Why Do Kestrels Wings Look More Rounded?

An American Kestrel clinging to a section of telephone wire is a common roadside sight across much of North America. This species is the smallest of North America's falcons (genus Falco) and also the one we are most likely to encounter. Falcons are generally characterized as fast fliers and described as having pointed wingtips. However, in my experience American Kestrels are decidedly weaker fliers than other falcons. Their flight style can be described as more buoyant and their wingbeats have a bit of a rowing motion. Additionally, their wingtips often look blunt and somewhat rounded at the tip when fully extended.

It's funny what you discover when editing photos. While struggling to find a keeper among a bunch of modest to dismal quality flight shots I'd taken of an American Kestrel, I noticed several shots in which the bird appeared to have rather rounded wingtips. This rounded shape has always been one of those nagging sub-conscious birding questions that I never invested the time to answer. But on this occasion I began taking a closer look at the wing shape and the feathers that create it.

The outermost 10 feathers on a falcon's wing are its primaries. These feathers, along with the secondaries and tertials, comprise the flight feathers of a bird. In general, a falcon's primaries vary in length, getting progressively longer towards the wingtip. When these feathers are fully extended the trailing edge of the wings is very even and fairly straight, which creates the somewhat pointed wingtip we associate with falcons. The outermost primary (P10) is foreshortened a bit, so P9 appears to be the longest feather on the wing of most falcons.

This female Peregrine Falcon, photographed at Chametla, Baja California Sur, Mexico on 13 January 2009, exhibits the wing shape of most falcons. Looking closely at the right wing, note the fairly straight trailing edge of the wing and that P9 (second to outermost wing feather) is bit longer than P8, which is just a bit longer than P7. The relative lengths of these feathers create the attenuated or pointed look of a falcon's wingtips.

The 8th and 9th primaries (P8 and P9) of American Kestrels are distinctly longer than the other primaries and in flight P8 generally looks longer than P9, which breaks up the clean line along the trailing edge of the outer wing. This makes the wing look somewhat more blunt or rounded at the tip.

This male American Kestrel, photographed near Brownsville, Linn Count, Oregon 20 February 2008, shows a rather blunt and rounded tip to the right wing. Note the relative lengths of P10 (the outermost), P9, P8, and P7. P8 and P9 are much longer than the two primaries that bracket them, and P8 (rather than P9) appears to be the longest of the flight feathers.

I've examined numerous flight shots of American Kestrels both in books and online and found that this relationship between the lengths of P8 and P9 is consistently evident.   

All photos taken by David Irons using a Canon EOS XSI 450D camera and an EF 100-400mm f/4.5-5.6 lens