YELLOW-BILLED MAGPIE 177 



sometimes by the birds when they suddenly change the course of their 

 flight and dive back downward to the tops of thickets of trees. Their 

 wings make a great roar, then all is silent. Within a short time, two 

 minutes in one example, the flock rises and proceeds normally on its way. 

 Departure of the magpies from the roost on a fall morning is some- 

 what as follows : The birds fly up from the roost in live oaks in a ravine 

 and enter large, leafless black oaks on a ridge. Calls are to be heard 

 often before any birds appear and as early as 20 minutes before they 

 finally leave the hill. The flight to the exposed trees is made singly 

 or in pairs, and the birds perch silently, with heads drawn down close 

 to their shoulders. They finally become vociferous, but the calls are 

 short and quiet compared with ones made later in the day. Calls in- 

 crease slightly or cease altogether before the birds leave. Gradually 

 the single birds begin to move about and combine to make small groups. 

 Sometimes all these groups unite into one big one, especially in disturbed 

 weather, as rain, fog, strong wind, or low temperature. Finally the 

 magpies leave the hill and fly, swiftly and low, to a wooded knoll on 

 the canyon floor, where the long columns of birds converge in the tops 

 of one or two trees. Calls become louder and more frequent than they 

 have been on the hill or in the flight. If the groups are small, 10 or 

 15 minutes are required for all the magpies to leave the hill; if large, 

 the time may be as little as 5 minutes. Next, the birds scatter from 

 the trees to the ground. 



Change in time of sunrise is followed closely by c.hange in time of 

 roost-leaving by the magpies, but there is considerable variation in 

 the time of the actual flight from the hill. On many mornings, espe- 

 cially in December, the magpies, instead of all going to the lower part 

 of the canyon, split up into groups, and some of them fly directly across 

 the canyon to the top of another sunlit, warm hill. From there they 

 move back later in the morning to the floor of the canyon. 



Behavior of magpie flocks at the start of their day on the Hastings 

 Reservation was observed on several occasions in the second half of 

 July in 1938. Once, at 5 :55 a.m., an observer suddenly became aware 

 of raucous magpie calls. A flock in compact formation arrived near 

 the center of the normal nesting area and split into three groups of 

 six or seven birds each. Twenty were counted in all. Two groups lit 

 in blue oaks, the third in a sycamore. All were calling loudly, harshly, 

 and continuously. There was a constant interchange of individuals be- 

 tween the groups. The birds moved into many trees but stayed in the 

 top branches. When they first arrived the din was terrible, as all 20 

 birds squawked loudly at the same time. Twenty-five minutes later 

 the birds had spread out and the calls were more widely spaced. 



