Black-billed magpies form monogamous pairs throughout the breeding season and may form lifelong bonds. In some regions black-billed magpies switch mates almost yearly. Courtship displays involve the male flashing his wings and flaring his tail at the female. Females will call loudly during their fertile period. Males respond by courtship-feeding of females. These behaviors continue from mating to incubation of the eggs. Since other males may be attracted by the loud calls of the fertile female, males vigilantly guard their mates to prevent extra-pair copulations.
Mating System: monogamous
Black-billed magpies breed from late March to early June, depending on location. There is usually one brood per season, although a second brood may be attempted if the first is not successful. Females lay up to 9 eggs (average of 6) which are greenish-gray with brown markings. The eggs are 3.3 x 2.3 cm and are sub-elliptical to oval. The first egg hatches within 25 days of being laid, hatching is asynchronous with usually only one chick hatching per day. The chicks are born without feathers and the eyes remain closed for 7 days. Fledging takes place between 24 to 30 days after hatching. The parents feed the chicks in or near the nest for the first 3 to 4 weeks and the young are beginning to fend for themselves by weeks 6 to 8. Young become independent at about 70 days. Females are likely to nest in their first year, whereas males may not breed until their second or third year.
Breeding interval: Black-billed magpies produce one brood yearly.
Breeding season: Black-billed magpies breed from late March to early June.
Range eggs per season: 9 (high) .
Average eggs per season: 6.
Range time to hatching: 25 (high) days.
Average fledging age: 27.5 days.
Range time to independence: 70 (high) days.
Average age at sexual or reproductive maturity (female): 1 years.
Average age at sexual or reproductive maturity (male): 2 years.
Key Reproductive Features: seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate)
Both sexes help to construct an elaborate, domed nest which may take between 5 and 7 weeks to complete. Males focus on building the 60 to 120 cm high dome, while females focus on the egg bowl, which is a mud cup lined with hair, grasses, bark strips, fibrous roots, and feathers. The female is the primary defender of the nest prior to and during egg laying, and males become the defender of the nest during incubation. Like other members of the corvid family, the female is the sole incubator and is almost totally reliant on the male to feed her during this period. Because the sexes have specialized roles, if either mate dies during incubation, the brood does not survive. Both male and female help to gather food for the nestlings after hatching. The female is presumed to eat the fecal sacs of the young. The post-fledging period, during which parents continue to feed their young, is much longer than in crows. This period may be instrumental in teaching the young how to recognize danger and what the most appropriate response is for a particular predator.
Parental Investment: altricial ; male parental care ; female parental care ; pre-fertilization (Provisioning, Protecting: Male, Female); pre-hatching/birth (Provisioning: Male, Female, Protecting: Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female, Protecting: Male, Female); post-independence association with parents; extended period of juvenile learning