In the article entitled "The Deep Fovea, Sideways Vision and Spiral Flight Packs in Raptors", Vance A.Tucker of the Department of biology, Duke University, North Carolina, published in November 2000, he talks about Falcons Eagles and Hawks, collectively known as raptors, as having two areas of the focal point of the retina (the fovea), that he calls the shallow fovea and the deep fovea. Through his work and through the work of other biologists it has been found that the deep fovea has a much higher acuity of sight, compared to the shallow fovea.
Birds in general tend to spend much of their time looking sideways at an object depending on the proximity of it. When an object is further away, between approximately 8 m and 21 m, they adopt an observational style which is more readily identifiable to us humans, as that kind of inquisitive head tilted, sideways look. Indeed as objects move further away again, approximately 80% of their gaze is through these angled positions.
When considering that birds of prey swooped down on to their victims from significant heights, air to ground speed increases, and therefore any changes in drag coefficients and aerodynamics through altering their wings, or indeed their head position would slow the descent and give prey animals more time to react. It seems therefore that raptors have modified their behaviour in such swoops and dives, so as to make an approach in what appears to be spiral pattern. What this means is that the raptor can continue at the fastest velocity, yet at the same time through sideways focus through their leading eye, they are able to take advantage of almost straight line speed without losing their abilities of extreme accuracy in vision.
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