. & Boles W.E. 2008, 'Higher-level avian systematics', in J  Kelly (eds), Systematics and Taxonomy of Australian Birds, CSIRO, Victoria. pp. 52-54.

• King, R.C. & Stansfield, W.D. 2006, A dictionary of genetics, 7th edn, Oxford University Press, New York. 

• Morris, F.T 1976, Birds of Prey of Australia, Lansdowne Editions, Melbourne. pp.14-23.

• Sibley, G. C. & Ahlquist, J. E. 1990, 'Structure and Properties of DNA', in J J Johnson (eds), Phylogeny and Classification of Birds: A study in Molecular Evolution, Yale University Press, United States of America. pp. 27-29.
• Sibley, G. C. & Ahlquist, J. E. 1990, 'A Chronological Survey of the Classification of Birds',  in J J Johnson (eds), Phylogeny and Classification of Birds: A study in Molecular Evolution, Yale University Press, United States of America. pp.187
• Sibley, G. C. & Ahlquist, J. E. 1990, 'Structure and Properties of DNA', in J J Johnson (eds), Phylogeny and Classification of Birds: A study in Molecular Evolution, Yale University Press, United States of America. pp. 245
• Weick, F. & L.H, Brown 1980, 'Topography of diurnal bird of prey (Heterospizias)', in P Parley (eds), Birds of Prey of the World, Collins, London. pp.12
• Weick, F. & L.H, Brown 1980, 'Key for identifying', in P Parley (eds), Birds of Prey of the World, Collins, London. pp.124

Scientific papers:

• Mayr, G. 2005, "The postcranial osteology and phylogenetic position of the Middle Eocene Messelastur gratulator Peters, 1994—a morphological link between owls (Strigiformes) and falconiform birds?", Journal of Vertebrate Paleontology, vol. 25, no. 3, pp. 639-640.

• Wink, M. and Sauer-Gürth, H., 2004. “Phylogenetic relationships in diurnal raptors based on nucleotide sequences of mitochondrial and nuclear marker genes”, Raptors worldwide. WWGBP, Berlin, pp.483-487.