174 BULLETIN 197, UNITED STATES NATIONAL MUSEUM 



haps are preferred to smaller animals, since one or two captures with 

 the aid of the impaling reactions may provide food for an entire day. 

 Secondly, in any habitat at any particular season there exists one or 

 more optimum types of prey, that is to say, animals that by reason of 

 their abundance in the habitat and the efficiency with which they may 

 be captured are most commonly used as food. Obviously, the effi- 

 ciency with which food is captured in turn depends on a delicately 

 adjusted equilibrium between the many protecting devices on the part 

 of the prey and the detecting powers of the shrikes. 



Behavior. — The most profitable method of foraging, although not 

 the one commonly observed, is what I have termed active hunting. This 

 activity occurs usually early in the morning and at dusk in the evening. 

 The actions as seen in gambeli consist principally of perching on 

 objects 6 inches to 6 feet above ground where prey clearly may be 

 seen moving within a radius of a few yards. At times the ground or 

 low bushes are resorted to, and the bird hops about in these places in 

 search of animals. The shrikes in moving rapidly from perch to perch 

 fly close to the ground. Instead of waiting indefinitely on a perch 

 until prey is sighted, if food is not secured from a certain post within 

 a minute or two, the bird moves on to another part of its territory. 

 Much or all of the territory may be covered in a short time as a result of 

 these tactics. 



Contrasted with active hunting is passive hunting, which is noted 

 commonly during a large part of the day at times other than when 

 adults are engaged in feeding young. It is conducted from relatively 

 high perches that at the same time may serve as territorial lookout 

 posts. Food is captured at distances from the perches of 10 to 150 

 feet, and there are extended intervals, often 10 to 30 minutes in dura- 

 tion, when no food is taken. The passive form of hunting occurs when 

 digestion and i^ellet formation are in progress, the results of the early 

 morning active hunting. 



A less common method of foraging is the capturing of insects in the 

 air, the birds maneuvering as do kingbirds, although in an awkward 

 fashion. Dragonflies are caught in this manner. The actions con- 

 sist of darting out into the air at insects as they fly past, the shrike 

 often towering many feet above its perch. 



The flight toward prey on or near the ground commonly is a nearly 

 vertical or diagonal plunge either with set wings or, if necessary, ac- 

 companied by rapid wing motion. Hovering frequently is observed 

 at the end of the approach flight and is either a searching device or 

 serves to allow the bird better to judge the succeeding stages in the 

 attack. Shrikes are adept at following prey that may fly or run from 

 them. They are aided in this by the short, rapidly moving wings and 

 long tail, which enable them to change rapidly the direction of their 

 flight. 



