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Although magpies usually pair off in 

 their first spring after hatching and may re- 

 main together for years, they need a terri- 

 tory to breed. In the Rivelin Valley, almost 

 all of the suitable habitat is carved up into 

 magpie territories, and with more than 

 seventy-five breeding pairs per square 

 mile, the breeding density here is among 

 the highest ever recorded for these birds. 

 Territories, and hence actual opportunities 

 for reproduction, are hard to come by. 

 Magpies may breed in their first spring 

 after hatching, but do so more usually in 

 their second. In contrast to most other 

 birds, they do not wait for a territory va- 



cancy to occur naturally, but go out as a 

 pair and actively try to create one. This 

 driving need for space, the prerequisite for 

 breeding, proved to be the key to the mag- 

 pie congregations. 



On bright, crisp mornings in late winter 

 and early spring, a high-ranking pair of 

 magpies from the nonbreeding flock may 

 leave their normal home range and fly de- 

 liberately into the heart of an occupied ter- 

 ritory. The territory owners' response is 

 immediate — they fly out to threaten and 

 chase the intruders. The raucous chatter- 

 ing that accompanies these encounters 

 rapidly attracts other magpies, both breed- 



ers and nonbreeders. Within a minute after 

 the two dominant birds invade a territory, 

 up to twenty magpies will be flitting about 

 in the treetops, calling noisily. After care- 

 fully observing my banded birds, I real- 

 ized that most of the action was between 

 the intruders and the territory owners; the 

 other birds were merely noisy spectators, 

 drawn into the melee only when they got 

 in the way of the protagonists. 



The usual outcome of such a gathering 

 was the eviction of the intruders within a 

 few minutes. When this occurred, all the 

 participants quickly dispersed and re- 

 sumed whatever they had previously been 



57 



