On Friday I gave you this unidentified specimen from the Horniman’s collections to take a look at. I had already had a go at working out what it is, but it never hurts to get a second opinion.
It’s actually a bit of a generic looking overall shape, perhaps reminiscent of a owl or a maybe a pheasant of some sort. However, the nares (nostrils) are very small and round and set in a bill that is sharp, shortish and very solidly constructed, which is something you only really see in a few passerines, some parrots and the falcons. The skull is too big for a passerine and the bill is totally the wrong overall shape for a parrot, which leaves us with a falcon – a fairly small one at that.
From there the shape of the palate and the proportions of the cranium led me to a species identification that I’m pleased to say agreed with that proposed by Tony Irwin and Wouter van Gestel (who eloquently explained the indicative characters that I mentioned above). We all think that this is the cranium of a Eurasian Kestrel Falco tinnunculus Linnaeus, 1758.
Kestrels are a familiar sight in the UK, often seen hovering along motorway verges as they hunt for voles and other small rodents, a behaviour I find quite fascinating from a biomechanical perspective.
Normally hovering is a very energetically demanding mode of flight, accompanied by complex and rapid wingbeat patterns. Hummingbirds take hovering to an extreme and their humerus is remarkably short and oddly shaped to do it – plus they feed on huge amounts of sugary nectar to maintain their levels of hyperactivity.
Kestrels manage to hover in a far more leisurely and energy efficient way, relying on wind power rather than burning huge amounts of fuel. Instead of flapping fast to keep enough air flowing over their wings to generate lift, they find a spot (often near a slope) where there is good persistent breeze that they simply fly into at the same speed as the breeze is blowing. If the breeze slows down they will flap a bit more to maintain their lift and if it speeds up they can glide and might even pull their wings in a bit to reduce the surface area over which lift is generated so they maintain their position.
Obviously the bird needs to have a remarkable set of wind speed sensors over its body to be able to maintain this sort of fine control, particularly if the wind isn’t uniform and the body has to constantly change position in response. These behaviours can all be seen in the video below – also watch out for Kestrels flying in more blustery conditions where the wind is a bit turbulent, you will see that their secondary flight feathers ruffle up, trapping vortices of air that roll up the back of the wing and would otherwise cause a stall. Fascinating stuff!
For the more scientific-minded readers there’s also an interesting paper by Videler et al. from 1983 that looks at Kestrel flight from an energetic perspective. Enjoy!