Black-billed magpies are common residents along the Colorado Front Range and we often observe pairs or small flocks on our Littleton farm. Yesterday, however, we had a full-fledged magpie invasion, consisting of thirty or more birds. All were adult in size and their feeding activity was erratic; indeed, they spent most of the afternoon noisily moving about the property.
Breeding magpie pairs are thought to be monogamous (at least for several seasons) and pair off by late winter to build their nest. Using the same territory each year, they generally produce six young which fledge by late summer. Local non-breeding birds, most of which are yearlings, account for about half of the magpie population and wander about in flocks, a behavior that improves their chance of survival and their opportunity to secure a breeding partner; such flocks also include older magpies that are widowed or were displaced from a previous partner. Magpies are also known to congregate at the site of a dead magpie (so called "magpie funerals") but I could not find a carcass to explain yesterday's gathering.
The flock moved on by early evening and not a single magpie visited the farm today. Presumably, they have moved on to greener pastures though most magpies remain within a few territories of their birth site in the course of their lives. Such a limited range is made possible by their aggressive nature and their omnivorous diet; the latter consists of insects, larvae, seeds, nuts, grain, eggs, fruit, carrion, nestlings, lizards, small snakes, frogs and mice (not to mention discarded human food).
