Long-term demographic data on the red-cockaded woodpecker

In cooperative breeding systems, inclusive fitness theory predicts that
non-breeding helpers more closely related to the breeders should be more
willing to provide costly alloparental care, and thus have more impact on
breeder fitness. In the red-cockaded woodpecker (Dryobates borealis), most
helpers are the breeders’ earlier offspring, but helpers do vary within
groups in both relatedness to the breeders (some even being unrelated) and
sex, and it can be difficult to parse their separate impacts on breeder
fitness. Moreover, most support for inclusive fitness theory has
been positive associations between relatedness and behavior, rather than
actual fitness consequences. We used functional linear models to evaluate
the per capita effects of helpers of different relatedness on eight
breeder fitness components measured for up to 41 years at three sites. In
support of inclusive fitness theory, helpers more related to the breeding
pair made greater contributions to six fitness components. However, male
helpers made equal contributions to increasing pre-fledging survival
regardless of relatedness. These findings suggest that both inclusive
fitness benefits and other, direct benefits may underlie helping behaviors
in the red-cockaded woodpecker. Our results also demonstrate the
application of an underused statistical approach to disentangle a complex
ecological phenomenon.