Ardea
Official journal of the Netherlands Ornithologists' Union

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Baeyens G. (1981) Functional aspects of serial monogamy: the Magpie pair-bond in relation to its territorial system. ARDEA 69 (2): 145-166
In a marked population of Magpies in Haren the establishment of territories by non-breeders and the filling up of vacancies was studied over a period of 4 years. If one pair member in a territory disappears, the remaining bird often remates with an individual which in its turn divorces its original mate. Two types of question are examined: 1) are non-breeders prevented to establish territories and reproduce by territoriality? 2) what are the possible advantages or disadvantages of switching mates? Non-breeders lived either in flocks or solitary. Between flock members a more or less linear rank order existed. High ranking juveniles were more likely to establish territories than low-ranking and solitary juveniles. Males seemed to establish closer to their natal nest than females. Two types of territory could be distinguished: class I which were permanently inhabited and class II which were only temporarily inhabited. Part of the study area never contained territories. Class I-territories were most suitable for breeding a.o. because Crow interference was lowest (due to higher human activity and more tree cover), and thus chances for successful reproduction highest. Vacancies in class I-territories were immediately taken over, often by occupants of class II-territories. Non-breeders also filled vacancies in class I-areas if they were given an (experimental) opportunity. When the original class I-owners, after temporary captivity, were returned to their original territory, they deprived the non-breeder substitutes again from their chances to reproduce. Breeding density in class I-areas was apparently limited by territoriality. It is still unknown which factors determined the breeding density of the population as a whole. When class II-territory owners moved into a class I-vacancy, they usually disrupted the bond with their previous mate and paired with the class I-territory owner that was still present. Switching mates, in class I-areas at least, did not lower the chances to breed successfully. The possible advantages of switching+ mates are discussed. An individual which divorces its mate in a class II-territory and remates with the class I-territory owner might gain if I) the new partner is healthier and/or co-operates better, 2) it passed on genes responsible for the' acquisition of better territories to its offspring, or 3) it allows the newcomer to save time and energy by copying its activities with regard to self maintenance and territory defence.


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