We provide evidence that this disease outbreak led to directional selection on both E. coli killing ability (a measure of non-specific constitutive immunity) and body condition. Although we did not discern a significant net change in these traits after the epidemic, we only have comprehensive data from male breeders in the population that were feeding young (of which nine of 32 died; 28%) and know little about the health-state of females or young-of-the-year (a demographic that suffered 51% mortality). Further, a power analysis shows that (for some measures) only a slightly larger sample size could have revealed significant net selection (see ). Also, a screening of survivors showed that 75% had antibodies against eastern equine encephalitis (EEE), strongly suggesting that this pathogen was responsible for the epidemic (R. K. Boughton unpublished data). For perspective, in previous years EEE antibody prevalence was approximately 10 per cent (R. K. Boughton unpublished data).
At the time of data collection a male breeder must provision his mate and offspring at the nest, while defending their territory. The demands remain elevated until the young reach nutritional independence. We speculate that the coincidence of high parental effort with the disease outbreak contributed to the impact of the pathogen upon our study population. Obviously, there are multiple other factors that are not considered here, such as additional components of the immune system (Martin et al. 2006
; Ardia & Schat 2008
) and genetic quality (O'Brien & Evermann 1988
) that could have contributed to survival.
Though speculative, there are a number of possible reasons that we found directional selection in only one of two measures of innate immunity. First, in a comparative study with birds, Millet et al. (2007)
found differences between BKA of S. aureus
and E. coli
within the same individuals. Differences that may be due to immunoredistribution (a temporary shifting of immune resources and function to different components of the immune system; Martin et al. 2006
), the effectiveness or efficiency of toll-like receptors specific for each E. coli
and S. aureus
(Janeway & Medzhitov 2002
), or a population-wide difference in exposure history to these bacteria (Keeler et al. 2007
). It is also important to note, that although there was no statistically significant directional selection for S. aureus
killing, the individuals that survived did
have slightly greater S. aureus
killing ability ().
Our findings that link an aspect of immunocompetence with survival agree with those of other studies (reviewed in Møller & Saino 2004
). Though used extensively in current ecoimmunology research, the BKA we used (see Millet et al. 2007
) has not been linked to fitness in free-living animals. Our results confirm that these measures can be indicators of individual quality and fitness, and support the findings of other avian studies (i.e. Råberg & Stjernman 2003
; Hanssen et al. 2004
) in that physiological condition during one life-history stage can carry over and influence survival during other life-history stages.