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.