COPULATION AND MATE GUARDING IN THE YELLOW-BILLED MAGPIE PICA-NUTTALLI AND A COMPARISON WITH THE BLACK-BILLED MAGPIE P-PICA

Yellow-billed magpies breed in loose colonies. Extra-pair courtship is frequent, and a comparison (1) between two adjacent areas with different inter-nest distances, and (2) with solitary, territorial European black-billed magpies, indicates that either an aggregated distribution of nests, or a relatively loose territorial system in yellow-billed magpies increases opportunities for extra-pair behaviour. Male yellow-billed magpies guard their partner by closely following them from five days before the first egg is laid. Pair copulations were infrequent, occurring 3 or 4 times per clutch, and mainly over days -5 to -3. Territoriality, which is most pronounced during the female's fertile period, may enable the male to guard his partner while simultaneously foraging to provide her with food.