Figure 1. Distribution of the Red Crossbill in North America.
Based on knowledge of major crossbill audio collections, conifer distributions, published literature, and documented records. Core Zone of Occurrence (solid dark purple) represents the area where key conifers are found and where birds can regularly be found utilizing those key conifers during most typical years -- Breeding is common in numbers the majority of years. Secondary Zone of Occurrence (solid light purple) represents area where lesser numbers of birds can be found many years with some regularity -- Small numbers of breeding birds present most years. Primary Zone of Irruption (solid dark blue, and it also sometime includes a light dashed purple line to represent area where some call types are known to irrupt to, and nest, occasionally in small numbers) represents area where birds commonly move to when cone crops on key conifers fail in their core zones of occurrence – these areas often harbor conifer species most similar to those found in core zones, and in some years upon arrival, if cone crops are good on appropriate conifers, they can nest in small numbers. Secondary Zone of Irruption (solid light blue) represents area where birds move to only when there’s widespread cone failures on many conifers they typical use in core zones -- nesting is rare. Solid light yellow represents tertiary zone of irruption where movement is uncommon and breeding rare and local, and it only pertains to Type 2. Dashed Blue represents area of irruptive migration. Solid light purple dots represent known local breeding.