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Over looked, or at least under reported, in our newspaper columns and newsletters were the raptors counted which were judged to also be migrating along with the 31,000 Turkey Vultures. Due to more experienced and very dedicated observers, a record 116 raptors were recorded. There were 17 Golden Eagles! As for hawks, 28 Red-tailed, 1 Cooper's, 2 Ferruginous, 37 Swainson's and 11 unidentified buteo species. In addition there were 2 Prairie Falcons and 14 Osprey.
These sightings deserve a separate column, especially the Golden Eagles, Swainson's Hawks and Osprey. However, instead we’ll introduce the bird we did not see - the Zone-tailed Hawk. Although one of the borderland buteos, it's has a wider range then the others, and from time to time is seen along the southern California coast in winter. This past spring and summer there were sightings of this bird along the central and northern coasts!
Why were we alerted to watch for this bird during the vulture count? Well, because they hang out with Turkey Vultures. Why would they do that? Please refer to your field guides. Note the grayish-white flight feathers and dark under wing coverts. Both fly with heads drooped and may be seen cutting lazy circles in the sky with wings in a moderate dihedral. Remind you of a Turkey Vulture?
As Sibley says, " Incredibly similar to Turkey Vulture, with which it often soars; distinguished only by details." The details are, narrower winged and longer tailed then Red-tail Hawks (yet similar in size), feathered head, uniformly bared flight feathers, one broad and one narrow white tail band (female has two of the narrow bands).
Zone-tail flight patterns are similar to harriers and vultures. That is, floating about with a moderate dihedral and tending to wobble a bit. Vulture-like, its flap cadence is slow and wing beat deep. When soaring high, as in migration, a Zone-tail's wings will be extended flat - vulture like. If it flies like a vulture, looks like a vulture, is it a vulture? No, however it is very likely exploiting its vulture-likeness.
Birds that are preyed upon by hawks recognize vultures as a non-threat. It is not a lack of vigilance on the part of birds, lizards and small mammals. In fact the Zone-tail's prey may very well see this mimic and dismiss the bird as non-threatening. By disguise and therefore surprise the signals do not go off.
Is this look-a-like disguise a product of convergent evolution, or the result of adaptations that mimic Turkey Vultures? We think the latter. The Turkey Vulture waits for something to die, while the Zone-tailed Hawk is an aggressive hunter of live prey to kill. The range of the Zone-tailed lies within the more extensive Turkey Vulture range. If there was no benefit in exploiting this look-a-likeness, then Zone-tails certainly would be found beyond the range of the Turkey Vulture.
Further, as Pete Dunne points out in "The Wind Masters", there probably are but 100 or so breeding pair of Zone-tails north of the Mexican border from Arizona to western Texas, and seldom do they nest within five miles of each other. If their hunting success is dependent on ruse, then over exposure would certainly diminish the effect.
High in the sky Zone-tails are the mirror image of a Turkey Vulture. Did we miss it on the count as it hid in one of those big-day multi-kettles of 800 or 1000 birds? Or was it one of the 11 unknown buteo species flying high and far from us over the ridges? In the June, 2000 edition of Birding, Dunne, Sibley and Sutton write, “the dapper Zone-tailed Hawk is enigmatic: relatively easy to find (in its range), but only if it can be identified.”
Good birding, gone birding.
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