PATTERNS OF NEST SITE, TERRITORY, AND MATE SWITCHING IN BLACK-BILLED MAGPIES (PICA-PICA)

A colour-marked population of black-billed magpies (Pica pica) was studied in Edmonton, Alberta, from 1982 to 1988. Between years, magpies changed nest sites within their territories significantly more often than they remained in the same site. Both sexes showed equally strong territory fidelity, with 72 % of the males and 70 % of the females nesting within their territories of the previous year. Breeding success in the previous year did not determine whether magpies switched nest sites within territories or changed territories, or how far they moved between years. In 1 year (1987), breeding failure (during a snowstorm) influenced the switching of neither nest sites nor territories. Although there was remarkable mate fidelity in some pairs (paired for 5 - 7 years), other magpies changed mates up to three times over the 7 years of the study. Both sexes showed a similar tendency to switch mates; 50 % of the males and 63 % of the females changed mates at least once in 7 years. Mate switching was independent of reproductive success in the previous year. There was a positive association between mate fidelity and territory fidelity in males: males that changed territories were also likely to change mates. These results are compared with those from other studies of black-billed magpies and discussed in the context of life-history strategies.