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.