Irruption of Crossbills

Over the past few weeks, red crossbills have been reported across Metro Denver and here in central Missouri; no doubt, these irruptive migrants have been spotted in many other areas of the U.S. as well.  Residents of Southern Canada, New England, the Great Lakes region and the Western Mountains, red crossbills occasionally appear at lower elevations or in more southern latitudes.

Irruptive species, while usually non-migratory, expand their territory when food (seeds, berries or prey) becomes scarce in their homeland; the scarcity may be do to normal fluctuations in prey populations, severe weather, insect blight, wildfire or, perhaps, climate change.  Red crossbills are among the more common irruptive migrants and have been known to nest outside their usual breeding range if they encounter sites with a large supply of pine cones; indeed, this species may nest during any season of the year.

On the other hand, irruptive species tend to be restless, moving about the landscape in search of food.  Here one day and gone the next, they are often a source of frustration for hopeful birdwatchers, as a friend and I found out yesterday.