doing. If the initiators of the invasion, and 

 indirectly of the gathering, were particu- 

 larly highly motivated, they might fly off 

 to another territory and start the process 

 again. I once watched one such pair start 

 no fewer than seven gatherings, one after 

 the other over a thirty-minute period, 

 being evicted each time. 



Once in a while the outcome is differ- 

 ent. If the territory owners are less than 

 forceful, the rituaUzed threats of the two 

 parties can end up as a serious fight. Dur- 

 ing such a battle, male grapples with male, 

 and female with female, with both sets of 

 birds on the ground with their feet firmly 

 interlocked. Eventually one will gain the 

 upper hand and begin to rain heavy blows 

 with its beak on its opponent's head. In 

 several cases I witnessed, the intruders de- 

 feated the owners and drove them from the 

 territory. The vanquished pair usually dis- 

 appeared (and were presumed dead), but 

 in one case, they were forced to swap 

 places with the invaders and had to hve 

 out the rest of their lives in the nonbreed- 

 ing flock, while the intruders settled into 

 the territory. 



What is going on is that just prior to the 

 breeding season, the dominant members 

 of the nonbreeding flock visit established 

 territories to assess how well they are de- 

 fended. In the majority of cases, territories 

 are under adequate guard and the intruders 

 retreat gracefully, albeit after a brief burst 

 of aggression. But territory tenure is lim- 

 ited—owners eventually age or become 

 sick and are less able to defend their patch. 

 These are just the opportunities intruders 

 are looking for, and once they find a weak- 

 ness, they are relentless in pressing home 

 their attack. 



Why does this territorial probing by 

 dominant nonbreeders provoke the rapid 

 and dramatic gathering of so many other 

 magpies? What is the advantage to those 

 that turn up as spectators? I believe that 

 these other birds can benefit by knowing 

 the outcome of a gathering. For example, 

 if the gathering results in a change of terri- 

 tory ownership, this sometimes precipi- 

 tates several other shifts in territory in a 

 domino effect, creating new breeding op- 



portunities for both estabUshed breeders 

 (hoping to move up market) and non- 

 breeders (hoping to obtain some space). 



Over the duration of my study, I found 

 that about one third of all territories were 

 acquired during a ceremonial gathering, 

 while another third were obtained simply 

 as one magpie replaced another that had 

 died in an occupied territory. The last third 

 were won by pairs squeezing themselves 

 in between the boundaries of existing ter- 

 ritories late in the season. The last strategy 

 was successful because it was undertaken 

 only after most other birds had started to 

 breed and when their territoriality was 

 waning. It was, however, the least produc- 

 tive strategy, because by the time latecom- 

 ers had estabUshed sufficient space to call 

 a territory, the breeding season was over. 

 Like other perching birds of similar body 

 size, 30 to 40 percent of breeding magpies 

 die between one year and the next, so only 

 a few individuals using the "squeezing" 



strategy will survive to see their tactic pay 

 off in terms of producing chicks during the 

 next year's breeding season. 



The only other bird species known to 

 similarly acquire territory through cere- 

 monial gatherings are the Eurasian carrion 

 crow and the acorn woodpecker in Califor- 

 nia, whose gatherings are referred to as 

 "power struggles." (Other bird species 

 form noisy aggregations, but for different 

 reasons; the ubiquitous house sparrow, for 

 example, performs communal sexual 

 chases.) More attention has been paid to 

 magpie gatherings than to the congrega- 

 tions of other species, perhaps because of 

 the magpie's striking plumage, brash man- 

 ner, and dramatic interactions, which have 

 earned it a place not only in the scientific 

 literature but also in local folklore. 



The acquisition of territory is only one 

 of several hurdles a magpie has to over- 

 come if it is to leave any descendants. 

 Once a pair have secured a territory, the 



58 Natural History 3/94 



