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.

Methods
We used long-term demographic monitoring data collected over 28 to 41 consecutive years at three sites: the Sandhills region in south-central North Carolina (1980–2020), Marine Corps Base Camp Lejeune on the central coast of North Carolina (1986–2020), and Eglin Air Force Base in the western panhandle of Florida (1993–2020). Monitoring methods are described in detail by Walters et al. (1988) (see also Appendix A for more details on monitoring). See Walters and Garcia (2016) for how individuals are assigned breeder and helper status.