Black-billed Magpie - Reproduction

Reproduction

Adult Black-billed Magpie pairs stay together year-round and often for life unless one dies, in which case the remaining magpie may find another mate. Divorces are possible: one South Dakota study found low rates of divorce (8%) but one 7-yr study in Alberta found divorce rates up to 63%.

Black-billed Magpies nest individually, frequently toward the top of deciduous or evergreen trees. Only the nest tree and its immediate surroundings are defended, and so it is possible for nests to be somewhat clumped in space. When this happens (usually in areas with a limited number of trees or with abundant food resources), a diffuse colony is formed. In this the Black-billed Magpie is intermediate between the European Magpie, whose nests are much more spread out because a large territory is defended around each nest, and the Yellow-billed Magpie, which is always loosely colonial.

Nests are loose but large accumulations of branches, twigs, mud, grass, rootlets, bark strips, vines, needles, and other materials, with branches and twigs constituting the base and framework. The nest cup is lined with fine rootlets, grass, and other soft material. Nests almost always include a hood or dome of loosely assembled twigs and branches, and usually have one or more side entrances. Nests are built by both sexes over 40–50 days, starting in February (though later in northern parts of the range). Old nests can be repaired and used, or a new nest can be built on top, with older nests thus reaching 120 cm high by 100 cm wide (48 inches high by 40 inches wide). Other bird species, including small hawks and owls, often use old magpie nests.

The breeding season for magpies is generally from late March to early July. They nest once a year, but may re-nest if their first attempt fails early. The female lays up to thirteen eggs, but the usual clutch size is six or seven. The eggs are greenish grey, marked with browns, and 33 mm (about 1.3 inch) long. Only the female incubates, for 16–21 days. The male feeds the female throughout incubation. Hatching is often asynchronous. Hatched young are altricial, brooded by the female but fed by both sexes. They fly 3-4 weeks after hatching, feed with adults for about two months, and then fly off to join other juvenile magpies. Fledging success (usually 3-4 young per nest) is lower than clutch size; this is not an unusual state of affairs in species with asynchronous hatching, as some nestlings often die of starvation.

Black-billed Magpies breed for the first time at 1 or 2 years of age. The lifespan of the species in the wild is about four to six years.