Y£LLOW-BILLED MAGPIB 175 



perching places oftenest used when the birds are not on the ground are 

 the larger-sized limbs of big trees. On telephone wires they appear to 

 have no difficulty in balancing. When coming to a stop the bird may 

 jerk its tail upward four or five times and then maintain its balance with 

 the tail held close to 45° below the horizontal. Foraging birds may jerk 

 their tails upward independently of need for balance. A slight jerk of 

 the tail often accompanies a vocal note. 



When starting to fly away after being disturbed the birds do not flap 

 evenly. Several spurts may be distinguished. In one example the rhythm 

 was three easy beats followed by two vigorous ones. The contrast was 

 audible as well as visible in a bird flying 25 feet overhead. On another 

 occasion this irregular flight consisted of five to eight rather weak wing 

 beats and then four or five rapid, strong beats. The flight of the several 

 individuals, however, was not synchronized. Another habit of flight 

 observed often is that of gliding downward for long distances, extending 

 the wings, and braking the speed at regular intervals. 



Flocks of yellow-billed magpies may be seen all through the nesting 

 period, but these are mainly temporary formations, probably accidental 

 assemblages. When the young birds leave their homes, however, the 

 flocking tendency soon becomes conspicuous again, and the birds form 

 definite aggregations that have the quality of permanency. The unit of 

 organization for this species then changes from the pair to the flock. 

 For the next half year the behavior of the flock is our chief concern 

 with this species. 



A striking example of responses of members of a flock to a certain 

 note of one of their species was observed in late November. At 4:30 

 P.M., after the birds had congregated and just before they started off 

 to roost, they were perching quietly in some oaks. A dead snag broke 

 under one of the magpies in the vicinity, causing it to fly away squawk- 

 ing in alarm. The flock, calling noisily, flew in a compact body to the 

 scene of the disturbance. The birds perched on the remaining limbs of 

 the snag and on nearby trees and then became silent. From there they 

 finally flew off, 15 and 20 minutes later, to roost. 



Roosting time is the occasion for congregation of the largest and most 

 compact flocks. The birds assemble in tree tops and then seem to await 

 the move of one to act as leader. Sometimes the group flies out to join 

 one or more individuals flying overhead ; sometimes they follow one of 

 their number as it makes a start from the perching tree. If one bird 

 turns back the whole group is apt to follow, whether the bird is ahead 

 of or behind the others. On such occasions they are likely to come 

 swooping back down to trees or thickets, swerving erratically, and wings 

 whistling. In these assemblages there is apparent a great reluctance to 



