Ambient temperature, but not paternity, is associated with immune response in Savannah Sparrows (Passerculus sandwichensis)

Females may engage in extrapair copulations to receive indirect benefits in the form of more immunocompetent young. Nestling quality is often assessed by examining immune function using the phytohemagglutinin (PHA) skin test, which can be a predictor of nestling recruitment. However, the PHA test can also be influenced by factors such as nest temperature. We tested the prediction that extrapair young mount greater immune responses to a PHA challenge and recruit at a greater rate than within-pair young in a wild population of Savannah Sparrows (Passerculus sandwichensis). We also investigated ambient temperature and its relationship with immune response. Extra- and within-pair young did not significantly differ with respect to PHA-induced immune response or with respect to recruitment. In contrast to other studies, we found that PHA-induced immune response was not associated with nestling recruitment rates. However, it was positively correlated with minimum ambient temperature for males hatched in 2002 and 2003, and for females in 2003 (with a similar, but marginally nonsignificant, trend in 2002). Immune response to PHA also was positively correlated with nestling mass for males hatched in 2002 and for females hatched in 2002 and 2003. We concluded that given the lack of relationship among immune responses, paternity, and recruitment rates in this population of Savannah Sparrows, nestling immune response to PHA injection should not be considered predictive of recruitment in populations where this relationship has not been explicitly explored, and environmental variables such as nest temperature may be an important covariate in immunoresponsiveness. Key words: extrapair paternity, Passerculus sandwichensis, phytohemagglutinin, recruitment, Savannah Sparrow, temperature.