serious business of breeding ensues, and 

 here, too, competition is rife and vigilance 

 necessary on the part of the male. Al- 

 though Darwin knew about the magpies' 

 gatherings and recognized the general sig- 

 nificance of reproductive competition, he 

 assumed that the females of monogamous 

 species — in which a mated pair raise 

 young — were strictly monogamous. As in 

 many species of birds long considered 

 faithful within pairs, the truth is more 

 complex, as revealed by a particular inci- 

 dent I witnessed one day at the beginning 

 of the breeding season. 



The pair I was observing had laid part 

 of their clutch, and the female was still fer- 

 tile. As she searched for insects in a field, 

 her mate sat on a nearby stone wall eyeing 

 her every move. To my surprise, the 

 male's head gradually sank onto his chest, 

 and he fell asleep in the spring sunshine. 

 No sooner had he stopped observing his 

 partner than the male from the neighbor- 



ing territory flew over and, without any of 

 the usual precopulatory niceties, mounted 

 the female. Although receptive, she 

 chanced to utter a cry and awake her 

 spouse. He swooped down to attack the in- 

 truder, who coolly retreated to his own 

 partner and territory. Calling noisily, the 

 wronged male then chased the female 

 back to their nest tree, and the two birds 

 disappeared into the dense vegetation. A 

 day or two later, I noticed the male build- 

 ing a new nest in a tree near the one that 

 housed their original nest. This action 

 would have been normal if the first clutch 

 had been taken by a predator, but on 

 checking, I found the partly completed 

 clutch intact. The male appeared to be 

 starting over and siring a new clutch of 

 eggs in order to avoid the risk of rearing 

 one or more of his neighbor's offspring. 



This incident was unusual only in that 

 the male fell asleep. Male magpies are es- 

 pecially keen to obtain sneaky matings 



Like their crow relatives, magpies will 

 harass hawks that approach the nest or 

 otherwise threaten their livelihood. In this 

 case, a magpie unsuccessfully attempted 

 to divert a buzzard from feeding on a 

 dead rabbit. 



with breeding females, but must also 

 guard their own mates to prevent being 

 cuckolded. Males do not take this threat — 

 known as extra-pair copulation — lightly. 

 During the time that his female can be fer- 

 tilized, a period of about one week, the 

 male stays within a few yards of her from 

 dawn until dusk, following her every 

 move. He remains close enough to inter- 

 cept any males trying to sneak a mating. 



Only already mated males, rather than 

 single males, sneak matings, and they do 

 so when their own females are just past the 

 fertile stage and the pressure of guarding 

 her is relieved. Females accept the atten- 

 tions of interiopers and readily mate with 

 them, but conversely, do not appear to 

 condone the extra-pair activities of their 

 own males. Each member of a magpie pair 

 appears to attempt to optimize its own 

 chances of copulating with more than one 

 partner, while retaining a mate with which 

 to rear chicks. On several occasions, I 

 placed a caged female bird in an occupied 

 territory. If the male territory owner ap- 

 proached this decoy bird alone, he invari- 

 ably started to court her, singing and try- 

 ing to mount her by placing his foot on her 

 back through the bars of the cage. If, how- 

 ever, he was caught in such behavior by 

 his partner, he instantly switched from 

 courting the decoy to displaying aggres- 

 sion toward her. 



The black-billed magpie was one of the 

 first bird species in which mate guarding 

 was described. Subsequent studies over 

 the past fifteen years have shown that such 

 behavior by males during their partners' 

 fertile period is standard in many birds. 

 However, guarding does not guarantee pa- 

 ternity. In many so-called monogamous 

 species, such as the reed bunting in Eura- 

 sia and the splendid fairy wren in Aus- 

 tralia, more than a third of aU the offspring 

 in a population are fathered through extra- 

 pair copulations, and some males help to 

 rear young that are not genetically their 

 own. We suspect that the same may be true 

 for magpies and hope to ascertain this by 

 testing for paternity through DNA finger- 

 printing. 



Over the course of the study, I have fol- 



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