Abbreviated inner primaries: A sex-linked dimorphism in the Acorn Woodpecker

In at least 15 species of woodpeckers, the inner primaries of nestlings are reduced in size. This trait is polymorphic in Acorn Woodpeckers (Melanerpes formicivorus) at Hastings Reservation in central coastal California, where most nestlings (90.1%) had reduced first and second primaries, 9.8% had only a reduced first primary, and 0.1% had no reduced primaries. More males (11.1%) had large second primaries than did females (5.3%), and females with this trait were less likely to survive their first year of life. Smaller inner primaries may provide a temporary energetic savings for nestlings that facilitate faster growth, but, if so, why this trait is not more common among other cavity-nesting species remains unknown. This trait appears to be more ecologically interesting than previously suspected and warrants study in other species and populations of woodpeckers.